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The freshwater mussels (mollusca: bivalvia: unionoida) of Nebraska.

This paper reports the results of the first statewide survey of the freshwater mussels of Nebraska. Survey goals were: (1) to document current distributions through collection of recent shells; (2) to document former distributions through collection of relict shells and examination of museum collections; (3) to identify changes in distribution; (4) to identify the primary natural and anthropomorphic factors impacting unionids; and (5) to develop a model to explain the documented distributions. The survey confirmed 30 unionid species and the exotic Corbicula fluminea for the state, and museum vouchers documented one additional unionid species. Analysis of museum records and an extensive literature search coupled with research in adjacent states identified 13 additional unionid species with known distributions near the Nebraska border. Some of these unionids may have formerly inhabited the state. Seven documented species have probably been extirpated, and the ranges of 15 others have contracted significantly. Only nine species are relatively stable at present, and one of these is extremely rare and federally endangered. Diversity is concentrated in the formerly glaciated portion of eastern Nebraska, and drops abruptly along the Missouri River to the east and in the Great Plains westward. The primary natural factors influencing unionid distributions in Nebraska include availability of perennially flowing waters, host fish diversity, substrate composition and stability, and formerly heavy sediment loads. Anthropomorphic factors are surface and subsurface irrigation withdrawals, construction of canals and reservoirs, channelization, erosion, intensive grazing, contaminants, and the introduction of invasive species. Unionid presence and diversity largely correlates with substrate stability. Stream orientation and morphology in portions of eastern Nebraska promote slow currents, and substrates composed of mud, sand, and rock provide greater stability than the shifting sand substrates common elsewhere in the state. In central and western Nebraska, unionid populations are concentrated in headwaters, backwaters, and side-channels of rivers, and canals, reservoirs, and small tributaries, while the generally shifting sand habitats of most rivers provide relatively little viable habitat. In the historic Missouri River, unstable substrates and (formerly) heavy sediment loads limited diversity to silt tolerant species, and viable habitat largely to backwaters, sloughs, attached lakes, side channels, and relatively few main channel environments. Today, upstream dams have greatly reduced the sediment load in the Missouri River, but unstable substrates restrict mussels to backwaters, side channels, pools below wing dams, revetments, and other areas sheltered from strong currents. Channelization and erosion/sedimentation have destroyed or drastically impacted stream habitats in eastern Nebraska resulting in the loss of much of the original unionid diversity. Surface and subsurface irrigation coupled with related declines in water tables have eliminated habitat in western Nebraska, though losses are partially compensated for some species by colonization of viable reaches of newly created impoundments and canals. Intensive grazing along streams and agricultural pollution further exacerbate habitat degradation and pressure remaining unionid populations. Invasive species directly compete with native mussels for sustenance and living space and may become serious threats to the survival of remaining unionid species in some habitats.

Introduction

The greatest unionoid diversity in the world is in North America (Bogan, 1993). The eastern, midwestern and southern United States contain the greatest diversity and populations of mussels, and are also the regions that have received the majority of attention, scientific and otherwise. Investigations in the Missouri River Basin from the Kansas River northward were minimal until the last quarter of the twentieth century. In Nebraska during the nineteenth and most of the twentieth centuries, research on mussels was largely limited to reports of their existence (William Clarke 1804 as cited in Moulton, 2003), a few isolated collections (Lea, 1858; Hayden, 1862; Tryon, 1868; Walker, 1906; Canfield and Wiebe, 1931), and a small number of museum vouchers often with only general locational data (i.e. state, river). The most extensive publication on the unionoids of Nebraska was that of Aughey (1877), however, aspersions on his academic reputation as well as questions on the accuracy of his identifications preclude reliance on that paper (Bolick, 1993; Hoke, 2000b).

Research on Nebraska mussels did not benefit directly from the flow of money into the field in the early 1900's propelled by commercial interest in mussels relating to the button industry. In 1900 there was one button manufacturer in Nebraska and two by 1905 (Wilson, 1910), however, Coker (1919) does not include Nebraska in his list of states where "the mussel fishery is pursued more or less actively."

The extent of research in adjacent states at the initiation of this study was similarly limited. In Kansas, Murray and Leonard (1962) listed only a few species occurring north of the Kansas River. The latest research in Missouri was that of Utterback (1914, 1915-16, 1917), and in South Dakota, the most recent publication was that of Over (1942). Little was known of the mussels of western Iowa (Keyes, 1888; Bovbjerg et al. 1970), and the Missouri River was reported to be devoid of molluscan life below the confluence of the Milk River in eastern Montana (Hayden, 1862; Utterback, 1914, 1915-1916, 1917; Coker and Southall, 1915; Over, 1915, 1942; Bartsch, 1916).

The general absence of research on Nebraska unionids in 1972 suggested the need for a statewide survey, and this study was initiated at that time. The goals of the study were: (1) to document existing distributions through a statewide survey of living and recent shell materials; (2) to document the extent of past distributions through collection of relict specimens and by searching museums for relevant vouchers; (3) to identify changes in the fauna through comparison of distributions revealed by recent and relict shells; (4) to identify and understand the primary factors influencing past and present distributions; and (5) to develop a model to account for changes in the fauna through time.

Before discussing the survey, it is necessary to briefly comment on the subjects of unionoid biology, and the physical and aquatic geography of Nebraska. A basic familiarity with these subjects is useful to fully understand the sections that follow.

Unionoid Biology and Life History

Freshwater mussels are the most imperiled faunal group in North America (Williams et al. 1992) with 213 of the 297 known taxa considered to be extinct, endangered, threatened or of special concern. The current condition of the fauna reflects the extent of habitat deterioration, impediments to reproductive cycles, and the introduction of invasive species that compete for food and space.

Mussels embed themselves in the substrates of streams and lakes and filter the water for sustenance. Freshwater mussels use gills for respiration. Unionoids are best adapted to flowing waters and most species do not thrive in lentic habitats such as lakes and ponds. Most species breed only during a relatively limited period of the year, and most have separate sexes (e.g. are dioecious). A few species are hermaphroditic, or occasionally hermaphroditic (van der Schalie and Lock, 1940; van der Schalie, 1966, 1970). Unionoids have a complex reproductive cycle that begins with fertilization of eggs in females by sperm injected into the water by upstream males and entering females with incurrent water. The fertilized eggs develop into the larval form (glochidia) in a specialized part of the gills, the marsupium. The next stage of the reproductive process involves the mandatory infection of a species-specific host fish or salamander with the parasitic larvae. The larvae attach to gills or fins of host fish or salamanders. The transformation of glochidia to juvenile mussel occurs largely during the parasitic period. After completion of the parasitic stage, the young mussels drop from their host, and continue to mature on their own. They reach sexual maturity at ages from 1 to 8 years, depending upon the species (Coker et al. 1921), and some may then continue to live for decades.

The unionoid reproductive cycle is susceptible to interruption at many points. For example, elimination of the host fish will over time, result in the elimination of dependent species of freshwater mussels. Similarly, displacement of the host fish from the immediate vicinity of female mussels during the breeding period can produce the same effect. Different species of mussels have evolved a number of methods to achieve infection of host fish. There are two distinct levels to this problem: (1) attraction of a suitable host fish to the immediate vicinity of the female mussel and (2) successful infection of the host with glochidia. Some mussels solve both problems by one method: broadcast dispersal of thousands of glochidia into the water column. Many unionoid species produce conglutinates, worm-like structures filled with glochidia, that resemble one of the foods in the host fish's diet. In eating the conglutinate, the fish becomes infected with glochidia. Some fish species utilize thin-shelled mussels for food, and in the process of obtaining nourishment, may be infected with glochidia. There are many other strategies to attract and infect host fish including lures (Kraemer, 1970), mucus nets (Watters and O'Dee, 1997), and assembly of a glochidia filled structure, a superconglutinate, resembling a fish used by the host as a primary food source (Haag et al. 1995). For more detailed information, the reader is referred to Coker et al. (1921) and Howells et al. (1996).

Physical and Aquatic Geography of Nebraska

Nebraska encompasses an area of about 200,356 [km.sup.2], extending about 695 km east to west, and at its maximum 354 km north to south (Fig. 1). Nebraska is the sixteenth largest state in geographical extent, and the only state that lies entirely within the confines of the Missouri River Basin. The surface rises gently from 256 meters above sea level in southeastern Nebraska to 1,653 meters in the northwest. Rainfall and humidity are greatest in the southeast and decline to the northwest. Climate is temperate in the east and semiarid in the west.

The surface area is contained within two major physiographic provinces, the Central Lowlands and the Great Plains (Fenneman, 1931). In Nebraska, the Central Lowlands is entirely within a lower level physiographic region, the Dissected Till Plains. This is an eroded Pre-Illinoian glaciated surface covered with up to 61 meters of windblown loess, and extends from the Missouri River inland for a maximum of about 160 km. Drainage patterns in this region are heavily influenced by glaciation, and their orientation reflects their former function as melt-water drainage routes (Wayne, 1985; Aber, 1999). The division between the two provinces is shown in Figure 1. The Great Plains is unglaciated and includes all areas west of the Dissected Till Plains. A unique area within the Great Plains, the Sand Hills, consists of windblown sand dunes covered by a light vegetation of grasses, and encompasses 62,000 [km.sup.2] in northwest and north central Nebraska. In northwest Nebraska the Pine Ridge Escarpment separates the White River and Hat Creek drainages from the rest of the state.

[FIGURE 1 OMITTED]

Nebraska is primarily drained by the Platte River, and its major tributaries, the Elkhorn, Loup, North Platte, and South Platte (Figure 2). Below the Platte, drainage is largely through tributaries of the Kansas River: the Big Blue, and Little Blue rivers in the southeast, and the Republican River in the west. In extreme southeastern Nebraska, the Big and Little Nemaha rivers flow directly into the Missouri River. North of the Platte tributaries (i.e. the Elkhorn and Loup rivers), the Niobrara River flows across northern Nebraska to the Missouri River, while the White River in northwest Nebraska flows into South Dakota, and Hat Creek, a tributary of the Cheyenne River, drains a small area in the extreme northwestern corner of the state. The Cheyenne and White rivers ultimately flow into the Missouri River in central South Dakota. The Sand Hills region contains some 2,500 small, often alkaline lakes. In much of this region, there is no true drainage system and moisture is absorbed into the sandy soils (McCarraher, 1977).

In this paper, three aquatic/geographic provinces are recognized as distinct unionid habitat zones: the Dissected Till Plains; Great Plains; and the Missouri River and floodplain. The two former zones are as described by Fenneman (1931) above except for the exclusion of the Missouri River and floodplain. In this paper, the term "eastern Nebraska" refers to the region including both the Dissected Till Plains and the Missouri River and floodplain, and "western Nebraska" is synonymous with the Great Plains.

Methods

The methods and equipment utilized in this survey were strongly influenced by practical considerations. Chief among these was the general absence of external financial aid or any on the ground support. In this environment, a personal vehicle was utilized rather than a four-wheel drive vehicle, and use of a boat was precluded. These factors in turn, limited access in the sandy, unpaved roads in the Sand Hills, and along the unchannelized and channelized Missouri River respectively. The solitary nature of the survey limited the personal risks I was willing to take along the Missouri River as well. Logistical constraints due to the location of the author's physical residence during much of the survey (i.e. central Iowa 1981-1983 and St. Louis 1983-present), the extent of the survey area, and a field work season of only two or three weeks per year, precluded the use of time consuming collection techniques such as scuba and snorkeling, the effort that could be devoted to any one site, and largely dictated the intervals between sites. Logistics also increased the time needed to complete the study.

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Initial work was conducted in 1972 as a part of the requirements of a graduate class in zoology and later the same year as a funded independent research project. Based upon these initial efforts, the study was expanded to encompass the entire state. In the absence of previous studies and in an effort to obtain some general knowledge of the geographic extent of the unionid fauna of Nebraska, a questionnaire was mailed to Nebraska Game and Parks Commission conservation officers in 1976 requesting information on the known location of mussel populations in the state. Responses received formed the basis for much of the early collection effort, and indicated the existence of mussels in the Missouri River, thought to be uninhabitable for mussels at the time (Hayden, 1862; Utterback, 1914, 1915-1916, 1917; Coker and Southall, 1915; Over, 1915, 1942; Bartsch, 1916). To gain a general view of distributions in other upper Missouri Basin states, conservation officers in Colorado, Iowa, Montana, South Dakota, and Wyoming were queried by mail in 1978, and the responses provided valuable insight on regional unionoid distributions.

