Secondary Production of Two Lotic Snails (Pleuroceridae:Elimia)

Abstract

We investigated secondary production by two stream gastropods, Elimia clara and E. cahawbensis (Pleuroceridae). Monthly production was estimated for each of five size classes for both species using the instantaneous growth method. Both laboratory and field growth rates were estimated monthly for each size class from individuals kept in growth chambers. Growth rates in the field and in the laboratory did not differ significantly (p > 0.05). Laboratory growth rates were used to construct growth curves, which indicated that individuals of E. clara and E. cahawbensis may live for 11 and 10 yr, respectively. Growth and production varied widely from month to month for each species while biomass remained relatively constant. These snails have relatively high standing stocks when compared with most lotic invertebrates: E. clara with 5.4 g AFDM/m² and E. cahawbensis with 2.3 g AFDM/m². Because of slow growth, annual production was low when compared with other estimates for snails: 0.8 and 1.6 g AFDM/m² for E. clara (field and laboratory estimates, respectively) and 0.6 and 0.7 g AFDM/m² for E. cahawbensis. Annual turnover of biomass was slow (approximately 0.3 for both species) owing to the long life cycle.