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Early life history and distribution of pistolgrip (Tritogonia verrucosa (Rafinesque, 1820)) in Minnesota and Wisconsin.

INTRODUCTION

"The heavy waves will break many of the fine strings on which the young clams are fastened and wash them to the shore. Millions of such little tiny shells are at times piled up to six inches deep on the shores of Lake Pepin (Mississippi River) after a storm." Paul Harder, The History of the Fresh Water Clam or Musselshell, ca. 1908

Understanding the life history and distribution of a rare species are fundamental to returning a rare species to historic levels. Freshwater mussels are among the most threatened groups of organisms in the United States (Master et al., 2000) and there is a growing effort to coordinate freshwater mussel conservation through a national strategy (The National Native Mussel Conservation Committee, 1998). Improving our understanding of biological needs and distribution of mussels are ranked among the highest goals in this strategy.

An important part of understanding the life history needs of unionoid mussels is knowledge of the host requirement(s) of the larvae (glochidia). Most unionoid bivalves attach to a host fish in order to metamorphose from a glochidium into a juvenile (Williams et al., 2008). Several mussel species exhibit elaborate behaviors to draw hosts to their young (Barnhart et al., 2008). Natural glochidia hosts are usually identified through two kinds of studies, (1) laboratory trials where potential hosts are exposed to glochidia to determine if metamorphosis occurs (Zale and Neves, 1982; Jones et al., 2004), and (2) identification of glochidia, or preferably juveniles, recovered from naturally infested hosts using scanning electron microscopy (Kennedy and Haag, 2005; Allen et al., 2007) or molecular analysis (Kneeland and Rhymer, 2007). Recently, knowledge of host fish(es) has been used to strengthen conservation efforts through propagating rare mussel species (Jones et al., 2004; Barnhart, 2006; McLeod, 2006).

Culturing rare juvenile unionids for conservation purposes requires knowledge of how and when to find gravid mussels. Currently, glochidia for propagation efforts are obtained from wild stocks (Jones et al., 2004). Most mussel species brood mature glochidia for only a portion of the year (Williams et al., 2008). Many Amblemine mussels are short-term brooders, bearing glochidia in the spring or early summer, although a few species brood in the fall (Parmalee and Bogan, 1998; Williams et al., 2008). In addition to knowing when females are brooding glochidia it is useful to people searching for these animals to know if they exhibit visible characteristics distinguishable from other species or non-brooding animals. For example, several mussels in the Tribe Lampsilini (Campbell et al., 2005) display ornate mantle extensions to draw the attention of their hosts, while some members of the Tribe Quadrulini hold glochidia in an inflated mantle pouch or magazine that is readily visible (Barnhart et al., 2008).

The pistolgrip, Tritogonia verrucosa (Rafinesque, 1820), is a rare mussel in many parts of its range where conservation efforts would benefit from improved understanding of its distribution and biology. Pistolgrip is relatively widespread across central and southeast portions of the U.S. (Williams et al., 1993). However, it has declined to the point that it is listed as threatened in Minnesota and Wisconsin, endangered in Iowa and extirpated from North Carolina. While pistolgrip is known to live in the lower portions of some larger Minnesota and Wisconsin rivers (Baker, 1928; Cummings and Mayer, 1992; Sietman, 2003), a comparison between current and historic ranges is needed for recovery planning. The pistolgrip, a member of the Quadrulini, is apparently allied with mapleleaf mussels of the quadrula species group (Serb et al., 2003; Campbell et al., 2005). Similar to other quadrulines, pistolgrip display a mantle magazine where glochidia are held until they are presumably extracted by the host fish (Barnhart et al., 2008). Of the few studies on pistolgrip early life history, Jirka and Neves (1992) found them brooding glochidia between Apr. and early Jun, in the New River, West Virginia, although it is unclear if the brooding period is different in the upper Mississippi River, over 700 km to the north. Host suitability trials conducted in Texas using pistolgrip glochidia on three fish species showed that flathead catfish (Pylodictus olivaris) is a potential host (Howells, 1997), a result consistent with suspected catfish hosts for other members of the quadrula species group (Howard and Anson, 1922; Schwebach et al., 2002; Steingraeber et al., 2007). However, there are over 100 fish species living in Minnesota and Wisconsin (Fago, 1986; Hatch et al., 2003) that might also serve as hosts. Pistolgrip glochidia have been described using light microscopy (Surber, 1912), and more recently using scanning electron microscopy (Hoggarth, 1999; Kennedy and Haag, 2005), and this information can be used to identify unknown juvenile mussels collected from naturally infested fishes (Kennedy and Haag, 2005; Allen et al., 2007) or for taxonomic analysis (Rand and Wiles, 1982; Kwon et al., 1993). In light of the conservation needs for pistolgrip we undertook the following research objectives to: (1) examine gravid pistolgrip behavior, (2) describe conchological characters of glochidia, (3) identify suitable glochidia hosts and (4) assess the current distribution and status of pistolgrip in Minnesota and Wisconsin.

