Spatiotemporal Variation in Elimia comalensis (Gastropoda: Pleuroceridae) Density and Interspecific Associations with Exotic Thiarid Snails in the Upper San Marcos River, Texas

ABSTRACT Freshwater gastropods in the family Pleuroceridae are important components of North American freshwater ecosystems that can greatly influence ecological processes and biotic communities. Elimia is the most taxa rich among pleurocerid genera. Elimia comalensis Pilsbry, 1890, is the only pleurocerid species that occurs in Texas and is currently known to occupy artesian springs and spring-fed river systems associated with the Edwards Plateau region. Published literature on E. comalensis is limited, and little information is currently present regarding explicit patterns of distribution and abundance within most systems currently occupied. We examined temporal and spatial variation of E. comalensis densities in the upper San Marcos River from 2013 to 2016, as well as interspecific affinities between E. comalensis and two invasive thiarid snails. We observed significant annual differences in E. comalensis densities, with greater mean density occurring under low flow conditions in 2014. We also observed significant differences in densities among sites, with greater density occurring at sites with larger and more complex substratum particles. Elimia comalensis exhibited a strong positive relationship with the thiarid snail Tarebia granifera, although data suggest that interspecific competition is limited as these species co-occur at high densities. As anthropogenic impacts continue to impact Edwards Plateau spring systems, further research on the distribution and population dynamics of ecologically important pleurocerid snails, such as E. comalensis, is needed to aid in the conservation of these unique ecosystems.