The study was qualitative in nature, and no attempt was made to obtain quantitative data. Sites were generally collected at intervals of 16 to 32 km, though this goal was not possible to achieve in northwestern Nebraska due to access problems. In some areas, intervals were contracted when collecting conditions were exceptional or the possibility of recovering formerly occurring species seemed likely. The scope of the work was initially statewide, and later expanded to include northern Kansas. An attempt was made to sample essentially all freshwater habitats including: major and minor rivers, creeks, canals, lakes, and reservoirs. Unless otherwise noted, specimens at all sites were collected by or directly donated to the author.

Sites were sampled by hand and with the use of a garden rake, usually during periods of low water. Fresh dead specimens were collected in preference to live material, and few live specimens were retained. At least one specimen of each species at every site was retained to document the fauna collected. Relict specimens were actively collected to document species not represented by live animals or recent shells at sampling locations, and with the hope of collecting species extirpated from the state.

Most sites were collected upstream from road access points, usually bridges. In some western portions of the state, particularly the Sand Hills region, limited road access and a general unwillingness to grant access for collection, severely restricted sampling activities. Sites were recorded on USGS maps (scale 1:250,000). Field notes were recorded at each site noting date, site number, time of day, relative productivity, and observations relevant to the presence or absence of unionids. A photographic record was also produced at most locales. Sites were collected until diversity plateaued or the accessible area had been investigated. Specimens were placed in plastic bags by site as collected with an accompanying site number, and later cleaned, identified, and recorded by species on site inventory sheets. All specimens were then wrapped in soft paper, repackaged in clean sacks with a site number for each bag, and stored in labeled boxes until specimens could be deposited in research museums.

Initially, all identifications were confirmed by either Dr. Harold Murray, then of Trinity University, or Dr. David H. Stansbery, The Ohio State University Museum of Biological Diversity. Subsequently, only unusual specimens were confirmed. At least one specimen of every species recovered in the survey was identified or confirmed by The Ohio State University Museum of Biological Diversity. The taxonomy employed in this study follows Turgeon et al. (1998).

Vouchers for sites collected prior to 1991 have been deposited at The Ohio State University Museum of Biological Diversity, Columbus, Ohio. Most specimens collected after 1990 are currently in the possession of the author. Some post 1990 specimens have been deposited at the University of Michigan Museum of Zoology, Ann Arbor, Michigan and arrangements have been made for others to be deposited at the National Museum of Natural History, Washington, D. C. It is anticipated that all remaining vouchers will be donated to research museums within the next few years.

The relative dearth of information on the unionid fauna of contiguous states, led to significant sampling in reaches of Nebraska rivers beyond the borders of the state. This effort was concentrated in northern Kansas, where the Republican, Little Blue, Big Blue, and Big Nemaha rivers flow from or into Nebraska, and was further expanded to encompass most of the Kansas River Basin including the Smokey Hill, Solomon, and Saline rivers (Hoke, 1996, 1997b, 2004, 2005b). Similarly, the Missouri River and floodplain were first collected along the Nebraska border, and subsequently, eastward and southward to its juncture with the Mississippi River above St. Louis, Missouri (Hoke, 1983, 2005a, 2005c, 2009). A large portion of the impounded Missouri River was sampled: in South Dakota--Lake Oahe (Hoke, 2003); North Dakota--Lake Sakakawea; and Montana--Fort Peck Lake. Limited work was also conducted in streams in Wyoming (Hoke, 1979), Montana, and eastern Colorado. These regional collections were supplemented with an extensive search of museum holdings at research institutions throughout the United States. These facilities are given in a later section of this paper. During this process vouchers relating to Aughey's (1877) report on Nebraska mollusks were located and evaluated (Hoke, 2000b).

The general absence of reliable early research on the streams of Nebraska, and their current condition suggested that some former species might not be represented among the vouchers obtained in this study. To identify species that may have formerly occurred, an extensive literature search of early and recent publications from surrounding states was conducted. These sources are: for Colorado, Cockerell (1889), Ellis (1916), Henderson (1920, 1924), Brandauer and Wu (1978), and Cordeiro (1999); for Kansas, Call (1885a-c, 1886, 1887), Popenoe (1885), Scammon (1906), Murray and Leonard (1962). Miller and Hibbard (1972), Liechti and Huggins (1977), Schuster and DuBois (1979), Bleam et al. (1998), Angelo et al. (2009); for Iowa, Keyes (1888), Bovbjerg et al. (1970), Rausch and Bovbjerg (1973), Frest (1987), Arbuckle et al. (2000); for Missouri, Utterback (1914, 1915-1916, 1917), Oesch (1995); for South Dakota, Coker and Southall (1915), Over (1915, 1928, 1942), Henderson (1927), Skadsen (1998), Backlund (2000), Perkins and Backlund (2000, 2003), Shearer et al. (2005); and for Wyoming, Henderson (1924), Beetle (1989), Cvancara (2005), Edwards (2010). References for other states in the Missouri River Basin northward from Nebraska were examined including Minnesota, Sietman (2003); Montana, Henderson (1924, 1936), Gangloff and Gustafson (2000); and North Dakota, Cvancara (1975, 1983), Dyke (2000). Several general works on unionoid distributions were consulted as well (Simpson, 1900; Burch, 1975; Cummings and Mayer, 1992).

Mussel shells are often recovered from archaeological sites in Nebraska and these may be useful in documenting former ranges of the species represented. In this paper, it was decided to exclude such data in plotting distributions due to the difficulty of associating archaeological shell materials with a definite point of origin. Native American bands often traveled great distances during the year. Thus, shells may have originated from the nearest stream, or from a stream many kilometers from the locale of archaeological deposition. Further, it is difficult to rule out the possibility shells were obtained through trade. Archaeological specimens are often modified for use as tools or for ornamentation, and these were often traded. Unless unmodified shell materials are found in quantity at an archaeological site, it is speculative to assume these originated nearby.

Results

Survey activities were conducted in Nebraska between 1972 and 2000. This effort was complimented by extensive collecting in northern Kansas (1983-2002) and scattered sampling in western Iowa, northwestern Missouri, eastern Colorado, and Wyoming. Every river in Nebraska was sampled as well as many perennial creeks and canal systems, and a representative group of lentic habitats. During the survey period, a number of papers were published by others on mussels in Nebraska, or along the Nebraska--South Dakota border (Baxa, 1981; Roedel, 1990; Lingle, 1992; Freeman and Perkins, 1992, 1997; Clausen and Havlik, 1994; Peyton and Maher, 1995; Perkins and Backlund, 2000; Schainost, 2003; Shearer et al. 2005). Much of the data from this study was published as a response to those publications; however, the goal of this survey was always to produce a statewide paper. The survey resulted in the publication of eleven major papers (Hoke, 1983, 1994b, 1995, 1996, 1997b, 2000b, 2004, 2005a, 2005b, 2005c, 2009); two reports to state agencies (Hoke, 1979, 2003), and three minor publications (Hoke, 1994a; 1997a; 2000a). The Nebraska portion of these papers and additional unpublished material are presented in this paper.

The survey of Nebraska unionids encompassed sampling at 680 locales (Table 1). These included 392 productive locales (Fig. 3), and 288 non-productive locales (Fig. 4). The most consistently productive region was the Missouri River and its associated oxbow lakes where 88.4% of all sites tested yielded live mussels or recent shell materials. The least productive drainages were the Niobrara (19.4%), Loup (34.3%) and South Platte (35.0%). Survey activities in northern Kansas included sampling at 276 locales; most of these have been previously reported (Hoke, 1996, 1997b, 2004, 2005b, 2009).

The study documented 31 unionid species for Nebraska (Table 2). Thirty species were recovered during the survey, including two federally endangered species, Leptodea leptodon and Pleurobema clava. These species were first documented from the Missouri River Basin in this study. Villosa lienosa was confirmed for the state and the Missouri River Basin by a single nineteenth century museum voucher collected by Hayden and first reported by Tryon (1868). Mussels were recovered from the Missouri River, where unionids were previously reported to be absent, and the first survey of the channelized Missouri River was completed as well (Hoke, 1983, 2009). Collections at 276 locales in northern Kansas documented one additional species for Kansas reaches of the Big Blue and Little Blue River. Quadrula fragosa (Conrad, 1835), a federally endangered species, was collected as relic valves from two sites below the Nebraska border (Hoke, 1997a, 2004, 2005a). In addition, Lasmigona compressa, previously unknown in Kansas, was collected from a reach of the South Fork of the Big Nemaha River, and Anodonta suborbiculata last reported from northern Kansas by Scammon (1906) was recovered from Browning Lake on the Kansas-Missouri border in northeastern Kansas (Hoke, 1996, 2009).

[FIGURE 3 OMITTED]

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The Nebraska portion of the study produced 1,068 unionid species occurrences or species-specific geographic occurrences (Table 3), and four occurrences of the introduced bivalve Corbicula fluminea (Muller, 1774). The occurrences documented for Nebraska in this study are plotted by species on 32 maps (Figs. 5-36) and presented in Appendix 1. Occurrences shown along the Missouri River include collections on Iowa, Missouri, Nebraska, and South Dakota shores. The maps include results from other studies to the extent they enhance the distributions delineated in the current study. Sixty-five occurences from other studies are included on these maps. Also plotted, are the locales of 53 relevant museum vouchers. Data from non-survey sources utilized in species maps is given by figure in Appendix 2.

Survey occurrences were highly concentrated among a relatively small number of species. Anodontoides ferussacianus, Lasmigona c. complanata, Pyganodon grandis, Quadrula quadrula, and Uniomerus tetralasmus were the most frequently recovered unionids and accounted for 582 occurrences, 54.5 percent of all occurrences. The most widely distributed unionid was P. grandis with 193 occurrences, 18.1 percent of total occurrences. In contrast, the seventeen least common species included only 148 occurrences, 15 percent of study occurrences.

Diversity at productive sites ranged from one to a maximum of 19 species, and averaged 2.72 species per site. The most diverse site encountered was on Logan Creek in the Elkhorn Basin. Diversity was greatest in the Dissected Till Plains and abruptly declined in the Missouri River and floodplains to the east and the Great Plains westward. Streams in the Dissected Till Plains once supported populations of 29 of the 31 species documented for Nebraska. The Big Blue, Nemaha, and Elkhorn drainages yielded from 20 to 28 species each. In contrast, this study and other recent research confirmed only 16 species for the Missouri River and floodplain adjacent to Nebraska, including two species (Anodonta suborbiculata and Leptodea leptodon) that appear to be restricted to that region (Hoke, 1983, 2009; Clarke, 1996; Backlund, 2000; Perkins and Backlund, 2000; Shearer et al. 2005; Duggan, 2009a). The Great Plains supported 17 species, including one, Potamilus alatus, that is probably recently introduced to the region, and another, Strophitus undulatus, recovered in other studies (Peyton and Maher, 1995; Freeman and Perkins, 1997). Northwestern portions of the Great Plains were significantly less diverse with only five species, and no mussels were recovered from the Sandhills lakes.

The current condition of the Nebraska mussel fauna is poor, and largely reflects the extent of habitat destruction in eastern Nebraska. Seven species (Actinonaias ligamentina, Lasmigona compressa, Ligumia recta, Obovaria olivaria, Pleurobema clava, Potamilus purpuratus, and Villosa lienosa), 23% of the Nebraska fauna, were documented only by relict shells or early museum vouchers, and have probably been extirpated from the state. Most of these species were represented by only a handful of occurrences, perhaps indicating their former rarity. Pleurobema clava was documented by a single relict valve. In contrast, L. recta was once relatively widespread in the Dissected Till Plains. The mussel was also formerly widespread in Kansas, but Angelo et al. (2009) found only one live specimen in their survey, and indicated the species "now lacks reproductively viable populations" in that state.

