ABUNDANCE AND MICRODISTRIBUTION OF FRESH-WATER GASTROPODS IN 3 STREAMS OF MOOREA, FRENCH-POLYNESIA

The microdistribution and abundance of freshwater gastropods were stud ied in lower reaches of three streams on Moorea, French Polynesia, dur ing austral spring (AS) and austral fall (AF) 1994-1996 Seven species were examined: Neritina canalis Lamarck, Neritina turrita Gmelin, Clit hon spinosa Budgin, Septaria porcellana Linnaeus, Neritina auriculata Lamarck, Melanoides tuberculata Muller, and Thiara granifera Lamarck; the first five species are in the Neritidae, the latter two in the Thi aridae. Most gastropod species had higher densities during AF 1996 tha n AS 1995 (p = 0.0001) with higher densities being found in riffle tha n in pool habitats (p = 0.0001). No relationship between substrate typ e and either species presence-abundance or shell size was observed. A stepwise multiple regression indicated that velocity, depth, and subst rate roughness were determinants of gastropod abundance for some speci es within these streams. A distinct microhabitat preference for the bo ttoms of rocks was observed for C. spinosa and N. canalis The limited range of N. auriculata's occurrence within streams results from its sa linity tolerance and response to hydraulic stress.