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

We used laboratory experiments to assess the degree of, and the underl ying mechanism for, density dependence in the grazing rate of the pulm onate gastroped Physella virgata. Both fecal pellet production and upt ake and incorporation of C-14 radioisotopes from labeled periphyton we re used as indices of grazing rates Pronounced density-dependent reduc tions in grazing rate were observed, especially at densities above 4 s nails/-25 cm(2) periphyton grazing area. Radioisotope experiments also indicated that proportions of ingested C-14 periphyton retained in sn ail tissue and respired as carbon dioxide increased at higher densitie s, 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 wer e ''pre-conditioned'' with snails grazing at several densities actuall y stimulated grazing in subsequently added snails, suggesting that sub strate-borne cues are also not responsible for density-dependent reduc tions in grazing rate. Behavioral interference (in the form of shell-s haking after contacts with other snails) did, however, increase at hig her densities, and may be partially reponsible for depressed grazing r ates.