The distributional ranges of fifteen extant species have contracted, often significantly. Eleven species largely restricted to the Dissected Till Plains in eastern Nebraska have been heavily impacted. Arcidens confragosus, Truncilla donaciformis and Truncilla truncata, appear to be absent from Nebraska interior streams and are now present only in the Missouri River. Amblema plicata and Fusconaia flava may only survive in the Nebraska interior in upper reaches of the Nemaha Basin. Tritogonia verrucosa appears to be restricted to the lower Big Blue Basin (Schainost, 2003) and the Missouri River. Live populations of Potamilus alatus, formerly widely distributed in eastern Nebraska, are largely confined to the Missouri River, and possibly the upper Nemaha Basin. A museum voucher of this species from the Calamus Reservoir in the Loup Basin probably represents a recent colonization via glochidia infected fish. Lampsilis teres may be absent from the Big Blue Basin, and live populations are uncommon in its remaining range as well. Ligumia subrostrata is generally absent from the rivers and creeks of eastern Nebraska where relict shells indicate it was formerly common. The sole fresh dead occurrence in this survey was from the headwaters of the South Fork Big Nemaha River in northeastern Kansas (Hoke, 1996). Additional populations may persist in the upper Nemaha Basin in Nebraska and in southeastern Nebraska farm ponds. The latter habitats were not extensively examined in this study due to the difficulty of obtaining access. Murray and Leonard (1962) reported over 1,500 specimens of L. subrostrata from a drained one-acre lake on the University of Kansas campus at Lawrence. Quadrula p. pustulosa, formerly common, has largely disappeared from the Nemaha and Little Blue basins as well as from most eastern Nebraska creeks. In this survey live populations were concentrated in the Big Blue Basin, while isolated individuals continue to survive throughout the range documented in this study. Strophitus undulatus was recovered only as relict shells in eastern Nebraska and may now be absent in this region. Live specimens have been recovered from the Middle Platte and upper Niobrara Basins (Peyton and Maher, 1995; Freeman and Perkins, 1997).

The ranges of four, more ubiquitous, unionid species have also contracted significantly. Live populations of Anodontoidesferussacianus were largely limited to the Great Plains, though relict specimens were well distributed in the Dissected Till Plains. This unionid has also nearly disappeared from the Republican Basin in Nebraska, where the species was represented by only a few scattered populations (Hoke, 1997b). The near absence of A. ferussacianus from eastern Nebraska may relate to generally turbid stream conditions in that region. The presence of live populations was associated with relatively clear water, and the presence of only relict specimens was generally associated with turbidity. Lampsilis cardium was recovered live or as a fresh shell from the Big Blue and Elkhorn basins. The only recent collection of fresh shells of L. cardium and Lampsilis siliquoidea from the Little Blue River were specimens at the University of Nebraska--Kearny Biology Department collected from one site in 1967. Live Lampsilis siliquoidea were only collected from the Big Blue and West Fork Big Blue rivers in this study, and fresh shells were also recovered from the unchannelized Missouri River. The distribution of Uniomerus tetralasmus is declining due to a near absence of water in portions of the upper Republican Basin, where the westernmost populations were documented in this study.

The ranges of Anodonta suborbiculata, Lasmigona c. complanata, Leptodea fragilis, Leptodea leptodon, Potamilus ohiensis, Pyganodon grandis, Quadrula quadrula, Toxolasma parvus, and Utterbackia imbecillis appear relatively stable at present. This conclusion is based upon the recovery of live animals or fresh dead shells throughout the ranges documented in this study. Many of these unionids have been recovered, sometimes in great numbers, from artificial lentic habitats and or irrigation canals. The distributions of P. grandis and U. imbecillis may have expanded with the construction of ponds and reservoirs throughout the state. The most western locales for both species were from such habitats. The distribution of A. suborbiculata is concentrated in oxbow lakes and other slow current habitats along the Missouri River. Most of these environments have disappeared with channelization of the river. The sole Nebraska occurrence of L. leptodon and a second survey occurrence in the Missouri River in eastern Missouri were as fresh shells. Though stable as defined above, L. leptodon is exceedingly rare.

Three species reported for Nebraska either from published sources or represented by extant vouchers, have been excluded from the list of confirmed species due to questions on the reliability of the records upon which their confirmation depends. Canfield and Wiebe (1931) reported the "butterfly mussel", the common name for Ellipsaria lineolata (Rafinesque, 1820), from the Big Blue River at Crete, Nebraska during a fisheries survey of the river. Since E. lineolata is quite distinctive and easily recognized and Canfield had worked with people from the Fairport Biological Station, a focus of research on freshwater mussels in the early twentieth century, it is quite possible the report is valid. There are, however, no vouchers and acceptance of the report would extend the species known range rather significantly. The nearest Missouri River tributary with this species is the Blackwater River in central Missouri (Oesch, 1995), where the mussel was last collected by Utterback (1915-1916). Lampsilis higginsii (Lea, 1857) is represented by a voucher at the National Museum of Natural History (No 512459) reportedly collected at West Point, Nebraska, and a single fresh valve recently collected from the Missouri River below Gavins Point Dam on the Nebraska--South Dakota border (Shearer et al. 2005). Though I have previously accepted the West Point record as evidence of the species in Nebraska (Hoke, 1994b), greater familiarity with museum records and habitat discrepancies have led me to question this record. Lampsilis higginsii is typically a big river species, and its range is largely restricted to portions of the Upper Mississippi River and the St. Croix River in Wisconsin (Havlik, 1980). In contrast, West Point, Nebraska is on the Elkhorn River, a small and generally shallow river, near the eastern boundary of the Great Plains. The Missouri River specimen may be a valid occurrence, and may represent a recent colonization. It is also possible the valve was discarded by a visitor to the region, since it was recovered from an area frequented by thousands of outdoor enthusiasts each year. Pending recovery of additional specimens, this species should probably be considered a questionable component of the current Nebraska fauna. It is highly doubtful L. higginsii was present in the Missouri River before the closing of the upper Missouri River dams after 1936. The river was simply too turbid and the substrates and channel too unstable for the species. A third unionid, Alasmidonta marginata Say, 1820 is supported by a voucher at the University of Nebraska State Museum (No. 27-1-26-87) with the location given "Nemaha" which could refer to either the Little Nemaha River near the town of Nemaha, Nebraska or to an unspecified location on either the Little Nemaha or Big Nemaha rivers. The specimen location and identification are congruent with Aughey's 1877 paper, and may be valid, but I am increasingly reluctant to place any reliance on Aughey's vouchers or report. Though I have previously accepted this record as valid (Hoke, 1996), based upon research on Aughey (Hoke, 2000b) and (again) more experience with museum records, I am uncertain one can rely on the veracity of this record, and have accordingly classified this species as unconfirmed for Nebraska. The locales reported for the three species are plotted in Fig. 37.

[FIGURE 37 OMITTED]

Review of the unionoid literature of the Missouri River Basin combined with a search for vouchers at museum collections suggest as many as thirteen additional species may once have been present in eastern Nebraska (Table 4). Alasmidonta marginata, Cyclonaias tuberculata, Elliptio dilatata, Lasmigona costata, Obliquaria reyqexa, Plethobasus cyphyus, Pleurobema sintoxia, Quadrula fragosa, and Quadrula metanevra are documented from the Missouri River Basin above the Platte River confluence with the Missouri in western Iowa and/or eastern South Dakota. A former presence in eastern Nebraska would be only a slight extension of documented historic ranges. Specimens of A. marginata, O. reflexa, P. sintoxia, and P. cyphyus, probably collected by Aughey (1877), are in the collection at the University of Nebraska State Museum, though without reliable associated data, these cannot be confirmed for the state. Relict specimens of Q. fragosa were recovered from both the Big Blue and Little Blue rivers at points about 35 km below the Kansas-Nebraska border increasing the possibility of a former presence in the state (Hoke, 1997a, 2004, 2005b). This federally endangered species is now considered extirpated from Kansas (Angelo et al. 2009), though it appears to have once been rather broadly distributed in eastern Kansas (Bleam et al. 1998). An intensive collection effort focusing upon recovery of relict shells in the Big Nemaha and Big Blue basins might succeed in confirming at least some of these species for Nebraska. All these unionids are at the westward limits of their ranges, and it is unlikely extant populations will be found in the decimated streams of eastern Nebraska. The nearest documented occurrences of these species are illustrated in Fig. 38.

A large number of unionoid species were almost certainly reported erroneously for Nebraska by Aughey (1877). Appendix 3 lists the unionoids reported by Aughey with currently recognized names and distributions as originally given. Many of these are highly improbable. At least one of Aughey's (1877) unsupported unionoids, Cumberlandia monodonta, has been included in the literature (Simpson, 1900; Walker, 1910; Burch, 1975) though usually with some skepticism. Given the many questions concerning Aughey's scholarship and unionoid identifications (Bolick, 1993; Hoke, 2000b) all information given by Aughey is suspect. Records probably collected by Aughey and utilized in this paper are only for species recovered in this survey, within ranges established in this study.

[FIGURE 38 OMITTED]

Discussion

There are no previous statewide studies of Nebraska mussels to compare against the current study to evaluate the effectiveness of the methods employed, however, in a series of papers on the Big Blue, Elkhorn, Little Blue, Missouri, Nemaha, Niobrara, Platte, and Upper Republican basins (Hoke, 1983, 1994b, 1995, 1996, 1997b, 2004, 2005a, 2005b, 2009), portions of this survey were compared to other published reports from those regions, and were found to compare favorably to those studies. In instances where other studies have documented species where these were not collected in this study, this appears to be attributable to intensive collection of comparatively limited stream reaches, ideal collecting conditions, or collections of isolated populations.

A number of surveys have been completed in or adjacent to Nebraska since publication of my earlier papers, and deserve to be discussed. Peyton and Maher (1995) collected seven species from 28 sites in four counties in the Middle Platte Basin including one, Strophitus undulatus, not recovered from the 37 sites sampled in the same area in this study (Hoke, 1995). The former study was quantitative and conducted when irrigation canals and reservoirs were either dewatered or drawn down, and involved prolonged sampling efforts (Peyton and Maher, 1995). Of the 8,804 unionids sampled under those conditions, only 22 specimens (0.25%) were S. undulatus. Schainost (2003) recovered a single live Tritogonia verrucosa under extreme low water conditions from a site on the Big Blue River not sampled in this survey. The author is aware of one study of the Platte River and two of unchannelized reaches of the Missouri River conducted recently by the Nebraska Game and Parks Commission. The Commission did not provide copies of these surveys for review, so they cannot be analyzed nor cited in this paper; however, Duggan (2009a) noted one survey did recover a single specimen of Leptodea leptodon from the Missouri River.

Recent studies by the South Dakota Department of Game, Fish, and Parks in the unchannelized Missouri River along the Nebraska-South Dakota border (Perkins and Backlund, 2000; Shearer et al. 2005) may be compared against results from this survey (Hoke, 1983, 2009), and one unpublished collection. Perkins and Backlund (2000) recovered two Anodonta suborbiculata, four Lampsilis teres, and two Arcidens confragosus in the 2,064 specimens collected from 47 sites below Lewis and Clark Lake. The two former species were recovered in this survey, though from channelized reaches further downstream (Hoke, 1983; 2009). Arcidens confragosus was not collected from the Missouri River in this study, but Leptodea leptodon collected in this survey was not recovered by Perkins and Backlund (2000). Shearer et al. (2005) found one Potamilus alatus, and two Utterbackia imbecillis among the 247 specimens recovered from 49 locales above Lewis and Clark Lake. Neither species was collected from that reach in this survey, though both were recovered from downstream reaches, and the former upstream in the Lake Oahe impoundment as well (Hoke, 2003). Shearer et al. (2005) also reported Amblema plicata, thought to be a recent colonization, from the reach below Gavins Point Dam.

Survey results from relevant portions of the Big Blue, Little Blue, Nemaha, and upper Kansas basins and the Missouri River (Hoke, 1996, 1997b, 2004, 2005b, 2009) may be compared with results of several recently published Kansas surveys. Bergman et al. (2000) recovered 19 species from the upper Kansas drainage, including one, Potamilus alatus, not recovered from the basin in this survey (Hoke, 1997b). However this survey found 19 species including one, Lampsilis siliquoidea, not recovered by Bergman et al. (2000), and significantly more occurrences of all species for which information was given, than the former study. Unfortunately Bergman et al. (2000) do not present data by river, collection locales are not given, and occurrences are only presented for a few species, so further comparisons are not possible. Angelo et al. (2009) presented results of an eighteen-year survey of Kansas unionoids conducted by the Kansas Department of Health and Environment and the Kansas Department of Wildlife and Parks. Those surveys documented a number of range extensions in the upper Kansas Basin, and two relict occurrences of one species, Obliquaria reflexa, not found in my earlier surveys. The range extensions reflect a greater number of collection sites in western Kansas, 297 vs. 181. In contrast, Anodonta suborbiculata, Arcidens confragosus, Lasmigona compressa, and Quadrula fragosa, reported from northeastern Kansas in this study (Hoke, 1996, 2004, 2005b, 2009), were not recovered from the region by Angelo et al. (2009), though more sites were sampled in that effort (over 230) than the 95 locales in this survey.

