Relationship between stream velocity & depth and snail size distribution & density of the balcones elimia, elimia comalensis (pilsbry, 1890) (gastropoda: pleuroceridae) in comal springs, texas

-Pleurocerid snails are important components of aquatic ecosystems and the majority of species are threatened or endangered. This study describes aspects of the life-history of Elimia comalensis, specifically population density in relationship to water velocity and depth, and seasonal change. Also examined are spatial segregation of different E. comalensis age groups and the relationship of snail size (proxy for age) to flow, depth, and seasonal change. The study was carried out in the lotic portion of spring run 3, Comal Springs, New Braunfels, Texas, by quadrat sampling at 10 m intervals from the spring head to Landa Lake in Fall, Winter, and Spring 2005-2006. The length of snails was strongly influenced by physical characteristics of the stream with stream depth, water velocity, and distance from the spring head accounting for -43% of the individual variability for shell length (p <0.001). A significant relationship between snail size, stream depth and water velocity was found, with spatial segregation of juvenile snails into the deepest, fastest moving water. Snail population densities did not change across sampling seasons (p = 0.285). Pleuroceridae is a highly endangered family of freshwater gastropods with >50% of the species endangered or extinct (Brown et al. 2008). Threats to pleurocerids include water extraction, impoundments, point-source pollution, and invasive species (Brown et al. 2008). Pleurocerids are also significant functional components of aquatic ecosystems affecting periphyton community structure, abundance and diversity of other macroinvertebrate taxa, and populations of predators (Brown et al. 2008). As such, pleurocerids represent a threatened and important component of stream 184 THE TEXAS JOURNAL OF SCIENCE-VOL. 63, NO. 3,2011 ecosystems, but specific life history information for many species is lacking. Elimia is the most diverse genus of pleurocerid snails with over 100 species ranging from Texas northward into the Midwest and northeastern United States and Canada (Brown et al. 2008). Elimia species are periphyton grazers and detritus shredders (Mulholland et al. 1985) that reproduce perennially (Dillon 2000) and may show sexual dimorphism (Dillon 1984a). Elimia species inhabit both lakes and rivers with species preferring lotic habitats. Published life history studies of Elimia have been limited to a few species (Dillon & Davis 1980; Dillon 1984b; Huryn et al. 1994; Huryn et al. 1995; Huryn & Denny 1997), and only one (Tolley Jordan & Owen 2008) includes E. comalensis, the subject of this study. The Balcones Elimia, Elimia comalensis (Pilsbry 1890), is the only pleurocerid known from Texas and one of the few to occur west of the Mississippi River. This study describes some aspects of the life-history of E. comalensis in Comal Springs, New Braunfels, Texas. Specifically, this study examines E. comalensis population density in relationship to water velocity, water depth, and seasonal change. Previous work in other Elimia species, including E. comalensis, suggested significant relationships between water velocity and depth on population density (Johnson & Brown 1997; Huryn & Denny 1997; Tolley-Jordan & Owen 2008). In addition to those factors, this study examined spatial segregation of different age groups of E. comalensis and the relationship of snail size (proxy for age) to water velocity, depth, and seasonal change. Recent molecular work on E. comalensis revealed this species is native to the Comal River drainage, but widely distributed in West Texas springs through human-mediated introductions in the early 20th century (Hayes et al. 2007). Hayes et al. (2007) observed significant differences in morphology among populations occupying different drainage basins, but with no apparent population genetic