Life cycle and production of the exotic snail Physa venustula (Pulmonata :Physidae) in the Naposta Grande stream, Southern Pampas, Argentina

The life cycle of Physa venustula Gould, 1848 (Physidae) in Naposta Grande stream is semelparous and subannual, with two or three poorly defined cohor ts per year. Oviposition and recruitment are almost continuous. Juvenile mo rtality is high up to the length of 3.5 mm and decreases in subadults and a dults. Density fluctuations are tied to hydrodynamic conditions and drift e vents rather than to reproductive activity. Density and biomass in vegetate d bottoms were higher than in sandy-mud ones. Annual somatic production est imated by the size-frequency method was low (0.095 g(dry weight).m(-2)) and turnover rate high (8.10 yr(-1)) when compared to those of other Physidae.