A population study of epigean and subterranean Potamolithus snails from southeast Brazil (Mollusca: Gastropoda: Hydrobiidae)

Abstract

Population and reproductive biology of four Potamolithus species, two epigean (P. ribeirensis and Potamolithus sp. 1) and two subterranean (P. troglobius and Potamolithus sp. 2), from the Upper Ribeira Valley, southeast Brazil, were studied, from June 1996 to June 1997, using 1 m 2 quadrats and counting all individuals found in these quadrats. P. ribeirensis showed the highest variation in population densities (from 0 – after floods – to 467 ind.m −2 ), with average ranges from 16.0 to 284.8 ind.m −2 . The densities of the two troglobitic species (average ranges forPotamolithus sp. 2 and P. troglobius, from 17.6 to 58.4 and from 0 to 61.5 ind.m −2 , respectively), higher than those of Potamolithus sp. 1 (average from 0 to 9.3 ind.m −2 ), were high for cave organisms. P. ribeirensis and the two troglobitic species showed higher densities in warmer, wetter months, while Potamolithus sp. 1 showed higher densities in cooler, less rainy months. The troglobitic species were non-seasonal, since high percentages of mature females were observed throughout the year. In the epigean species, the reproductive periods showed no detectable pattern: in 1996 P. ribeirensis had a peak in winter and in the next year (1997) in spring and summer. Potamolithus sp. 1 had reproductive peaks in fall and winter.