METHODS

GRAVID FEMALE BEHAVIOR AND GLOCHIDIAL MORPHOLOGY

We examined gravid pistolgrip behaviors in the St. Croix River and at the University of Minnesota's Wet Laboratory (UMN) between 1997-2008. Observations were made on at least eight females in the St. Croix River during daylight hours bimonthly between May-Nov. 1997 and Apr.-Oct. 1998 after spring peak discharge in early Apr. Also, we photographed pistolgrip in the St. Croix River primarily at night approximately bimonthly between May-Jul. 2004-2007 to study the gravidity period. We identified mussels using criteria described in Sietman (2003) and female pistolgrip were identified as those that possessed a broad posterior margin that extended much further beyond the posterior ridge of a valve than the posterior margin of males (Baker, 1928; Cummings and Mayer, 1992; Parmalee and Bogan, 1998). Between 1997-1998 we collected females and males using SCUBA and snorkel equipment and checked for gravidity. Gravid animals were identified as those that had gills swollen with eggs, glochidia or conglutinates or held conglutinates beneath a swollen posterior mantle. The maturity of young held in gills was not examined. On 26-27 May 2005, we shot digital images of gravid pistolgrip in the St. Croix River and an adjacent ruler using a Nikon Coolpix 5200 camera with an underwater housing to determine if mantle size of individual mussels changed diurnally. The area ([mm.sup.2]) within the outline of swollen mantle profiles (lateral view) was measured using Image J v. 1.41o (National Institutes of Health, U.S.A., http://rsb.info.nih.gov/ij). Treatment effects of day vs. night on magazine surface area were tested by paired t-test (unequal variances). Statistical analyses were conducted using JMP v. 3.2 statistical software (SAS Institute, Cary, NC). We also studied and photographed gravid pistolgrip behavior at the UMN between May-Jun. 1995-1998, 20042008. To examine whether gravidity behavior was related to water temperature we obtained water temperature data (recorded between 7-9 am at Prescott, WI) from Minnesota Metropolitan Council Environmental Services.

We examined morphological characteristics of pistolgrip glochidia from 11 females collected using SCUBA on 28 May 2005. Mature glochidia were flushed from the mantle magazine with a stream of water and preserved in 95% ethanol. Females were returned to their collection site. Glochidia were washed with 100% ethanol two to four times and mounted on a scanning electron microscope specimen stub using conductive, double-faced adhesive tape. We covered specimens with gold using a Fullham GMS-76M Sputter Coater and viewed them with a Hitachi $3500N variable pressure scanning electron microscope with Quartz PCI digital imaging software (Quartz Imaging Corporation, Vancouver, BC, Canada). Image files of lateral views of glochidia were printed and measured to the nearest mm. Measures of height, length and hinge length (Hoggarth, 1999) were converted to microns using an electronically generated scale bar printed with each image. We measured 3-8 glochidia from each female, although not all dimensions were measurable for each glochidium. Differences in glochidia valve dimensions among female mussels were analyzed by multiple means comparison tests (Tukey-Kramer HSD).