Recent and historic collections from portions of the North and South Platte Basins in Wyoming and Colorado are compared with results from this and other studies in western Nebraska in Table 5. Museum vouchers from Colorado indicate a number of species not recovered from the South Platte Basin in this survey, however, recent collections in Colorado suggest only one species, Pyganodon grandis, is currently present (Cordeiro, 1999). Recent and historic collections in Wyoming have recovered a number of species from the North Platte Basin not collected in this study. This may indicate a deficiency in this survey or merely reflect the impact of extensive water diversions above Nebraska reaches. The fact that the North Platte River in Nebraska is currently only one to two-tenths as wide as in 1865 may suggest upstream withdrawals are largely responsible (Williams, 1978). Further, at least some of the upstream diversity (i.e. Potamilus ohiensis and Utterbackia imbecillis) may be due to recent introductions.

On the basis of the analysis above, I believe the results of this study fairly reflect the composition of the unionid fauna of the regions surveyed. Differences from other studies appear to be due to concentrated collecting in limited stream reaches (Peyton and Maher, 1995; Perkins and Backlund, 2000; Shearer et al. 2005; Angelo et al. 2009), superior collecting conditions (Peyton and Maher, 1995; Schainost, 2003), colonization subsequent to my original survey (Shearer et al. (2005), better access to stream collection areas (Perkins and Backlund, 2000; Shearer et al. (2005), and possibly more favorable conditions above Nebraska reaches (Cvancara, 2005; Edwards, 2010). These largely reflect the financial and logistical constraints of this study.

A number of species recovered in this survey are far beyond their generally known ranges. Angelo et al. (2009) have questioned the identification of one of these, Potamilus purpuratus, from the Kansas drainage in Kansas (Scammon, 1906; Liechti and Huggins, 1977; Hoke, 2005b) due to a similarity of shell traits in some specimens of Potamilus alatus and P. purpuratus, and chosen to assign the preceding specimens to P. alatus in their recent publication. While this treatment may be more aesthetically appealing than the former presence of populations of a species rarely reported within the Missouri River Basin, the initial identifications are retained in this paper. The identifications of all specimens of this species were verified by The Ohio State Museum of Biological Diversity, as was a museum specimen from Papillion Creek that is clearly P. purpuratus.

The former presence of Potamilus purpuratus in Nebraska and northern Kansas is less disconcerting when viewed with the perspective of survey results and some knowledge of the minimal extent of historical collections in lower Missouri River Basin tributaries. Three species (Leptodea leptodon, Pleurobema clava, Quadrula fragosa) were first documented from the Missouri River Basin in this survey (Hoke, 1983, 1997a), and new Nebraska and Kansas records for Lasmigona compressa are the first from the basin since Tryon's (1868) publication. A single museum voucher documents the former presence of Villosa lienosa in the Missouri River Basin. These species are or appear to have been extremely uncommon in the region. Rivers and streams in northeastern Kansas, northern Missouri, southern and western Iowa, and Nebraska were not adequately sampled before severe habitat degradation due to erosion from the tilling of the native prairies and subsequent extensive channelization in and before the early twentieth century, and much of this region remains poorly sampled today. Given the level of research in the basin, it is not surprising relatively uncommon components of the unionid fauna were not collected. As rivers in the basin are subjected to more thorough examination, additional specimens of some of these species have been recovered (Bleam et al. 1998; Skadsen, 1998; Perkins and Backlund, 2003; Hoke, 2009; Duggan, 2009a). This suggests knowledge of the former richness of the unionid fauna of the lower Missouri River Basin, particularly the more uncommon species, is deficient and probably reflects only the limited extent of early research in the region.

The distributions documented in this study reflect an amalgam of environmental conditions, both natural and anthropogenic, as well as a temporal component evidenced by collections of relict shells. The natural factors are discussed in depth in the first subsection below and presented as a model of the pre-settlement unionid distributions of Nebraska. This model utilizes results from creeks and rivers in the current study supplemented by observations of early explorers and travelers to postulate a view of unionid habit and distributions before the agricultural development of the state. Necessarily, this view is quite general. Historical changes in the aquatic habitats of Nebraska are developed in a subsequent subsection.

Natural (Pre-Settlement) Aquatic Environments in Nebraska

Unionid distributions in Nebraska were and continue to be heavily impacted by geologic, hydrologic, and climatic parameters. Relevant factors include precipitation, drainage development, stream permanency, current velocity, sediment load, host fish diversity, and substrate composition and stability. These factors are increasingly unfavorable for unionids westward in the Great Plains, and relatively favorable in the Dissected Till Plains. Heavy sediment loads were detrimental to mussels in the historic Platte and Missouri rivers. Given perennial stream flow and host fish availability, the factor that most consistently limits unionid presence throughout the state is substrate stability. Mussels were usually absent entirely from unstable substrates, and when found in such habitats did not survive over time.

Substrate instability is largely a product of substrate composition and current velocity, and can be augmented by the presence of heavy sediment loads. Sandy substrates coupled with rapid currents produce shifting sand bottoms, and these have long been known to negatively impact unionids (Baker, 1928; Murray and Leonard, 1962). The deleterious impact of unstable substrates have been repeatedly emphasized in this study (Hoke, 1994a, 1994b, 1995, 1996, 1997b, 2005a, 2009), and the importance of substrate stability has been recognized as a key determinant of regional and local mussel distributions in other studies as well (Holland-Bartels, 1990; Brim Box and Mossa, 1999; Strayer, 1999; Gangloff and Feminella, 2007).

In the Great Plains, precipitation and drainage development increasingly decline westward (Table 6). Rivers are generally wide and shallow with few of the deeper holes that provide habitat for diverse populations of potential host fish. Rivers running from western Nebraska generally have sandy substrates and tend to flow in relatively straight lines or gradual arcs with little meandering (Fennemann, 1931). This general orientation combined with the steady decrease in elevation to the east promotes relatively rapid currents and generally shifting sand substrates. In this study, Great Plains river habitats supporting unionids were headwater areas, side channels, backwaters, and sloughs. Mussels were usually uncommon in main channel habitats (Hoke, 1994a, 1994b, 1995, 1997b, 2005a).

In the eastern United States, large rivers tend to have greater diversity than smaller streams (Watters, 1992). In contrast, in this study, western Nebraska rivers were often found to contain less diversity than their tributaries. This reflects the impact of the predominantly shifting sand substrates characteristic of these rivers. In the middle and lower Loup system, of forty-one sites sampled in river habitats only two produced mussels and these were in habitats sheltered from upstream currents. In the Loup and upper Republican basins the greatest diversity is in tributary creeks, while basin rivers support almost no mussels (Hoke, 1994a, 1997b). Productive reaches of tributary creeks have slow currents, and substrates of mud, sand, or mud and sand. These tributaries may be viewed as mussel refugia in the predominant adverse shifting sand environments of the major western drainages. Mussel distributions in such drainages often consist of discontinuous populations in widely separated patches of viable habitat.

Mussels are susceptible to removal from substrates, especially shifting sand substrates, by current shear during peak flow events (Strayer, 1999; Gangloff and Feminella, 2007). Seasonal peak flows formerly occurred on the Platte and Missouri Rivers due to melting of snow on the High Plains to the west and northwest and to melt water from the Rocky Mountains. Both rivers carried large quantities of sediment from natural erosion of the eastern slopes of the Rocky Mountains as well as from the plains to the east (Hayden, 1862; Mattes, 1969), and were known for rapid currents (Meriwether Lewis, 1804 as given in Moulton, 2003; Schneiders, 1999). Flash floods also served to scour the unstable sandy substrates of Great Plains rivers. The upper Republican basin was particularly susceptible to flash flooding, a phenomenon that claimed hundreds of lives (Hoffman, 1983b; Mohlman, 1993), and occasioned the construction of six dams in the region after 1935. Nebraska rivers with predominantly shifting sand substrates are the middle and lower reaches of the Elkhorn, Little Blue, Loup system, and Niobrara rivers, and all Nebraska reaches of the North Platte, Platte, Republican, and South Platte rivers.

The substrates of the historic Missouri River were perhaps the ultimate example of instability. The Missouri River was known for turbidity, rapid deposition of sediment, shifting sand and mud bars, frequent channel movement, and a rapid current (Hayden, 1862; Utterback, 1914, 1915-1916, 1917; Over, 1915; Schneiders, 1999), factors that undoubtedly produced unstable substrates. Below the confluence with the Platte River, the Missouri was essentially a braided stream due to the quantity of sand input from the Platte River (Pierce, 1983). The Missouri was reported to be uninhabitable for freshwater mussels due to the heavy load of silt and other sediment in its waters (Hayden, 1862; Coker and Southall, 1915; Over, 1915, 1942; Utterback, 1915-1916, 1917; Bartsch, 1916) though support for this view appears more anecdotal than research based, and is contradicted by Simpson (1900) and a small number of early vouchers from South Dakota reaches (Hoke, 2009). There were, however, many backwaters, chutes, sloughs, attached lakes, and detached lakes along the river and probably some isolated habitats in the main channel where currents were slow and substrates stable and these may have supported some silt-tolerant unionid species (Hoke, 2009). Historically, unionids were found in slow water habitats in and along the Missouri River (Simpson, 1900; Utterback, 1915-1916, 1917), and were probably largely absent from the main channel. Two species, Lasmigona c. complanata and Pyganodon grandis, are documented by early vouchers from Crystal Lake, a Missouri River oxbow, in northeastern Nebraska. Given the near absence of early collection efforts, it is likely this is not a complete species list for the early Missouri River and associated oxbows along the Nebraska border.

In contrast to western Nebraska streams and the Missouri River, streams in the Dissected Till Plains had slower currents, more diverse substrates, lighter sediment loads, and greater substrate stability. The Nemaha and Elkhorn rivers and many eastern Nebraska creeks were once characterized by significant meandering. Meanders promote formation of deep holes that provide habitat diversity for mussels and potential host fish and also decrease stream gradients and current velocity. Further, many streams in eastern Nebraska flow north to south, betraying their glacial origin and promoting slower currents. For example, the Big Blue River flows at right angles to the general east to west uplift that underlies the state, flowing more along than across contours, resulting in lower stream gradients and current velocities than in rivers in the Great Plains.

Stream substrates in the Dissected Till Plains ranged from pebble and cobble to mud and mixtures of mud and sand. Substrate diversity in this region correlates with the presence of Pre-Illinoian glacial debris. Over time rock washed from overlying till and became a component of stream substrates. The Big Blue, Big Nemaha, and Little Nemaha cut through bedrock at numerous points in southeastern Nebraska, and rock washed from these cuts was incorporated into substrates below. Many streams in the region were also relatively clear. In 1804, William Clark (as cited by Grier, 1983) reported the Elkhorn River as "about one hundred yards wide, with clear water and a gravelly channel". The Big Blue River was named for the blue color of some of its deeper pools (Andreas, 1882). In 1804, William Clark noted the Big Nemaha was "a Butifull River of Clear water of about 80 yards wide" (Moulton, 2003). The former diversity and stability of substrates and clarity of water is reflected in the diversity of current and relict unionid species recovered in this survey or represented by nineteenth century museum vouchers.

The Dissected Till Plains may be viewed as a region of relatively favorable mussel habitat surrounded by more adverse habitat zones, the Great Plains to the west and the Missouri River eastward. This pattern is repeated in the Kansas drainage in northern Kansas, and in southern South Dakota, though it is less pronounced in the former, and more accentuated in the later. In both regions, unionid diversity is concentrated in formerly glaciated areas, and diminishes in the Great Plains and along the Missouri River.

Post Settlement Modification of Aquatic Habitat

While natural factors still exert the predominant influence upon unionid distributions in Nebraska, agricultural development has significantly modified many aquatic habitats. Observations during this survey, supplemented by conversations with landowners, and an extensive literature review suggest anthropomorphic activities with the greatest effect upon Nebraska mussels include surface and subsurface irrigation, construction of impoundments and canals, stream channelization, erosion/sedimentation, intensive grazing, contaminants, and the introduction of invasive species.

Withdrawal of water from streams for irrigation of croplands is common throughout Nebraska, but is most developed in the Great Plains, where storage reservoirs have been constructed on rivers and major streams, and water is transported by extensive canal systems from these impoundments or directly from rivers to croplands. Surface water diversions are most significant in the Middle, North, and South Platte basins, and in the Republican Basin, and result in severe degradation of stream habitats (Bentall, 1982; Hoffman, 1983a, 1983b; Hoke, 1995, 1997b). Severe declines from upstream irrigation withdrawals are documented for the North and South Platte rivers as early as the late 1870's and 1880's (Bentall, 1982). Anecdotal accounts from western Kansas and eastern Colorado describe similar dramatic flow decreases for central plains rivers and streams in those regions (Mead, 1896; Henderson, 1920).

