A key to the common seed shrimp (Crustacea: Ostracoda) of the playa lakes of the Llano Estacado region of northwestern Texas.
Faunal diversity of several playas of eastern New Mexico and western Texas has been studied by Sublette & Sublette (1967) and MacKay et al. (1990). Playas are typically dominated by crustaceans during the early stages of the recharge cycle (one to four weeks) and by insects during the later stages. The crustacean fauna include members of the Branchiopoda (clam shrimp, fairy shrimp and tadpole shrimp), Cladocera, Copepoda and Ostracoda, all of which have eggs or larval stages that are resistant to desiccation and extreme temperatures. Because of the temporary and unpredictable nature of the playa wet phase, crustacean inhabitants can be considered as fugitive species (Hutchinson 1951). Eggs must hatch, larvae develop, and adults reproduce in a few days or weeks before the playa dries.
The ostracod component in playas often differ significantly (Horne 1996). Because of their infrequent encounter, small size and overall similarity in appearance, they are often unreported or remain unidentified and listed only as "ostracods" or "ostracod species" (Merickel & Wangberg 1981; MacKay et al. 1990). However, ostracods often represent an important biotic component of playas of this region of northwest Texas. Most freshwater species are cosmopolitan in distribution (Tressler 1947; 1954; 1959; Ferguson 1967a; 1967b; Havel et al. 1990). Their resistant stages allow for easy dispersal (Proctor 1964; Proctor & Malone 1965; Horne 1966; 1993).
Descriptions of Texas ostracods are available in references by Ferguson (1967a; 1967b) and Tressler (1954). Delorme's numerous papers (1967; 1969; 1970a; 1970b; 1970c; 1970d) on the freshwater ostracods of Canada also are useful when working with this faunal group.
Although research emphasis today has tended to shift toward molecular studies, morphological characters are the link between extant and fossil species (Gutentag & Benson 1962; Wheeler et al. 2004). Morphological characters are those commonly used in field studies and in laboratory identifications, and also are the ones used in this paper.
This study provides playa researchers of the Llano Estacado with a single listing (Table 1) of ostracod species likely encountered as well as a taxonomic key based primarily upon the external characteristics of the carapace of mature specimens. Ten species representing eight genera in five families of the class Ostracoda are given in the following key.
The playa lakes included in this study are located in a 43 county geographical area bordered on the north by the Canadian River and on the east and west by escarpments (Osterkamp & Wood 1987). The identification key presented here is based on specimens from playas of the counties of Andrews, Crosby, Dawson, Gaines, Garza, Lubbock, Lynn, Martin and Terry (Horne 1996). Following the classification of pluvial lake basins by Reeve's (1966), all of the playas are type VI, except for the saline playas which are type IV.
USE OF THE KEY
Only a dissecting microscope with low magnification is required for ostracod identification. No dissection or examination of appendage morphology is necessary for identification to species level. Scanning electron micrographs are presented here to facilitate and confirm identification (Figure 1).
[FIGURE 1 OMITTED]
[FIGURE 1 OMITTED]
The 7th instar of Candona patzucaro (Fig. 1c) is included in couplet 4a due to the ability of this stage to undergo desiccation and therefore appear earlier than other species of ostracods following the initial filling (or recharge) of the playa habitat. Two of the larger ostracods, Megalocypris ingens and Megalocypris pseudoingens, are very similar in appearance and size, but occur in different types of playas. Megalocypris ingens occurs only in saline lakes, while M. pseudoingens inhabits freshwater playas. Delorme (1969) considered M. pseudoingens as uncommon. Although all of the species have been collected from the ephemeral playas of the Southern High Plains of Texas (Home 1993; 1996), many also occur in stock tanks, watering troughs and other permanent bodies of water. Specimens are deposited in the Museum of Natural History (USNM No. 413603).