HOST SUITABILITY

We conducted pistolgrip glochidia host suitability trials using standard protocol (Zale and Neves, 1982; Hove et al., 2000). Gravid pistolgrip were collected from the St. Croix River, Polk County, WI and held at the UMN in large beakers within flow-through aquaria between 17-23 C. Fishes used in host suitability trials were collected from rivers, lakes and streams throughout Minnesota and held at the UMN for at least 20 d before being exposed to glochidia. We used freshly released, mature glochidia (i.e., fully formed valves and adductor muscle) for host suitability trials in 1995-1998, 2004 and 2008. We used a dissecting microscope to study glochidia health and attachment to fish and siphonate. Glochidia were used in trials if >70% of 15-30 glochidia closed in response to sodium chloride diffusing from nearby salt crystals. Fish were exposed to glochidia in a vigorously aerated container for 90 s to 24 h depending on the rate that >5 glochidia were observed attached to one set of gills. We found that flathead catfish can be over-exposed to pistolgrip glochidia very quickly, ultimately resulting in fish mortality (M. Hove and B. Sietman, pers. obs.). Subsequently, we diluted the glochidia concentration from all the glochidia from 6 female pistolgrip placed in approximately 20 1 water and exposing the fish for 5 min to half the amount of glochidia in the same volume of water for 5-10 s and closely monitored fish for gasping or equilibrium during exposure and improved flathead catfish survival. Inoculated fishes were held in 40-200 1 single pass, flow-through aquaria at water temperatures generally following rising spring and summer temperatures in the St. Croix River (20 [+ or -] 3 C). Small fishes and catostomids were held in suspended nets to prevent them from possibly consuming glochidia or juveniles from the aquarium floor. Throughout the study fish were given 15 min to feed daily before food was usually removed from the aquarium. Aquaria were siphoned 2-3 times a week to retrieve and count glochidia and juveniles until all young mussels were released from the fish. Fish were inspected for encapsulated glochidia once or twice during the first 10 d of the trial and inspected again after three checks of siphonate yielded no glochidia or juveniles. If glochidia were found on a fish's gills, the trial was continued using this procedure until all glochidia had either been sloughed or metamorphosed into juveniles. Juvenile mussels were identified by the presence of valve growth beyond the glochidial valve and presence of an actively moving foot or paired adductor muscles. Those species that facilitated pistolgrip metamorphosis were considered suitable host species. Extra siphoning was done for a portion of flathead catfish Trial 3. For 13 d of the juvenile release period we siphoned the aquarium twice every 24 h, at 7:30 am and 7:30 pm, to determine if there was a difference in release between night (sunset approx. 8:30 pm in St. Paul, MN) and day (sunrise approx. 6:00 am). We measured glochidial and juvenile valve lengths of nine freshly released juveniles from one of the flathead catfish trials to determine growth during attachment. Treatment effects of day vs. night on juvenile release rates were tested using analysis of variance. Unionid and fish nomenclature follows Turgeon et al. (1998) and Nelson et al. (2004), respectively.

DISTRIBUTION

We gathered information from several sources to describe recent pistolgrip status and distribution in Minnesota and Wisconsin. Most data were from Minnesota (MN DNR) and Wisconsin (WI DNR) departments of natural resources surveys completed from 1999 to 2009 and 1980 to 2008, respectively. Surveying methods in Minnesota are described in Allen et al. (2007). Methods for Wisconsin samples were similar, except some sites were also quantitatively sampled using quadrats. Border waters (Mississippi and St. Croix rivers) were sampled both by MN DNR and WI DNR. We also include data from post 1980 surveys on the Cannon (Davis, 1987), Zumbro (Bright et al., 1989) and Minnesota (Bright et al., 1990) rivers, Minnesota, Chippewa River, Wisconsin (Balding, 1992; Balding and Balding, 1996) and Mississippi River (Havlik, 1983; Hornbach et al., 1992). Additionally, we recognize occurrences of 5 live pistolgrip specimens from the Mississippi River not collected by us but were verified by us or other experts (M. Farr, M. Havlik and D. Helms, pers. comm.), museum specimens or photographic evidence. All data used to evaluate the current status of pistolgrip were collected from 1980 to 2009. To examine the historical distribution of pistolgrip, beyond empty shells that we collected, we compiled data from museum records (University of Minnesota's James Ford Bell Museum of Natural History, JFBM, Milwaukee Public Museum, Ohio State University Museum of Biological Diversity, OSUM, Illinois Natural History Survey Mollusk Collection, INHS) and a literature search (Grier and Mueller, 1922, 1923; Baker, 1928; van der Schalie and van der Schalie, 1950; Fink, 1966; Havlik and Stansbery, 1978; Mathiak, 1979; Fuller, 1980; Thiel, 1981). We deposited voucher specimens at JFBM, INHS and OSUM.

Results

GRAVID FEMALE BEHAVIOR AND GLOCHIDIAL MORPHOLOGY

St. Croix River pistolgrip brooded young in all four gills during spring and early summer. We likely missed the beginning of the gravidity period in 1997 as our first observation on 7 May revealed 89% of females bearing glochidia. Gravid females were observed between 7 May-3 Jun. 1997 and 28 Apr.-8 Jun. 1998 at 13-22 C water temperature (Fig. 1). The percentage of gravid females peaked to 100% on May 20 in 1997 and 83% on Apr. 28 in 1998. Between May-Jul. 2004-2007 we observed pistolgrip with partially inflated mantle magazines in late Apr. and early May, but fully inflated magazines were only observed between mid-May to early Jul. During any given year, fully inflated magazines were observed for approximately 5-6 wk.

