Variation in the life history and abundance of three populations of Bruneau hot springsnails (Pyrgulopsis bruneauensis)

Bruneau hot springsnail density size class structure, recruitment, and mort ality were measured monthly over approximately 2 years and compared to envi ronmental variables at 3 hot spring sites in southwestern Idaho. Food resou rces (attached algae) and water chemistry were similar among sites, but tem perature, population density, and size structure differed significantly: De nsity was highest at a warm, fairly constant temperature site. A cooler hig hly variable temperature site and a site where temperatures frequently appr oached or exceeded thermal maxima tolerance limits had lower densities. Siz e class structure varied seasonally and distinctly among sites. with recrui tment occurring year-round at temperatures <36<degrees>C. Mortality affecte d different size classes at different sites, with smaller snails incurring greatest mortality. at site 3 (highest temperature variability). Growth rat es were positively correlated with temperatures up to 36 degreesC. Mean sna il size differed among sites and also appeared to be related to temperature , with die variable ;temperature sites having larger snails. Although these snail populations are found in a range of water temperatures, they appear best adapted to springs with mean temperatures between 32 degrees and 33 de greesC and low thermal variance.