KEY TO SPECIES
1. Lateral surface of adult carapace smooth 2 or nearly smooth (w/o heavy pits and/or grooves) 1a. Lateral surface of adult carapace highly 9 sculptured (heavily or obviously pitted with one or more grooves) 2. Length of adult carapace less than 2.0 mm 3 2a. Length of the adult carapace greater 7 than 2.0 mm 3. Length of adult carapace less than 2.0 4 but greater than 1.0 mm 3a. Length of adult carapace less than 5 1.0 mm 4. Carapace greater than 1.0 mm, but less Heterocypris antillensis than 1.7 mm ovate in sideview; venter (Fig. 1d). convex or flat right valve bears small rounded denticles. Marginal zone of the anterior, anteroventral and posteroventral regions of the right valve bear small rounded denticles 4a. Carapace white, greater than 1.0 mm, but Candona patzucaro less than 1.7 mm, ovate in sideview; (Figs. 1a, 1b, & 1c). venter convex or flat, characteristic posterior-lateral surface, faint, but distinct reticulation, female with blunt posterior aspects, male more rounded Cypridopsis vidua 5. Carapace inflated in dorsal view, tumid, (Fig. 1f). width about equal to height, subtriangular in side view, surface pitted, shell length less than 0.8mm 5a. Carapace subovate (disc-like) or 6 crescentric (hatchet-like) in side view; valves compressed in dorsal view 6. Carapace crescentric/hatchet shaped in Potamocypris smaragdina side view, surface setaceous; valve (Fig. 1i). overlap distinctive, shell length less than 0.75 mm 6a. Carapace subovate or disc-like in side Physocypria globula view, surface not setaceous, shell (Fig. 1e). length less than 0.6 mm 7. Length of shell 3.3-3.5 mm, tubular- Megalocypris gnathostoma shaped, subtrapezoidal anterior and (Fig. 1j). posteriorly rounded, setae along margins except dorsally 7a. Length of carapace 3.0-4.0 mm, 8 freshwater or saline 8. Shell subtrapezoidal; anterior end Megalocypris pseudoingens rounded, posterior end slightly rounded (Fig. 11). to truncated, uncommon, found only in freshwater 8a. Shell subtrapezoidal; both anterior and Megalocypris ingens posterior ends rounded, not truncated; (Fig. 1k). restricted to saline playas 9. Lateral shell surface pitted with Pelocypris distinct surface reticulations, two (=Pseudoillocypris) median sulci, may contain lateral umbo- tuberculatum (Fig. 1m). shaped protuberances, oblong 1.4 to 1.8 mm, calcareous distinct tubercles, spines along anterior and posterior margins, prominent on the anterodorsal margins of both valves 9a. Lateral carapace surface reticulate, Limnocythere sanctipatrici valves <0.3 mm, bears three to four (Figs. 1g & 1h). lateral protuberances, contains two median sulci, may or may not be tuberculate
Appreciation is expressed to Joe Koke, Texas State University, San Marcos, Texas, for assisting in preparing scanning electron micrographs, Vernon W. Proctor, Texas Tech University, Lubbock, Texas, for his many discussions on playa lakes, and Richard Forester, USGS, Denver, Colorado, for confirmation and identification of ostracods to species.
Delorme, L. D. 1967. Field keys and methods of collecting freshwater ostracods in Canada. Can. J. Zool., 45:1275-1281.
Delorme, L. D. 1969. On the identity of ostracode genera Cypriconcha and Megalocypris. Can. J. Zool., 47:271-281.
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FRH at: FH01@txstate.edu
Francis R. Horne and Ned E. Strenth
Department of Biology, Texas State University
San Marcos, Texas 78666 and
Department of Biology, Angelo State University
San Angelo, Texas 76909
Table 1. Listing of species of ostracods known to occur in the playa lakes of the Llano Estacado region of northwest Texas. FAMILY SPECIES Cyprididae Subfamily Cypridinae Heterocypris antillensis Megalocypris ingens Megalocypris pseudoingens Megalocypris gnathostoma Subfamily Cypridopsinae Cypridopsis vidua Potamocypris smaragdina Cyclocyprididae Physocypria globula Candonidae Candona patzucaro Llyocoprididae Pelocypris tuberculatum Entocytheridae Limnocythere sanctipatrici
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|Author:||Horne, Francis R.; Strenth, Ned E.|
|Publication:||The Texas Journal of Science|
|Date:||Nov 1, 2007|
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