In this study, mussels were encountered only in the eastern portion of the South Platte Basin, largely from the Sutherland canal and associated reservoirs supplied primarily with water from the North Platte River. Western portions of the basin were not productive reflecting the near absence of water in the South Platte River during sampling (Hoke, 1995). Mussels were restricted in the North Platte Basin to reservoirs, ponds, canals, and a few backwater areas along the North Platte in this survey, and diversity was limited. Mussels continue to survive in some reaches of local creeks in Great Plains basins, though these habitats too are often tapped for surface irrigation. Water rights allocated along many smaller Nebraska streams exceed total flow during normal years (Table 6), and many Great Plains streams that formerly provided habitat for mussels are now nearly unpopulated (e.g. Shell Creek, Wood River).

Large-scale water diversion projects are more limited in the Niobrara and upper Loup basins where ranching is often the most important agricultural activity, however, in the lower Loup Basin, diversion of water is increasing, and two new reservoirs were constructed during this study. Some mussels are present in headwater reaches of the Niobrara and Loup system rivers, and in some creeks in the Loup Basin, though both habitats are in decline (Hoke, 1994a, 2005a). Surface water withdrawal in eastern Nebraska is less pervasive due to relatively greater precipitation.

Subsurface water withdrawals have an increasing impact upon unionid habitat throughout western Nebraska. The advent of center pivot irrigation technology makes possible the irrigation of lands that were formerly suitable only for dry land farming or grazing. Intensive development of wells in the Republican, Little Blue, and upper Big Blue basins, and in Box Butte County in the Niobrara Basin is associated with significant declines in water tables (Mack et al. 1996a, 1996b). Headwaters and many tributaries of the Little Blue River and upper Big Blue River now dry up completely in some years (Hoke, 2004, 2005b). Huntoon (1974) forecast continued declines ha the water of the upper Big Blue Basin under all scenarios short of transbasin water infusion. The impact of subsurface withdrawals is not included in the figures for degraded stream kilometers given in Table 6, since the greatest increase in center pivot systems occurred after those figures were compiled. Between 1972 and 1988 the number of center pivot systems in use in Nebraska increased from 2,725 to 27,617 or over 913 percent (Sheffield, 1993a). The number of registered irrigation wells in the state rose from 42,899 in 1972 to 75,363 in 1991, an increase of 32,464 wells or 76 percent (Sheffield, 1993b). Nearly all the increase was in the Great Plains habitat zone (Mack et al. 1996a).

Construction of impoundments and canals has created some new aquatic habitats for mussels in Nebraska (Figure 39). Thirteen unionid species have been collected from these artificial lentic and lotic environments (Table 7). Most species were represented by only a few survey occurrences; however, some species appear to thrive in these habitats. Uniomerus tetralasmus is common in farm ponds and reservoirs in the Dissected Till Plains. Anodontoides ferussacianus, Lasmigona c. complanata, Potamilus ohiensis and Quadrula quadrula are quite common in some Great Plains canals, and with one exception, A. ferussacianus, in many western reservoirs as well (Freeman and Perkins, 1992; Peyton and Maher, 1995; Hoke, 1995, 1997b). Pyganodon grandis is common in all viable artificial habitats in the state, and many occurrences of Utterbackia imbecillis in this study were from artificial habitats as well.

The portion of these artificial habitats utilized by mussels is often quite limited. Most reaches of canals are dewatered when irrigation activities are completed, and the levels of some irrigation reservoirs drop substantially during the summer as water is withdrawn for irrigation. The Davis Creek Reservoir fluctuates by 13 meters during some years. The great extent of seasonal drops limits the value of some Nebraska reservoirs to only a few species, and particularly favors Pyganodon grandis the most widely distributed mussel in this study (Riegle, 1967).

A positive aspect of reservoir construction in and adjacent to Nebraska is the great decrease in sediment loads and turbidity of the Platte and Missouri rivers. Sediment from the Rocky Mountains formerly transported through Nebraska by the Platte River or eastward by the Missouri River now settles beneath the waters of upstream reservoirs, and downstream flows are less turbid than in the past. Leptodea leptodon and Lampsilis teres, are reported to be relatively intolerant to silt (Parmalee and Bogan, 1998; Brim Box and Mossa, 1999), but occur today in the Missouri River, where silt content was formerly high (Hoke, 1983, 2000a, 2009; Perkins and Backlund, 2000; Duggan, 2009a).

Reservoir dams control and moderate downstream flows, and prevent or lessen high flows that can scour river substrates and remove embedded mussels. In the upper reaches of the Middle Platte River, the diversion of 82% of upstream flow (Bentall, 1982), decreases the magnitude of peak flow events below and thus limits associated scouring of substrates. Some mussels now inhabit the river's main channel in this area.

Reaches immediately below dams can be quite productive for mussels. Current velocity increases below dams, but is at a minimum in pools immediately below dams. In a survey of 47 sites in the unchannelized Missouri River below the Gavins Point Dam, the three sites immediately below the dam produced 71% of all live and dead unionids collected (Perkins and Backlund, 2000). In the Great Plains, pools below dams can provide refugia from the generally rapid currents and associated unstable substrates characteristic of the region, and may support mussels while lower reaches may be largely unoccupied.

Construction of low dams along the Big and Little Blue rivers in southeastern Nebraska, a region formerly rich in unionid diversity, probably had an extremely adverse impact upon mussels. Subsequent to settlement of the area, at least 35 dams were constructed on the Big Blue River and 14 on the Little Blue River (Bouc, 1983). In total these dams converted a significant portion of both rivers into lentic habitat, rendering inundated reaches unsuitable for many unionid species (Vaughn and Taylor, 1999). Kansas and lower Nebraska reaches once supported a diverse fauna of 22 species and 24 species for the Little Blue and Big Blue rivers respectively (Hoke 2004, 2005b), however, diversity is lower in most Nebraska reaches. Though most of the dams have been destroyed, probably increasing viable lotic habitat in upstream reaches, remaining dams may preclude repopulation of lotic unionid species by blocking the upstream movement of infected host fish (Watters, 1996).

[FIGURE 39 OMITTED]

One of the most destructive aspects of agricultural development in Nebraska is stream channelization. The focus of stream channelization is in eastern Nebraska, which includes the region of greatest unionid diversity (Table 6). The Big Nemaha, Elkhorn, and Little Nemaha rivers and many larger creeks were extensively channelized during the late nineteenth and early twentieth centuries (Bentall et al. 1971; Delich, 1983). Straight channels cut along these formerly meandering streams, increased stream gradients, and current velocity, and destroyed deeper holes that provided habitat for diverse populations of potential host fish. Table 8 illustrates the extent of the loss in selected streams in the Nemaha and Elkhorn Basins. The direct effect of this process on mussels in Nebraska rivers is not documented, however, Missouri River tributaries in northern Missouri (the Grand, Chariton, Nodaway, and Tarkio rivers) and the Vermillion River in southeastern South Dakota, were channelized during the early 1900's and thousands of stranded mussels were reported in the by-passed channels at that time (Utterback, 1914; Coker and Southall, 1915).

Today, channelized stream reaches in the Dissected Till Plains contain few living mussels. Channelized portions of the Nemaha rivers are largely devoid of any live mussels. These reaches now often have shifting sand substrates, and in the summer, a more or less uniform depth of around 0.3 meters. Substrates in the middle and lower Elkhorn River are now predominantly shifting sand and were not productive in any survey (Clausen and Havlik, 1994; Hoke 1994b). Frest (1987) reported similarly catastrophic destruction of habitat and limited diversity in the dredged rivers of western Iowa, and noted "mussels were either completely eradicated (the common finding) or reduced to a single species nearly everywhere examined." The only exception in that study were five sites on a six mile reach in the headwaters of the Boyer River where fifteen species were recovered, attesting to the former diversity of the western Iowa mussel fauna.

Channelization of the Missouri River below Ponca State Park in northeastern Nebraska was completed in the mid twentieth century. This process directly or indirectly eliminated most of the natural slow water stable substrate habitats along the river such as chutes, side-channels, sloughs, backwaters, and attached lakes and led to the near total elimination of adjacent riparian forest habitat (Schneiders, 1999). Today, the lands bordering the river are often farmed nearly to the edge of riverbanks. The channelized Missouri River is three times as swift as in former times (Schneiders, 1999), and the river bottom was described in one research study as a "biological semi desert" of moving waves of sand (Sayre and Kennedy, 1978). Remaining slow water areas, now primarily pools below wing dams, support a number of unionid species, and rock revetments (when interstices have filled with sediment) provide stable and possibly significant habitat along the river as well (Hoke, 2009).

Relatively few unionid species are tolerant to excessive amounts of silt. Ellis (1931, 1937) attributed the demise of many formerly productive mussel beds to accumulations of silt on stream bottoms. Erosion is a significant problem throughout Nebraska (Table 6), but its impact on mussels is most significant in the Dissected Till Plains. Rainfall is heaviest in eastern Nebraska, soils are highly erodable, and the region is intensively farmed, often to the edge of stream banks, resulting in significant erosion of topsoil (Clausen and Havlik, 1994; Hoke, 1994b, 1996, 2004, 2005b). Rivers once reported to be relatively clear are now quite turbid. In some instances, streambeds are blanketed with a foot or more of silt, covering earlier pebble or cobble substrates (Hoke 1994b), and thus destroying the value of these habitats for many mussel species. Silt fills up deeper holes in rivers and streams and eliminates habitat for many fish species (Bliss and Schainost, 1973a, 1973b, 1973c, 1973f). The combined impact of channelization and erosion/siltation were probably lethal to many unionid populations in eastern Nebraska.

Grazing of domesticated livestock along and in rivers and streams is detrimental to unionids throughout Nebraska, though its impact is most significant in the Great Plains. In contrast, in much of southeastern Nebraska, streams may be deeply incised into the surface, and domestic livestock are often denied stream access. The animals compact and level substrates, crush unionids, and trample and consume streamside vegetation that anchors soil along stream banks. This process increases soil erosion, which fills the deeper holes and consequently degrades habitat for many fish species. In the sandy soils of the Niobrara and Loup basins, cattle cave in stream banks ultimately widening streams, and decreasing average depth. The animals crush relict shells, and after a period of time, no evidence remains to document the former presence of mussels in many habitats (Hoke, 2004). Many small creeks that formerly supported mussels are now largely depopulated, while reaches of the same streams, protected from livestock by fencing, may continue to support isolated populations of some species (Hoke, 1994b).

The impact of urine and feces expelled directly into streams by livestock can be severe. Profit (1967) notes runoff from feedlots decreases dissolved oxygen, and increases the amount of ammonia and fecal coliform bacteria in affected streams. Feedlot runoff is associated with fish kills (Profit, 1967, Profit and Edwards, 1973), and is a particularly serious problem in the Elkhorn River Basin (Clausen and Havlik, 1994; Hoke, 1994b), and probably in the Little Blue River and some of the small creeks paralleling the Platte River in east central Nebraska as well. Starrett (1971) found no living mussels in the upper Illinois River where ammonia nitrogen levels were in excess of 6 ppm, but noted mussels began to appear in downstream reaches where levels were diminished. High concentrations of dissolved ammonia are associated with an absence of live mussels (Environmental Protection Agency, 2009).

Sewage and industrial pollution were occasionally noted during this study and both were associated with an absence of living mussels in localized stream reaches. A more widespread problem is the impact of chemical and other agricultural contaminants on unionid mortality and reproduction. Though these are poorly understood, there is little doubt contaminants play a major role in the decline of freshwater mussels (Havlik and Marking, 1987). The effect of chemicals on unionid sperm is unstudied, and the impact of various chemicals and combinations of chemicals upon unionid life stages has also received little attention, though some recent studies suggest these may be severe (Jacobson et al. 1997; Conners and Black, 2004).

In this study, contaminants or evidence of them were occasionally noted in the form of algal blooms and associated destruction of unionid populations, however, I believe their greatest impact may be in the interruption of the unionid reproductive cycle. Nebraska streams often did not provide much evidence of recruitment. In many streams individual mussels were widely scattered, and it was unclear if enough individuals remained to successfully breed. It is possible this observation is the product of survey methods that were not designed to collect juvenile specimens; however, considerable recruitment was noted in the canals of central Nebraska.

