DENSITY-DEPENDENT INFLUENCES ON FEEDING AND METABOLISM IN A FRESH-WATER SNAIL

We used laboratory experiments to assess the degree of, and the underlying mechanism for, density dependence in the grazing rate of the pulmonate gastroped Physella virgata. Both fecal pellet production and uptake and incorporation of C-14 radioisotopes from labeled periphyton were used as indices of grazing rates Pronounced density-dependent reductions in grazing rate were observed, especially at densities above 4 snails/-25 cm(2) periphyton grazing area. Radioisotope experiments also indicated that proportions of ingested C-14 periphyton retained in snail tissue and respired as carbon dioxide increased at higher densities, suggesting that both assimilation efficiency and respiratory costs increase at higher densities. Constant replacement of water in aquaria did not remove density-dependent effects on grazing, suggesting that a dissolved metabolite is not responsible. Experiments where tiles were ''pre-conditioned'' with snails grazing at several densities actually stimulated grazing in subsequently added snails, suggesting that substrate-borne cues are also not responsible for density-dependent reductions in grazing rate. Behavioral interference (in the form of shell-shaking after contacts with other snails) did, however, increase at higher densities, and may be partially reponsible for depressed grazing rates.