Gravid pistolgrip presented an inflated posterior mantle that reached its greatest distension at night. On 27 May 2005 we inspected the magazine of thirteen gravid pistolgrip with full displays and observed conglutinates with mature glochidia (i.e., glochidia valves were fully formed and a high proportion of individuals closed upon exposure to sodium chloride) within the magazine cavity. Mande magazines were nearly three times larger (270%, range 160 to 410%) at night (mean lateral magazine area [+ or -] 1 SD = 8.1 [+ or -] 2.8 [cm.sup.2]) than the following early afternoon (3.1 [+ or -] 1.1 [cm.sup.2], t = 9.3, df = 14, P < 0.0001) (Fig. 2a, b). We observed similar changes in the degree of magazine distension at night versus the day in the laboratory but did not quantify these observations.

[FIGURE 1 OMITTED]

Gravid pistolgrip held conglutinates and glochidia in their marsupia or within the magazine. Pistolgrip released glochidia individually and in broken and whole conglutinates in the laboratory. Conglutinates were light yellow or occasionally white in color, frequently had an elongated rectangular outline that tapered to a rounded point on at least one end. Broken and whole conglutinates were 3-25 mm long and 9-3 mm wide (Fig. 3a). Pistolgrip glochidia had a subelliptical valve outline, rough surface and slightly concave dorsal margin (Fig. 3b). The ventral margins of the glochidial valves were lined with small lanceolate micropoints arranged in broken vertical rows along a band that narrowed dorsally to approximately one-half valve height (Fig. 3c). Occasionally we observed glochidia with a lateral gape and blunt supernumerary hooks. Glochidia length (mean [+ or -] 1 SD = 102 [micro]m [+ or -] 4, range = 92 to 111 [micro]m, n = 58) and height (119 [+ or -] 6 [micro]m, 108 to 132 [micro]m, n = 57) were significantly different among females (length, F = 3.03, df = 57, P = 0.005; height, F = 4.53, df = 56, P = 0.0002). Hinge length (49 [+ or -] 3 [micro]m, 44 to 56 [micro]m, n = 39) did not differ significantly among females (F = 0.96, df = 37, P = 0.5).

HOST SUITABILITY

Laboratory host suitability studies revealed pistolgrip glochidia metamorphose on some ictalurid species. We exposed 65 fish species (18 families) to pistolgrip glochidia and observed metamorphosis during flathead catfish and some yellow bullhead (Ameiurus natalis) and brown bullhead (A. nebulosus) trials (Table 1). No other fish species tested facilitated glochidia metamorphosis although glochidia grew while encapsulated during three black bullhead (Ameiurus melas) trials and one unusually long creek chub (Semotilus atromaculatus) trial. Juvenile pistolgrip released four to nine wk after encapsulation. Flathead catfish facilitated metamorphosis of dramatically more glochidia than other suitable hosts, which were similar in size (i.e., total length of suitable hosts ranged between 15-25 cm). In flathead catfish Trial 3, there was no difference in the number of juveniles released between night and day (t = 1.73, df = 24, P = 0.1). On average, glochidia grew 422% (from mean length [+ or -] 1 SD = 94 [micro]m [+ or -] 3 to 395 [+ or -] 19 [micro]m, n = 9) while encapsulated (Fig. 3d).

[FIGURE 2 OMITTED]

DISTRIBUTION

In Minnesota and Wisconsin, pistolgrip are, with few exceptions, restricted to the Mississippi River and lower reaches of its larger tributaries, and the Fox River, a tributary of Lake Michigan. Reproducing populations are present in the lower St. Croix, Chippewa, Black and Wisconsin rivers of the Mississippi River System, and the middle Wolf River of the Fox River Drainage (Fig. 4). Pistolgrip are presumably extirpated from the Minnesota River, as no live individuals have been collected during recent sampling. They are also very rare in the Upper Mississippi River. In the last 28 y, we are aware of only 26 live individuals collected from the Mississippi River, 24 of which were found in three small reaches; above the confluence with the St. Croix (15 individuals) and Wisconsin (two individuals) rivers and below the Black River confluence (seven individuals). In 2009, 13 live individuals were collected above the St. Croix River confluence below Lock and Dam 2, of which 69% were estimated at [less than or equal to] 5 years old based on a count of external annuli.

[FIGURE 3 OMITTED]

DISCUSSION

GRAVID FEMALE BEHAVIOR AND GLOCHIDIAL MORPHOLOGY

Pistolgrip gravidity period is similar to other species within the Quadrula genus living in the St. Croix River as well as pistolgrip populations to the south. The initiation of gravidity for St. Croix River pistolgrip, monkeyface (Q. metanevra) and pimpleback (Q. pustulosa) occurs in late Apr. or early May, and in late Aug. for winged mapleleaf (Q. fragosa) (Heath et al., 2001). The periods when St. Croix River pistolgrip and winged mapleleaf have been observed gravid (6-7 wk) are shorter than those observed for monkeyface and pimpleback (10-15 wk). Other studies have shown that pistolgrip are gravid between late Apr. and early Jun. (Sterki, 1907; Jirka and Neves, 1992), although one gravid individual was observed in Jan. during an unusually warm winter in Texas (Howells, 2000). Jirka and Neves (1992) observed gravid pistolgrip after water temperature reached 9 C in West Virginia. We observed gravid pistolgrip beginning when water temperature reached 14-15 C, although we likely missed the beginning of the gravidity period in 1997.