A key difference in the water quality of streams and canals is that the former contain runoff from cropland whereas the later do not. Many unionid species move into shallow water to reproduce in the late summer (Murray and Leonard, 1962), and warm, bank full return flows, laden with chemicals and sediment from irrigated fields may impact unionid reproduction at that critical time. Thus, the negligible recruitment observed in many streams may be associated with chemicals and sediment washed from cropland. Additional research is needed to validate or disprove this association.

Within the past twenty-five years, the Asiatic clam (Corbicula fluminea) and the zebra mussel (Dreissena polymorpha) have colonized much of the Missouri River and many tributary streams as well. Large populations of C. fluminea were present in Wilson Lake, an impoundment of the Saline River in northwestern Kansas, in 1983 (Hoke, 1997b), and the species was widely distributed in the Missouri River east of Kansas City, Missouri by 1990 (Hoke, 2009). In this study, C. fluminea was confined to a small number of locations in the Platte River system (Hoke, 1995), however, it is probably much more widespread at present. Corbicula fluminea is now widely distributed in northern Kansas, and the first colonies of the zebra mussel have also appeared (Angelo et al. 2009). The zebra mussel has recently colonized the Missouri River at Sioux City and below Gavins Point Dam, and one lake in eastern Nebraska (Missouri Department of Conservation, 2001; Duggan, 2009b). Both species have significant reproductive advantages over unionoids since larval forms are free swimming and do not need a parasitic stage to mature, and both compete directly with native mussels for food and bottom habitat. These bivalves may become a threat to native mussels in many Nebraska habitats.

A Model of Nebraska Unionid Distributions

Climatic, geologic, and hydrologic factors delineate three unionid faunal zones in Nebraska: the Dissected Till Plains, Great Plains and the Missouri River and floodplain. Given the importance of perennial stream flows and host fish diversity which decrease westward in the Great Plains, the primary factors influencing mussel distributions and diversity in Nebraska are substrate composition and stability. Shifting sand substrates are characteristic of the Missouri River and most reaches of rivers in the Great Plains, while generally diverse and stable substrates are relatively common in the Dissected Till Plains.

The best unionid habitat in Nebraska is the 160 km wide section of eastern Nebraska encompassed in the Dissected Till Plains habitat zone. This region receives more precipitation, has a more developed drainage, provides more diverse habitat for host fish, and contains more stable and diverse substrates than surrounding regions. Streams in the Dissected Till Plains once supported 29 of the 31 unionid species documented for Nebraska, and possibly others as well. Subsequent to settlement of the state, many streams in the region were subjected to extensive channelization, and the effects of poor farming practices led to severe erosion and resultant sedimentation, together destroying or degrading much of the best unionid habitat. A series of low dams inundated much habitat in the Big and Little Blue rivers. Runoff of chemical fertilizers, herbicides, and insecticides from adjacent cropland and competition from invasive species pose further challenges to unionid populations in this region and throughout Nebraska.

The Great Plains has probably always been relatively marginal habitat for mussels. Only seventeen unionid species were recovered from the Great Plains habitat zone, and one of these, Potamilus alatus, is recently introduced. Rivers in this region were and continue to be generally wide and shallow with shifting sand substrates. Unionid populations were largely restricted to small tributaries and the headwaters, backwaters, sloughs, and side channels of the major rivers. Subsequent to development of the region, many rivers were largely drained for irrigation, eliminating habitat in many slough and backwater areas. Intensive grazing also heavily impacts streams in the region. Small tributaries and headwaters of some rivers continue to provide much of the remaining viable habitat, though many are increasingly becoming degraded. A limited number of species have adapted to the canal and reservoir habitats created by agricultural development, but other species such as Lampsilis cardium and Lampsilis siliquoidea have largely been extirpated from the region, though populations persist in southeastern Wyoming (Cvancara, 2005; Edwards, 2010).

The Missouri River was historically noted for high silt content and unstable substrates and was generally believed to be uninhabitable for mussels (Hayden, 1862; Utterback, 1914, 1915-1916, 1917; Coker and Southall, 1915; Over, 1915, 1942; Bartsch, 1916). Scattered early vouchers and current research cast doubt on this assertion, and suggest viable habitat existed primarily in the slow waters and stable substrates of a few main stem habitats and in side channels, sloughs, backwaters, oxbows, and attached lakes (Hoke, 2009). All unionid species must have been relatively silt tolerant. The construction of upstream dams has greatly decreased the silt content of the river below and some silt intolerant species now live in these waters. Channelization of the lower river eliminated most natural stable substrate habitats but created many artificial slow water habitats such as pools below wing dams, and stable substrates in some revetments as well. These artificial habitats and surviving natural slow water habitats continue to support unionid populations in the river.

Appendix 1--Nebraska Freshwater Mussel Distributions.

Symbols indicate unionid occurrences and generalized data sources as follows: circles = survey occurrences; squares = museum vouchers; and triangles = other published papers. All sources of non-survey data utilized are given by figure in Appendix 2.

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Appendix 2--Sources of Non-Survey Data Utilized in Distribution Maps

Note: Literature citations, if any, are presented first and followed by museum vouchers. Designations of museums for records given are indicated as follows:

FMNH--Field Museum of Natural History, Chicago, Illinois

NMNH--National Museum of Natural History, Washington, D. C.

PANS--Philadelphia Academy of Natural Sciences, Philadelphia, Pennsylvania

UCMNH--University of Colorado Museum of Natural History, Boulder, Colorado

UMMZ--University of Michigan Museum of Zoology, Ann Arbor, Michigan

UNSM--University of Nebraska State Museum, Lincoln, Nebraska

UNOB--University of Nebraska--Omaha, Department of Biology, Omaha, Nebraska
Appendix 2--Sources of Non-Survey Data Utilized in Distribution Maps

Note: Literature citations, if any, are presented first and followed
by museum vouchers. Desigmations of museums for records given are
indicated as follows:

FMNH--Field Museum of Natural History, Chicago, Illinois

NMNH--National Museum of Natural History, Washington, D. C.

PANS--Philadelphia Academy of Natural Sciences, Philadelphia,
Pennsylvania

UCMNH--University of Colorado Museum of Natural History, Boulder,
Colorado

UMMZ--University of Michigan Museum of Zoology, Ann Arbor, Michigan

UNSM--University of Nebraska State Museunl, Lincoln, Nebraska

UNOB--University of Nebraska--Omaha, Department of Biology, Omaha,
Nebraska

Figure Non-Survey Sources

5 None.

6 Shearer et al. (2005). Musetnn vouchers: NMNH No. 512437 (West
 Point, Nebraska), and 540270 (Oak Creek at Lincoln, Nebraska);
 PANS No. 9456--(Nebraska City); UMMZ No. 99771--(Weeping Water
 Creek at Weeping Water, Nebraska).

7 Perkins and Backlund (2000). Museum voucher: UNSM No.
 00074.3--(Missouri River, Boyer Chute, Washington County,
 Nebraska).

8 Museunl voncher: NMNH No. 512240--(Aowa Creek).

9 Perkins and Backlund (2000).

10 Museum vouchers: NMNH Nos. 539968 & 514912--(West Point,
 Nebraska); UMMZ No. 93054--(Weeping Water Creek, Weeping Water,
 Nebraska).

11 Clausen and Havlik (1994); Schainost (2003). Museun'J voucher:
 NMNH No. 512425--(Aowa Creek, Ponca, Nebraska).

12 Freeman and Perkins (1992); Perkins and Backlund (2000);
 Schainost (2003). Museum vouchers: FMNH No. 59326--(Cambridge,
 Nebraska); NMNH No. 514994--(Big Blue River near Crete,
 Nebraska); UMMZ No. 87234--(Big Blue River at Crete, Nebraska);
 UNSM unnumbered--(Weeping Water Creek, Weeping Water,
 Nebraska).

13 Perkins and Backlund (2000). Museum vouchers: NMNH Nos. 540293
 & 540300--(West Point, Nebraska) and No. 738424-- (Nebraska
 City, Nebraska); PANS No. 54036--(Aowa Creek, Ponca,
 Nebraska).

14 Lingle (1993); Clausen and Havlik (1994); Perkins and Backlund
 (2000); Schainost (2003); Shearer et al., (2005). Museum
 vouchers: NMNH No. 477141--(Aowa Creek, Ponca, Nebraska); UMMZ
 No. 246766--(Cambridge, Nebraska); UNSM unnumbered-- (Weeping
 Water Creek, Weeping Water, Nebraska).

15 Tryon (1868). Museum voucher: NMNH No. 477051--(West Point,
 Nebraska).

16 Freeman and Perkins (1992); Lingle (1993); Clausen and llavlik
 (1994); Perkins and Backlund (2000); Schearer et al., (2005).
 Museum vouchers: UCMNH No. 36775--(Big Blue River near
 Barneston, Nebraska); UNSM No. 1991-10-KP3 (Missouri River,
 Boyer Chute) and unnumbered--(Weeping Water Creek, Weeping
 Water, Nebraska).

17 None.

18 Schainost (2003). Museun-vouchers: PANS No. 42389--(Falls
 City, Nebraska); UNSM No. 1158--(Big Blue River, Crete,
 Nebraska) and No. 35-1-26-87.5--(Big Blue River, Seward,
 Nebraska).

19 Clausen and Havlik (1994). Museum vouchers: NMNH No.
 477017--(Dead Mans Run, Lincoln, Nebraska) and No. 504856-
 (West Point, Nebraska); UMMZ No 89630--(Weeping Water Creek,
 Weeping Water, Nebraska).

20 Museum voucher: NMNH Nos. 514939 & 540279--(West Point,
 Nebraska).

21 None.

22 Freeman and Perkins (1992); Perkins and Backluud (2000);
 Shearer et al. (2005). Museum Vouchers: NMNH No. 477132--(Aowa
 Creek, Ponca, Nebraska) and No. 521412--(Papillion Creek,
 Douglas Co., Nebraska); UNSM--(Calamus Reservoir, Nebraska).

23 Lingle (1993); Peyton and Maher (1995)--(Lake Maloney);
 Perkins and Backlund (2000); Schainost (2003); Schearer et al.
 (2005). Museum vouchers: UCMNI I No. 40927--(Johnson Lake);
 UMMZ No. 7663--(Crystal Lake, Nebraska); UNSM No. 00076.1-
 (Missouri River, Boyer Chute) and unnumbered--(Weeping Water
 Creek, Weeping Water, Nebraska).

24 Museum voucher: UNSM No. 30-1-26-87--(Papillion, Nebraska).

25 Baxa (1981), Freeman and Perkins (1992), Perkins and Backlund
 (2000); Schainost (2003); Shearer et al., (2005). Museum
 vouchers: NMN11 No. 539910--(Aowa Creek, Ponca, Nebraska);
 PANS No. 131310--(Salt Creek, Lincoln, Nebraska); UNSM No.
 00075.1- (Missouri River, Boyer Chute) and unnumbered--
 (Weeping Water Creek, Weeping Water, Nebraska).

26 Clausen and Havlik (1994).

27 Freeman and Perkins (1992); Lingle (1993); Clausen and llavlik
 (1994); Perkins and Backlund (2000); Schearer et al., (2005).
 Museum vouchers: NMNH Nos. 514990 & 515020--(West Point,
 Nebraska); UNSM nnnun/bered--(Weeping Water Creek, Weeping
 Water, Nebraska).

28 Freeman and Perkins (1997); Peyton and Maher (1995)L Museum
 vouchers: PANS No. 56710--(Nebraska City, Nebraska).

29 Museum vouchers: NMNtl No 477094--(West Point, Nebraska); PANS
 No. 42149--(Falls City, Nebraska); UCMNI l--(Johnson Lake).

30 Schainost (2003). Museum voucher: UNSM 248-8-1 20--(Little
 Nemaha River, Nebraska); UNOB unnumbered--(Missouri River).

31 Perkins and Backlund (2000). Museum vouchers: NMNH No.
 510881--(West Point, Nebraska); PANS No. 56630--(Falls City,
 Nebraska); UNSM No. 1285--(West Point, Nebraska).

32 Perkins and Backlund (2000).

33 Roedel (1990): Lingle (1992). Museum vouchers: NMNH No.
 504853--(Big Blue River, Milford, Nebraska); UNSM unnumbered
 -(Weeping Water Creek, Weeping Water, Nebraska).

34 Baxa (1981); Roedel (1990); Clausen and Havlik (1994); Peyton
 and Maher (1995)1; Schearer et al., (2005). Museum vouchers:
 NMNH No. 519293--(Oak Creek, Lincoln, Nebraska); UNSM
 1991-12-KP3--(Missouri River chute).

