Diversity and distribution of freshwater gastropods in the Bayou Bartholomew drainage, Arkansas, U.S.A. Russell L. Minton 1 , John D. White 1 , David M. Hayes 1, *, M. Sean Chenoweth 2 , and Anna M. Hill 1 1 Department of Biology, University of Louisiana at Monroe, 700 University Avenue, Monroe, Louisiana 71209-0520, U.S.A., minton@ulm.edu 2 Department of Geosciences, University of Louisiana at Monroe, 700 University Avenue, Monroe, Louisiana 71209-0550, U.S.A. Abstract: Bayou Bartholomew is a low-gradient river system that drains much of southeastern Arkansas and northeastern Louisiana, U.S.A. As one of the few southeastern streams remaining un-impounded, the Arkansas reach of the bayou harbors a rich freshwater molluscan fauna. Collecting efforts have historically focused on documenting freshwater mussel and fish diversity, and there was no prior survey focusing on freshwater gastropods. This survey of the drainage yielded 13 gastropod species representing three orders and seven genera. Pulmonates were most abundant in low-order reaches of the drainage, while gill-breathing snails dominated higher-order reaches. Co-occurrence analyses indicated that pulmonates occurred significantly more often with other pulmonates than they did with gill- breathers; this trend was also observed in gill-breathers. Both stream order and predominant substrate influenced species richness and abundance. Our findings were consistent with other published studies on freshwater snail distribution but may be confounded by drought conditions experienced during the survey. Key words: snails, ecology, drought, co-occurrence The southern United States harbors one of the most diverse freshwater mollusc assemblages in the world (Wil- liams et al. 1993, Neves et al. 1997). Of the over 800 species of freshwater gastropods in North America (Lysne et al. 2008), up to 40 are reported from Arkansas (Hayes 2008, NatureServe 2008), depending on source. Like many fresh- water taxa, snails are experiencing declines due to habitat degradation, pollution, and anthropogenic effects (Lydeard et al. 2004, Perez and Minton 2008). Few stream systems in the United States with abundant snail resources have re- mained unaltered or unexploited. Those rare, minimally im- pacted systems offer a glimpse into the natural condition that existed prior to widespread impoundment, channeliza- tion, and other human influences. Bayou Bartholomew originates in loess hills west of Pine Bluff, Arkansas and flows 457 km through Jefferson, Lin- coln, Drew, Desha, and Ashley counties in Arkansas and Morehouse Parish in Louisiana before its confluence with the Ouachita River near Sterlington, Louisiana. It is the only non-channelized river in southeast Arkansas and northeast Louisiana. Bayou Bartholomew is a low-gradient, Yazoo- type watershed occupying approx. 20% of the Ouachita River basin and draining over 400,000 ha in southeast Ar- kansas and northeast Louisiana (Broom 1973), including parts of the Arkansas River floodplain (Saucier 1994). Up- stream reaches of the drainage are subject to frequent drying and modification due to low water levels; single reaches can become unconnected chains of pools during summer months. Most of Bayou Bartholomew occurs within the Mis- sissippi Alluvial Basin ecoregion characterized by fine tex- tured and fertile alluvial soils well suited to agricultural de- velopment (Alley 2005). Agricultural fields, row crops, and pastures dominate land-use in Bayou Bartholomew’s water- shed. There is a narrow riparian zone along the river, in most cases less than 50 m wide, dominated by bottomland hardwood species such as water tupelo (Nyssa aquatica), bald cypress (Taxodium distichum), and maples (Acer spp.). Erosion, sedimentation, agriculture and urban nutrient in- puts, and irrigation water withdrawals have been the main stressors of the Bayou Bartholomew stream ecosystem for many years (Alley 2005). Most research conducted on Bayou Bartholomew has focused on fishes (Thomas 1976, Hutchins 1988, Pezold et al. 2002) and mussels (George and Vidrine 1993, Pezold et al. 2002, Brooks et al. 2005). These surveys found that Bayou Bartholomew harbors a diverse mussel assemblage in both Arkansas and Louisiana. However, exact figures for the number of gastropod taxa in Bayou Bartholomew are lack- ing. The objectives of this project were (1) to assess the current status, diversity, and distribution of snail species in the Arkansas portion of Bayou Bartholomew and (2) to pro- vide baseline data for monitoring these species in the future. We additionally explored potential population structure in * Current Address: Department of Biological Sciences, Arkansas State University, P.O. Box 599, State University, Arkansas 72467- 0599, U.S.A. Amer. Malac. Bull. 26: 171-177 (2008) 171