St. Croix River pistolgrip produced unique-sized glochidia. Most upper Mississippi River (UMR) species in Minnesota and Wisconsin produce glochidia that are 200-350 [micro]m in height (Surber, 1912; Hoggarth, 1999). Pistolgrip glochidial valve height (mean [+ or -] 1 SD, range) is 119 [+ or -] 6, 108-132 [micro]m, distinguishing them from UMR mussel species with the next smaller and larger glochidial valves, winged mapleleaf (102 [+ or -] 3, 96-107 [micro]m, n = 24) and Wabash pigtoe (167 [+ or -] 5, 158-178 [micro]m, n = 27) (M. Hove, pers. obs.), respectively. We observed variation in glochidia size between females emphasizing the need to measure multiple glochidia from several females. The dimensions of pistolgrip glochidia we measured are similar to those reported from a St. Croix River specimen in Kennedy and Haag (2005), whereas glochidia dimensions reported in Surber (1912) (locality unclear) and Hoggarth (1999) from Ohio are smaller. These observations illustrate the importance of determining local glochidia valve dimensions for individual studies.

[FIGURE 4 OMITTED]

Data from our study could be useful in understanding how pistolgrip fits in quadruline phylogeny as these relationships are unresolved (Serb et al., 2003; Graf and Cummings, 2007; Williams et al., 2008). Ortmann (1912) describes a close alignment between Tritogonia and Quadrula, and Serb et al. (2003) proposed placing T. verrucosa within Quadrula based on DNA analysis. Of the three primary quadruline clades, the metanevra, pustulosa and quadrula species groups (Serb et al., 2003), recent molecular based studies place T. verrucosa as sister to the quadrula species group (Serb et al., 2003; Campbell et al., 2005). Pistolgrip also shares morphological similarities with the quadrula species group. Pistolgrip glochidia (average height = 119 [micro]m) are similar in size to Q. fragosa (102 [micro]m) and Q. quadrula (92 [micro]m), as compared to the larger glochidia of Q. metanevra (208 [micro]m) and Q. pustulosa (300 [micro]m) (M. Hove, pers. obs.; Utterback, 1915). The mantle magazine of pistolgrip and Q. fragosa is larger than Q. pustulosa and Q. cylindrical (Barnhart et al., 2008). We have observed "reflexive release" of glochidia (Barnhart et al., 2008) by Q. metanevra and Q. pustulosa but not among T. verrucosa, Q. fragosa or Q. quadrula. Laboratory host suitability trials show that members of the metanevra group use Cyprinids as hosts (Yeager and Neves, 1986; Yeager and Saylor, 1995; Crownhart et al., 2006) compared to suspected use of Ictalurids by members of the quadrula and pustulosa groups and pistolgrip (Howard, 1914; Coker et al., 1921; Howells, 1997; Schwebach et al., 2002; Haag and Warren, 2003; Steingraeber et al., 2007). In the St. Croix River pistolgrip and Q. fragosa brood glochidia for a relatively short period of time, approximately 6 wk, compared to at least 11 wk for Q. pustulosa and Q. metanevra (Heath et al., 2001). Including these types of data in future studies could be useful in resolving questions of quadruline phylogeny and classification.

Diel rhythms of mantle displaying behavior have been reported for unionids in the Tribe Lampsilini (Rypel, 2008; Haag and Warren, 2000), but ours is the first study to show this behavior in the Tribe Quadrulini. Two lampsiline species, yellow sandshell (Lampsilis teres) (Rypel, 2008) and Alabama rainbow (Villosa nebulosa) (Haag and Warren, 2000), were shown to display only at night, whereas southern rainbow (V. vibex) displayed more during daytime (Haag and Warren, 2000). The pattern of displaying behavior, i.e., night vs. day, is hypothesized to correspond to activities of host fishes such that the likelihood of mussel-host encounters is increased (Rypel, 2008). We showed that pistolgrip magazines are significantly more inflated at night, corresponding to the time when flathead catfish are most active, between dusk and dawn (Daughtery and Sutton, 2005a). St. Croix River pistolgrip usually emerge from the substrate during the brooding period (M. Davis, M. Hove, B. Sietman, pets. obs.). Perhaps brooding pistolgrip release chemical attractants (e.g., free amino acids (Caprio et al., 1993)) to attract foraging catfish (Pepi and Hove, 1997). We hypothesize that the diel behavior of pistolgrip is an adaptation that increases the likelihood of glochidia encountering a host.