35 Tryon (1868). Museum voucher: PANS No 56468--(Fails City,
 Nebraska).

36 C = Corbicula.fluminea South Dakota Game, Fish and Parks
 (2011); D = Dreissena polymorpha (Pallas, 1771) Missouri
 Department of Conservation (2001), Duggan (2009b).

37 A = Alasmidozta matxinata UNSM.No. 27-1-26-87 (Nemaha); E =
 Ellipsaria lineolata Canfield and Wiebe (1931)--Big Blue River
 at Crete, Nebraska; and L = Lampilis higginsii Shearer et al.,
 (2005), NMNH No. 512459--(West Point, Nebraska).

(1) Exact locale given by Peyton to author via E-mail communication.

Appendix 3--Unionoid Mollusks and Distributions Reported for Nebraska
by Aughey (1877).

 Basin

Species (l) Blue Elkhorn Nemaha Republican

Actinonaias ligamentina X -- X --
Actinonaias pectorosa X X X --
 (Conrad, 1834)
Alasmidonta marginata -- X X --
Amblema plicata -- -- X --
Anodonta suborbiculata X X -- --
Anodontoides ferussacianus -- -- X --
Cumberlandia monodonta (Say, X X -- --
 1829)
Elliptio complanata X X X X
 (Lightfoot, 1786)
Elliptio congaraea (Lea, 1831) -- -- X --
Elliptio crassidens (Lamarck, X -- -- --
 1819)
Elliptio dilatata X -- X --
Elliptio spinosa (Lea, 1836) -- -- -- --
Epioblasma flexuosa X X -- --
 (Rafinesque, 1820)
Epioblasma o. obliquata -- -- X --
 (Rafinesque, 1820)
Epioblasma personata (Say, X -- -- --
 1829)
Epioblasma torulosa -- -- -- X
 gubernaculum (Reeve, 1865)
Epioblasma t. torulosa -- -- X --
 (Rafinesque, 1820)
Epioblasma triquetra X -- X --
 (Rafinesque, 1820)
Fusconaia flava X X X X
Glebula rotundata (Lamarck, -- -- X --
 1819)
Lampsilis cardium X -- X --
Lampsilis fasciola Rafinesque, X -- X --
 1820
Lampsilis higginsii (Lea, 1857) -- -- X --
Lampsilis ovata (Say, 1817) X X -- --
Lampsilis siliquoidea X -- X --
Lampsilis teres X X X --
Lasmigona c. complanata -- -- X --
Lasmigona compressa -- -- X --
Leptodea fragilis X X -- --
Leptodea leptodon -- -- X --
Leptodea ochracen (Say, 1817) X -- X --
Ligumia recta -- -- X --
Ligumia subrostrata X X X X
Megalonaias nervosa -- -- X --
 (Rafinesque, 1820)
Obliquaria reflexa X X X --
 (Rafinesque, 1820)
Obovaria subrotunda -- -- X --
 (Rafinesque, 1820)
Pleurobema clava X -- X --
Pleiirobeina sintoxia X X X --
 (Rafinesque, 1820)
Potamilus alatus X X X --
Potamilus capax (Green, 1832) X X -- --
Potamilus ohiensis X -- X --
Potamilus pupuratus -- -- X --
Pyganodon grandis X X X X
Quadrula c. cylindrica (Say, X -- X --
 1817)
Quadrula fragosa (Conrad, 1835) X -- -- --
Quadrula metanevra -- X X --
 (Rafinesque, 1820)
Quadrula p. pustulosa -- -- -- --
Quadrula quadrula X X X --
Strophitus undulatus X -- X --
Toxolasma parvus X X -- --
Tritogonia verrucosa X -- X --
Truncilla donaciforniis X -- -- --
Trunicilla truncata -- -- X --
Uniomerus tetralasmus -- -- X --
Utterbackia inibecillis -- -- X --
Venustaconcha ellipsiformis -- -- -- --
 (Conrad, 1836)
Villosa lienosa X -- X --

Total Species by Basin 34 19 42 5

 Basin

Species (l) Bow Other

Actinonaias ligamentina X Tomaz
Actinonaias pectorosa -- --
 (Conrad, 1834)
Alasmidonta marginata X --
Amblema plicata -- --
Anodonta suborbiculata -- --
Anodontoides ferussacianus -- Middle
Cumberlandia monodonta (Say, -- --
 1829)
Elliptio complanata -- Papillion
 (Lightfoot, 1786)
Elliptio congaraea (Lea, 1831) -- --
Elliptio crassidens (Lamarck, -- --
 1819)
Elliptio dilatata -- --
Elliptio spinosa (Lea, 1836) X --
Epioblasma flexuosa -- --
 (Rafinesque, 1820)
Epioblasma o. obliquata -- --
 (Rafinesque, 1820)
Epioblasma personata (Say, -- --
 1829)
Epioblasma torulosa -- --
 gubernaculum (Reeve, 1865)
Epioblasma t. torulosa -- --
 (Rafinesque, 1820)
Epioblasma triquetra -- --
 (Rafinesque, 1820)
Fusconaia flava X X
Glebula rotundata (Lamarck, -- --
 1819)
Lampsilis cardium -- --
Lampsilis fasciola Rafinesque, -- --
 1820
Lampsilis higginsii (Lea, 1857) -- --
Lampsilis ovata (Say, 1817) -- --
Lampsilis siliquoidea X Oak
Lampsilis teres -- --
Lasmigona c. complanata -- --
Lasmigona compressa -- --
Leptodea fragilis X --
Leptodea leptodon -- --
Leptodea ochracen (Say, 1817) -- --
Ligumia recta -- --
Ligumia subrostrata -- --
Megalonaias nervosa -- --
 (Rafinesque, 1820)
Obliquaria reflexa -- Papillion
 (Rafinesque, 1820)
Obovaria subrotunda -- --
 (Rafinesque, 1820)
Pleurobema clava -- --
Pleiirobeina sintoxia -- Papillion, Logan
 (Rafinesque, 1820)
Potamilus alatus -- --
Potamilus capax (Green, 1832) -- --
Potamilus ohiensis -- --
Potamilus pupuratus -- --
Pyganodon grandis X Iowa, Elk, Oak
Quadrula c. cylindrica (Say, -- Logan
 1817)
Quadrula fragosa (Conrad, 1835) X --
Quadrula metanevra -- --
 (Rafinesque, 1820)
Quadrula p. pustulosa X Iowa
Quadrula quadrula -- --
Strophitus undulatus -- --
Toxolasma parvus -- --
Tritogonia verrucosa -- --
Truncilla donaciforniis -- --
Trunicilla truncata -- --
Uniomerus tetralasmus -- --
Utterbackia inibecillis -- --
Venustaconcha ellipsiformis X --
 (Conrad, 1836)
Villosa lienosa -- --

Total Species by Basin 10 10

(1) Original names converted to current nomenclature as given in
Turgeon et al. (1998).


Acknowledgements

A number of individuals and institutions contributed significantly to this project. This study began as a research project for a graduate course on the Zoology of the Great Plains taught by Dr. Carl Gugler, University of Nebraska--Lincoln, who provided valuable encouragement for several years thereafter. The Department of Anthropology, University of Nebraska, Lincoln, provided a stipend to study unionid distributions in the Republican River Basin in the summer of 1972. Bob Thomas, former Fisheries Chief, at the Nebraska Game and Parks Commission donated field notes and vouchers from his collection efforts, provided encouragement during the early years of the study, and also provided the contact information for state conservation officers that was utilized to mail a questionnaire on mussel locations in the state. The Colorado Division of Wildlife, Iowa Department of Natural Resources, Montana Department of Fish, Wildlife and Parks, South Dakota Department of Game, Fish and Parks and the Wyoming Game and Fish Department provided contact information utilized to conduct mail surveys on freshwater mussels in those states. Dr. Harold Murray, then of Trinity University, corroborated identifications early in the study, and Dr. David H. Stansbery corroborated and occasionally corrected my identifications of Nebraska vouchers, provided specimens for a synoptic collection, and also provided meals and a place to stay during numerous visits to The Ohio State University Museum of Biological Diversity in Columbus, Ohio.

I am indebted to the following institutions and their staffs for access to collections and other courtesies extended: Field Museum of Natural History, Chicago, Illinois; Illinois Natural History Survey, Champaign, Illinois; Kansas Biological Survey, Lawrence, Kansas; Missouri Department of Conservation, Jefferson City, Missouri; National Museum of Natural History, Washington D. C., Philadelphia Academy of Natural Sciences, Philadelphia, Pennsylvania; The Ohio State University Museum of Biological Diversity, Columbus, Ohio; University of Colorado Museum of Natural History, Boulder, Colorado; University of Michigan Museum of Zoology, Ann Arbor, Michigan; University of Nebraska--Kearney, Department of Biology, Kearney, Nebraska; University of Nebraska--Omaha, Department of Biology, Omaha, Nebraska; University of Nebraska State Museum, Lincoln, Nebraska; and Wayne State College, Department of Biology, Wayne, Nebraska. Gordon Edwards of the Wyoming Game and Fish Department provided access to the Wyoming mussel database. Mark M. Peyton provided a copy of field notes with legal descriptions of collection sites and findings from research in the canals and lakes along the Platte River in Dawson, Gosper, and Lincoln counties.

I am also indebted to landowners across the state for physical access to collection sites, and for information provided on the location of current and historic populations of unionids in Nebraska. A number of these individuals provided specimens for this study. Finally, thanks are extended to the editor and the two anonymous reviewers for useful comments on the initial version of this paper.

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(1) Exact locale given by Peyton to author via E-mail communication.

Ellet Hoke

Midwest Malacology, Inc.

Correspondence: Ellet Hoke, 1878 Ridgeview Circle Drive, Manchester, MO 63021 ellethoke@charter.net 636-391-9459
Table 1. Productivity of Nebraska Collection Sites by

 Number (Percent)

Basin Productive Not Productive Total

Big Blue 45 (73.8) 16 (26.2) 61
Elkhorn 55 (54.5) 46 (45.5) 101
Little Blue 12 (42.9) 16 (57.1) 28
Loup 35 (34.3) 67 (65.7) 102
Lower Platte 37 (84.1) 7 (15.9) 44
Middle Platte 52 (69.3) 23 (30.7) 75
Missouri 23 (88.5) 3 (11.5) 26
Missouri Tributaries 5 (50.0) 5 (50.0) 10
Nemaha 55 (82.1) 12 (17.9) 67
Niobrara 7 (19.4) 29 (80.6) 36
North Platte 12 (66.7) 6 (33.3) 18
Republican 45 (51.7) 42 (48.3) 87
South Platte 7 (35.0) 13 (65.0) 20
White/Hat Creek 2 (40.0) 3 (60.0) 5

Total Sites 392 (57.6) 288 (42.4) 680

Table 2. Unionid Species Collected or Documented for
Nebraska

 Current
Formal Taxonomy Status (l)

Unionidae
 Ambleminae
 Amblema plicata (Say, 1817) D
 Fusconaia flava (Rafinesque, 1820) D
 Pleurobema clava (Lamarck, 1819) X, E
 Quadrada p. pustulosa (Lea, 1831) D
 Quadrada quadrada (Rafinesque, 1820) S
 Tritogonia verrucosa (Rafinesque, 1820) D
 Uniomerus tetralasmas (Say, 1831) D
 Anodontinae
 Anodonta suborbicadata Say, 1831 S
 Ataodontoides ferussacianus (Lea, 1834) D
 Arcidens confragosus (Say, 1829) D
 Lasmigona c. complanata (Barnes, 1823) S
 Lasmigona compressa (Lea, 1829) X
 Pyganodon grandis (Say, 1829) S
 Strophitus undulafs (Say, 1817) D
 Utterbackia imbecillis (Say, 1829) S
 Lampsilinae
 Actimnaias ligamentina (Lamarck, 1819) X
 Lampsilis cardium Rafinesque, 1820 D
 Lampsilis siliquoidea (Barnes, 1823) D
 Lampsilis teres (Rafinesque, 1820) D
 Leptodea fragilis (Rafinesque, 1820) S
 Leptodea leptodon (Rafinesque, 1820) S, E
 Ligumia recta (Lamarck, 1819) X
 Ligumia subrostrata (Say, 1831) D
 Obovaria olivaria (Rafinesque, 1820) X
 Potamilus alatus (Say, 1817) D
 Potamilus ohiensis (Rafinesque, 1820) S
 Potamilus purpuratus (Lamarck, 1819) X
 Toxolasma parvus (Barnes, 1823) S
 Trencilla donaciformis (Lea, 1828) D
 Trencilla truncata Rafinesque, 1820 D
 Villosa lienosa (Conrad, 1834) X

(1) D = declining; E = endangered; S = stable; X = extirpated

Table 3. Nebraska Unionid Species Occurrences

Species Number Percent

Actinonaias ligamentina 2 0.19
Amblema plicata 35 3.28
Anodonta suborbiculata 4 0.37
Anodontoides ferussacianus 92 8.61
Arcidens confragosus 2 0.19
Fusconaia flava 42 3.93
Lampsilis cardium 52 4.87
Lampsilis siliquoidea 35 3.28
Lampsilis teres 25 2.34
Lasmigona c. complanata 114 10.67
Lasmigona compressa 2 0.19
Leptodea fragilis 33 3.09
Leptodea leptodon 1 0.09
Ligumia recta 16 1.50
Ligumia subrostrata 23 2.15
Obovaria olivaria 4 0.37
Pleurobema clava 1 0.09
Potamilus alatus 11 1.03
Potamilus ohiensis 60 5.62
Potamilus purpuratus 2 0.19
Pyganodon grandis 193 18.07
Quadrula p. pustulosa 51 4.78
Quadrula quadrula 118 11.05
Strophitus undulates 30 2.81
Toxolasina parvus 11 1.03
Tritogonia verrucosa 11 1.03
Truncilla donaciformis 5 0.47
Truncilla truncata 7 0.66
Uniomerus tetralasmus 65 6.09
Utterbackia imbecillis 21 1.97
Villosa lienosal -- --

Total Occurrences 1,068 100.00

(1) Represented only by a museum voucher.