HOST SUITABILITY

Fish are the primary dispersal mechanism for unionids (Smith, 1985; Vaughn, 1997; McLain and Ross, 2005) and gravid pistolgrip behavior and timing appear well suited to attracting catfish hosts. We confirmed flathead catfish as suitable hosts for pistolgrip glochidia, as reported by Howells (1997) and showed for the first time that yellow and brown bullhead can facilitate glochidia metamorphosis in the laboratory. Although we didn't test several potential host fishes and additional trials should be conducted, where few individuals have been tested it appears catfishes are important glochidial hosts to pistolgrip. We presume flathead catfish are more important hosts than bullhead species because flathead catfish that were slightly larger than the bullheads produce dramatically more juvenile pistolgrip, and because pistolgrip and flathead catfish live in the main flow of large rivers whereas yellow and brown bullheads tend to live in streams, ponds, lakes and river backwaters (Pflieger, 1977; Becker, 1983). Pistolgrip gravidity period occurs between late Apr.-early Jul., a time of the year when flathead catfish are most active. Flathead catfish are most active when they leave over-winter habitats and begin spawning behavior (Daughtery and Sutton, 2005a; Vokoun and Rabeni, 2005) in the spring (May-Jun.) during which water temperature rises above 10 C. As Jul. approaches, flathead catfish move to summer and fall habitats and restrict movement (Vokoun and Rabeni, 2005). During the summer flathead catfish are more active during the dusk, night and dawn than during the day (Daughtery and Sutton, 2005b). Adult flathead catfish are thought to feed opportunistically on live fish but also on invertebrates including crayfish and bivalves (Laher and Boles, 1980; Weller and Robins, 1999; Herndon and Waters, 2000). We have shown that pistolgrip are gravid in May and Jun. after water temperature climbs above 10 C, gravid pistolgrip mantles are more distended at night than during the day and juvenile pistolgrip generally release in the laboratory 1 to 2 mo after encapsulation or around the end of Jul. when flathead catfish are settling into their summer habitat and moving less. These associations lead us to recommend that pistolgrip conservation efforts include sustainable flathead catfish management.

DISTRIBUTION

The pistolgrip is considered a medium and large river species (Cummings and Mayer, 1992), and this characterization is largely borne out in our study (Fig. 4). The distribution of pistolgrip in the upper Midwest is influenced by the co-varying factors of flathead catfish distribution (presumed primary host) and geologic features that limit host distribution. The overlapping distributions of pistolgrip and flathead catfish are nearly identical in Minnesota and Wisconsin (Becker, 1983; K. Schmidt, pers. comm.), being restricted primarily to large river main stems and the lower reaches of main tributaries. Barrier falls on the Mississippi River at Minneapolis-St. Paul and St. Croix River at St. Croix Falls (Fago, 1986; Hatch et al., 2003) are abrupt distributional limits for both flathead catfish and pistolgrip. The distributional limit of pistolgrip and flathead catfish in the Chippewa, Black and Wisconsin rivers mirrors the Ironton Escarpment (Schultz, 2004) suggesting this feature is also an impediment to upstream dispersal of flathead catfish.

The pathway of entry into the Lake Michigan drainage for flathead catfish, and therefore pistolgrip, is likely a connection between the Fox and Wisconsin rivers (Becker, 1983). Natural connections between these rivers likely occurred during the late Wisconsin stage of glaciations, and subsequently during high water events when the Wisconsin River occasionally flowed across the divide at Portage, Wisconsin, into the Fox River (van der Schalie, 1939). Alternatively, the canal at Portage, constructed in the mid 1800s, could have allowed flathead catfish into the Fox River (Becker, 1983), which might explain their presence above Lake Winnebago only. We are unaware of prehistoric records for these species in this region, although pistolgrip were reported from the Fox River by Baker (1928).

The current distribution of pistolgrip in the upper Midwest illustrates this species' sensitivity to disturbance, as well as the potential for populations to recover. Although viable populations of pistolgrip remain in the lower reaches of tributary streams, it has been extirpated from the Minnesota River and nearly so in the Mississippi River. The Minnesota River is a highly impacted drainage and has a severely degraded mussel fauna (Sietman, 2007). Beyond scattered individuals, pistolgrip have not been reported from the upper Mississippi River main stem in several decades (Finke, 1965; Fuller, 1980; Thiel, 1981; Duncan and Thiel, 1983; Havlik, 1983; Hornbach et al., 1992; Hart et al., 2002; Sietman et al., 2004), and we know of no significant population occurring in the Mississippi River downstream to St. Louis, Missouri. Fuller (1980) reported the species was nearly extirpated from the upper Mississippi River. Below Minneapolis-St. Paul, possibly as far downstream as Red Wing, Minnesota, it is doubtful that any mussels survived late 19th early 20th century pollution (Fuller, 1980; Scarpino, 1985; Johnson and Aasen, 1989; Fremling, 2005). Therefore, our recent finding of recruitment in the Mississippi River above its confluence with the St. Croix River represents one of a few examples of mussel recovery following extirpation (Sietman et al., 2001) and is particularly promising given the pistolgrip's apparent sensitivity to disturbance. This population is presumably being recruited from the St. Croix, which has as relatively substantial population, and likely reflects improvements in water quality in the Mississippi River associated with the Clean Water Act (Johnson and Aasen, 1989).

SUMMARY

A comprehensive conservation strategy benefits from a variety of information. Pistolgrip appear well adapted, although narrowly, to some catfishes as glochidia hosts. Since pistolgrip appears particularly well adapted to using flathead catfish as a host, a top carnivore harvested throughout its range, conservation efforts should include monitoring host populations. Propagating pistolgrip is initiated easily as one searches for brooding animals with overt display behavior but planning is required when considering the species' relatively short brooding period and the susceptibility of flathead catfish to lethal glochidia exposure. Pistolgrip distribution is limited in Minnesota and Wisconsin but we identified populations that could be used to establish new populations once suitable habitat has been identified. Addressing pistolgrip life history needs and expanding habitat should help de-list this species in several states over time.

Acknowledgments.--We thank D. Allen, R. Benjamin, M. Berg, R. Bright, A. Crownhart, T. Deneka, J. DeVore, B. Dickinson, M. Endris, M. Farr, D. Graf, T. Griffith, R. Hart, M. Havlik, D. Helms, V. Kanodia, D. Kelner, R. Kenyon, B. Knudsen, M. Kohn, C. Lee, M. Marzec, C. Nelson, B. O'Gorman, J. Sieracki, A. Stoneman, C. Sullivan, M. Tenpas, N. Ward, K. Yngve, biology teachers and students at Grantsburg, Amery, Breck and Webster high schools and MN and WI DNRs for assisting with research and manuscript preparation. Financial support was provided by UMN Undergraduate Research Opportunities Program, Macalester College, MN Nongame Wildlife Tax Checkoff and the Reinvest in MN Program through the MN DNR's Natural Heritage and Nongame Research Program, the MN Legislature, ML 1997 Chapter 216, Section 15, Subdivision 15b as recommended by the Legislative Commission on MN Resources (LCMR) from the MN Environmental and Natural Resources Trust Fund (ENRTF) and 1999 Minnesota Laws, Chapter 231, Section 16, Subdivision 15 (a), as recommended by the LCMR from the MN ENRTF, St. Croix National Scenic Riverway, National Park Service, Federal Wildlife Conservation and Restoration Program Authorized by the Commerce, Justice and State Appropriations Act of 2001, Title IX, Public Law 106-553 and the U.S.F.W.S. who provided funds through MN's State Wildlife Grants Program, WI DNR Bureau of Endangered Resources, UMN Biological Sciences Summer Internship Program, Breck High School Summer Research Program, UMN, Department of Fisheries, Wildlife and Conservation Biology and American Fisheries Society Hutton Scholarship.

SUBMITTED 3 APRIL 2009

ACCEPTED 14 SEPTEMBER 2010

LITERATURE CITED

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MARK C. HOVE (1)

Biology Department, Macalester College, 1600 Grand Avenue, Saint Paul, Minnesota 55105 and Department of Fisheries, Wildlife and Conservation Biology, University of Minnesota, !980 Folwell Avenue, Saint Paul 55108

BERNARD E. SIETMAN AND JOSH E. BAKELAAR

Minnesota Department of Natural Resources, Division of Ecological Resources, 500 Lafayette Road, Saint Paul 55155

JENNIFER A. BURY

Department of Fisheries, Wildlife, and Conservation Biology, University of Minnesota, 1980 Folwell Avenue, Saint Paul 55108

DAVID J. HEATH

Wisconsin Department of Natural Resources, 3550 Mormon Coulee Road, La Crosse 54601

VANESSA E. PEPI AND JENNIFER E. KURTH

Department of Fisheries, Wildlife, and Conservation Biology, University of Minnesota, 1980 Folwell Avenue, Saint Paul 55108

J. MIKE DAVIS

Minnesota Department of Natural Resources, Division of Ecological Resources, 1801 South Oak Street, Lake City 55041