Table 4. Unionid Species That May Have Occurred in Nebraska Based On
Their Presence in Adjacent States

 State

 NW Missouri South
Species (l) Kansas Western Iowa Dakota Locale (2, 3)

Alasmidonta -- -- -- X (a) Little
 marginata Say, Sioux River
 1818
 -- -- X -- (b) Big Sioux
 River
 -- -- X -- (c) Boyer River

Cyclonaias -- -- X -- (d) Big Sioux
 tuberculata River
 (Rafinesque,
 1820)
Elliptio dilatata X -- -- -- (e) Wakarusa
 (Rafinesque, River
 1820)
 -- -- X -- (f) Big Sioux
 River
 -- -- X -- (g) Boyer River
Epioblasma X -- -- -- (h) Wakarusa
 triquetra River
 (Rafinesque,
 1820)
Lasmigona costata -- -- X -- (i) Little
 (Rafinesque, Sioux River
 1820)
Megalonaias -- X -- -- (j) Platte
 nervosa River,
 (Rafinesque, Missouri
 1820)
Obliquaria reflexa X -- -- -- (k) Smokey
 (Rafinesque, Hill River
 1820)
 -- -- -- X (l) Big Sioux
 River
 -- -- -- X (m) James River
Plethobasus -- -- X -- (n) Little
 cyphyus Sioux River
 (Rafinesque,
 1820)
Pleurobema X -- -- -- (o) Vermillion
 sintoxia
 (Rafinesque,
 1820) River
 -- -- X -- (p) Boyer River
 -- -- X -- (q) Little
 Sioux River
Ptychobranchus X -- -- -- (r) Wakarusa
 occidentalis River
 (Conrad, 1836)
Quadrula fragosa X -- -- -- (s) Little Blue
 (Conrad, 1835) River
 X -- -- -- (t) Big Blue
 River
 -- -- -- X (u) James
 River, SD
Quadrula metanevra -- -- X -- (v) Prairie
 (Rafinesque, Creek
 1820)
Simpsonaias -- -- X -- (w) Nishnabotna
 ambigua (Say, River
 1825)

(1) Frest (1987) reported the collection of Fusconaia ozarkensis
(Call, 1887) from the Boyer River in western Iowa. The species is
otherwise known from only a small number of streams in southern
Missouri, only one of which is a tributary of the Missouri River, and
also from one site in southeastern Kansas on the Missouri border
(Oesch, 1995; Angelo et al. 2009). Since (1) Arbuckle et al. (2000)
collected the same sites and recovered no specimens of F. ozarkensis;
and (2) all attempts to locate vouchers from Frest's (1987) study
have been unsuccessful, I have provisionally assumed the report of F.
ozarkensis from the Boyer River is incorrect, and the species is
accordingly excluded from this listing.

(2) Locales are plotted in Figure 38.

(3) Vouchers or literature references supporting these ranges are as
follows: (a) UMMZ as given by Frest (1987); (b) Skadsen (1998); (c)
Frest (1987); (d) NMNH--specimen given in Frest (1987); (e) Scammon
(1906); (f) NMNH as given by Frest (1987); (g) Frest (1987); (h)
Scammon (Murray and Leonard, 1962); (i) UMMZ Nos. 54804, 54843,
54846, 54862 and 7746 as given by Frest (1987); (j) Utterback (1917);
(k) Angelo et al. (2009); (l) Backlund (2000); (m) Perkins and
Backlund (2003); (n) UMMZ No. 233952; (o) UMMZ No. 153994 ; (p) Frest
(1987); (q) NMNH Nos. 528696 and 535258 ; (r) Call (1885b); (s) Hoke
(2004); (t) Hoke (2005b); (u) Perkins and Backlund (2003); (v)
Arbuckle et al. (2000); and (w) MCZ No. 564863 as given in Frest
(1987).

Table 5. Freshwater Mussels Reported From the South and North Platte
Basins in Colorado, Nebraska, and Wyoming

 Basin

 South Platte

Species NE CO WY

Anodontoides ferussacianus L H-M (c) R (f)
Lantpsilis cardium -- H? (a)
Lampsilis siliquoidea -- -- --
Lasmigona c. coinplanata L -- --
Leptodea fragilis H? (a) -- --
Ligumia recta -- -- --
Potantilus ohiensis L (b) -- --
Pyganodon grandis L L (d) --
Strophitus undulatus -- H-M (e) --
Utterbackia intbecillis L (b) -- R-M (g)

Species Reported 6? 4? 2

 Basin

 North Platte

Species NE WY

Anodontoides ferussacianus L R (j)
Lantpsilis cardium - R (j) + H (k)
Lampsilis siliquoidea RL (h) R (j)
Lasmigona c. coinplanata L --
Leptodea fragilis -- --
Ligumia recta -- R (j)
Potantilus ohiensis -- R (j)
Pyganodon grandis L --
Strophitus undulatus -- --
Utterbackia intbecillis L (i) --

Species Reported 5 5

Capital letters indicate condition and relative age of
source records: L = Live; R = recent; RL = relict; H =
historical report; R+H = recent and historical records; R
M = recent museum voucher; and H-M = historical
museum voucher.

All records without superscript references are from this
survey. Data sources by superscript are: (a) Cockerell
(1889); (b) Peyton and Maher (1995): (c) UCMNH, nos. 253,
258, 3606, 41594; (d) Cordeiro (1999); (e) Los Angeles County
Museum, nos. 140605, 140606 as given in Cordeiro
(1999); (f) Hoke (1995); (g) UCMNH, nos. 40916 and 40917;
(h) Freeman and Perkins (1992); (i) Baxa (1981); (j) Edwards
(2010), and (k) UCMNH, no. 8540.

Table 6. Nebraska Stream Condition Statistics In 1973 (1)

 Original Dewatered/
 Stream Channelized Polluted
Basin (km) (km) (km)

Big Blue 2,691.7 117.5 167.3
Elkhorn 2,492.8 467.7 72.4
Little Blue 1,488.6 31.9 168.9
Loup 2,734.2 117.9 48.3
Lower Platte 1,227.3 111.0 62.8
Middle Platte 1,089.7 76.6 265.5
Missouri Tributaries 1,088.2 40.7 40.2
Nemaha 2,320.8 264.0 11.3
Niobrara 2,523.4 22.0 32.2
North Platte 677.2 14.5 222.0
Republican 1,889.6 62.4 728.9
South Platte 464.5 13.7 186.6
White/Hat Creek 557.2 2.9 24.1

Total/Average 21,245.2 1,342.8 2,030.5
(6) Eastern Nebraska 9,820.8 1,000.9 354.0
(7)Western Nebraska 11,424.4 341.9 1,676.5

 Basin Stream
 Area Drainage Turbidity
Basin ([km.sup.2]) Development (2) Impact (3)

Big Blue 11,836 4.40 79
Elkhorn 18,130 7.27 77
Little Blue 6,863 4.61 91
Loup 39,446 14.43 60
Lower Platte 8,055 6.56 74
Middle Platte 13,287 12.19 91
Missouri Tributaries 7,641 7.02 67
Nemaha 7,148 3.08 47
Niobrara 30,743 12.18 20
North Platte 18,493 27.31 34
Republican 24,993 13.23 63
South Platte 8,158 17.56 25
White/Hat Creek 5,517 9.90 28

Total/Average 200,310 9.43
(6) Eastern Nebraska 52,810 5.38
(7)Western Nebraska 147,500 12.91

 Soil Water
Basin Stability (4) Rights

Big Blue H E
Elkhorn H E
Little Blue H E
Loup H E
Lower Platte H E
Middle Platte H E
Missouri Tributaries NG NG
Nemaha H E
Niobrara H E
North Platte NG E
Republican H NG
South Platte NG NG
White/Hat Creek NG E

Total/Average
(6) Eastern Nebraska
(7)Western Nebraska

(1) All data obtained or derived from Bliss and Schainost (1973a-m)

(2) Calculated as basin area (km2) divided by original stream km.

(3) The percent of streams impacted by turbidity.

(4) H = highly erodable; NG = not given in source materials.

(5) Extent water rights exceed total stream flow on small and
intermediate sized streams during years of low or normal flows:
E = exceed low or normal flows; NG = not given in source materials.

(6) Includes the Big Blue, Elkhorn, Lower Platte, Missouri
Tributaries, and Nemaha basins

(7) Includes the Little Blue, Loup, Middle Platte, Niobrara, North
Platte, Republican, South Platte, and White/Hat Creek basins

Table 7. Unionid Species Recovered From Canal,
Reservoir, and Pond Habitats in Nebraska *

 Sand
 Pits &
Species Canals Reservoirs Ponds

Anodontoides ferussacianus X (2,3) F X (2)
Lasmigona c. complanata X (2,3) X (3) --
Leptodea fragilis X X --
Potamilus alatus -- M --
Potamilus ohiensis X (2,3) X (2,3) --
Pyganodon grandis X (2,3) X (1,2,3) X
Quadrula quadrula X (2,3) X (3) --
Strophitus undulates P -- --
Toxolasma parvos -- X (3) --
Truncilla truncata -- X --
Uniomerus tetralasmus -- X X
Utterbackia imbecillis X (2,3) X (1,2,3) X

Total Species 8 11 4

* X = collected in this study; F=collected only by Freeman and
Perkins (1992); P = collected only by Peyton and Maher (1995); and
M = museum specimen, Nebraska State Museum. Superscript
numerals indicate species also collected in other studies as follows:
(1) Baxa (1981); (2) Freeman and Perkins (1992); (3) Peyton and Maher
(1995).

Table 8. Channelization of Selected Interior Nebraska
Streams--Nemaha and Elkhorn Basins

Basin Current Original Lost
Stream/ Segment (km) (km) (km) Percent

Nemaha Basin (1)
 Big Nemaha 31.5 61.1 29.6 48.4
 Big Nemaha, Middle
 Branch 34.4 36.2 1.8 4.9
 Big Nemaha, North
 Fork 109.4 164.3 54.9 33.4
 Big Nemaha, South
 Fork 49.7 80.9 31.2 38.6

 Total Big Nemaha 225.0 342.5 117.5 34.3

 Little Nemaha 122.5 160.6 38.1 23.7
 Little Nemaha,
 North Fork 56.7 62.3 5.6 9.0
 Little Nemaha, South
 Fork 40.6 47.3 6.7 14.2

 Total Little Nemaha 219.8 270.2 50.4 18.7

 All other streams in
 basin 1,612.0 1,708.1 96.1 5.6

 Total 2,056.8 2,320.8 264.0 11.4

Elkhorn Basin (2)
 Elhorn River 444.7 596.7 152.0 25.5
 Logan Creek 133.3 215.2 81.9 38.1
 All other streams in
 basin 1,447.1 1,680.9 233.8 13.9

 Total 2,025.1 2,492.8 467.7 18.8

(1) Nemaha Basin data from Bliss and Schainost (1973c)

(2) Elkhorn Basin data from Bliss and Schainost (19730
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Publication:Transactions of the Nebraska Academy of Sciences
Geographic Code:1U4IA
Date:Nov 1, 2011
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