DANIEL J. HORNBACH

Biology Department, Macalester College, 1600 Grand Avenue, Saint Paul, Minnesota 55105

AND

ANNE R. KAPUSCINSKI

Environmental Studies Program, Dartmouth College, 6182 Steele Hall, Hanover, New Hampshire 03755

(1) Corresponding author: e-mail: mark_hove@umn.edu
TABLE 1.--Fish species that facilitated pistolgrip glochidia
metamorphosis or glochidia growth

                                               Juvenile or
                                                glochidia      No.
                           No.         No.      recovery    juveniles
Species          Trial  inoculated  survivors  period (d)   recovered

Semotilus          1         6          6         21-23         0 *
  atromaculatus    2         6          6          1-5          0
                   3         4          4          1-5          0
                   4         1          1          1-5          0

Ameiurus melas     1         7          7         23-25         0 *
                   2         3          3         18-21         0
                   3         9          9          8-12         0
                   4        11         11         13-16         0 *
                   5        14         14         39-40         0 *
                   6         5          4          1-4          0

A. natalis         1         3          1         15-22        11
                   2         4          4         23-25         0 *
                   3         3          3         18-20         0
                   4         9          9         24-28         0 *

A. nebulosus       1         8          0         14-17         0 *
                   2         7          7         25-36         6 *
                   3         1          0          7-10         0
                   4         1          1         24-41       133 *

Pylodictis         1         4          4         27-63      4135 *
  olivaris         2         3          2         28-45       768 *
                   3         3          2         23-44      3964 *

* Glochidia growth observed

Fish species (number of trials, mean number of fish per trial, range
of days to rejection) that did not facilitate glochidia
metamorphosis: Acipenser fulvescens (1, 6, 1-5), Scaphirhynchus albus
(2, 1, 1-5), Lepisosteus osseus (1, 3, 1-5), Lepisosteus platostomus
(2, 3, 1-5), Amia calva (2, 4, 2-9), Carassius auratus (1, 2, 1-5),
Cyprinella lutrensis (1, 8, 1-5), Cyprinella spiloptera (1, 11, 1-
5), Cyprinus carpio (2, 4, 2-5), Hybognathus nuchalis (1, 1, 1-5),
Luxilus cornutus (3, 3, 1-5), Nocomis biguttatus (2, 6, 1-5),
Notropis atherinoides (1, 8, 1-5), N. dorsalis (1, 4, 1-5), N.
volucellus (2, 5, 1-5), Phoxinus eos (1, 10, 1-4), P. erythrogaster
(1, 12, 1-5), Pimephales notatus (2, 6, 1-5), P. promelas (3, 8, 1-
5), P. vigilax (1, 2, 1-5), Rhinichthys atratulus (3, 5, 1-5), R.
cataractae (2, 10, 5-11), Carpiodes cyprinus (1, 3, 5-9), Catostomus
commersonii (3, 7, 2-5), Hypentelium nigricans (1, 1, 1-5), Moxostoma
macrolepidotum (1, 2, 1-5), Ictalurus furcatus (2, 5, 1-5), L.
punctatus (5, 6, 4-8), Noturus exilis (2, 2, 5-24), N. flavos (4, 2,
1-5), N. gyrinus (3, 2, 5-12), Esox lucios (1, 10, 7-10), Umbra limi
(1, 14, 5-8), Percopsis omiscomaycus (1, 2, 1-4), Lota lota (2, 4, 4-
8), Labidesthes sicculus (1, 1, 1-5), Fundulus diaphanus (1, 14, 2-
5), Culaea inconstans (3, 7, 4-11), Cottus bairdii (1, 3, 8-12),
Morone chrysops (1, 1, 1-4), Ambloplites rupestris (4, 3, 1-5),
Lepomis cyanellus (1, 2, 1-5), L. gibbosus (2, 5, 1-5), L. humilis
(1, 9, 1-5), L. macrochirus (2, 4, 1-5), Micropterus dolomieu (3, 4,
1-5), M. salmoides (3, 5, 1-5), Pomoxis nigromaculatus (3, 2, 1-5),
Ftheostoma caeruleum (1, 9, 1-5), E. exile (1, 8, 4-7), E. flabellare
(2, 7, 4-7), E. nigrum (4, 8, 1-5), E. zonale (1, 2, 1-5), Perca
flavescens (2, 8, 4-8), Percina caprodes (3, 7, 4-7), P. maculata (4,
3, 4-7), P. phoxocephala (2, 3, 1-5), Sander canadensis (2, 5, 1-5),
S. vitreus (2, 6, 8-12) and Aplodinotus grunniens (1, 1, 1-5)
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Publication:The American Midland Naturalist
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Date:Apr 1, 2011
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