Filter-feeding in the Freshwater Prosobranch Snail Bithynia tentaculata: Bioenergetic Partitioning of Ingested Carbon and Nitrogen

Filter-feeding has been studied in specimens of Bithynia tentaculata from a population in Oneida Lake, New York. Chlorella vulgaris cultures provided filterable material for studies of feeding and assimilation in this freshwater prosobranch snail. Concomitant analyses of assimilation, respiration and excretion allowed assessment of bioenergetic partitioning. Weight-specific hourly rates for ingestion, egestion, respiration and nitrogen excretion were measured. These parameters provided data for calculation of assimilation and nonrespired assimilation rates. Such weight-specific rates were used to construct bioenergetic profiles for carbon and nitrogen allocation in postbreeding snails. Filtering rates were also measured at three different concentrations of filterable material. Results from filtering rate experiments show that individuals of Bithynia can increase weight-specific clearance rates as concentration of filterable material is decreased. The present study provides data which complement studies of grazing in specimens of Bithynia tentaculata. Relative to grazing, filter-feeding allows individuals of this species to accrue a higher net gain of nitrogen (protein) and carbon per respired costs. The data suggest an explanation for interpopulation differences in feeding modes. Further, substantive data have been gathered which may help explain the displacement of pleurocerids by Bithynia in some upstate New York habitats. INTRODUCTION The, literature presents mixed views about the feeding habits of populations of the freshwater prosobranch snail Bithynia tentaculata (see Fretter and Graham, 1962, and Mattice, 1970, for reviews). The ctenidium of Bithynia is large and the lamellae are triangular. Each lamella has a broad base and the tip overhangs a ciliated groove traversing the floor of the mantle cavity. Grazing undoubtedly occurs and the frequency of filter-feeding may depend on the abundance of other foods and the nature of the suspended particles (Fretter and Graham, 1962). The importance of filter-feeding in specimens of Bithynia tentaculata has been discussed by a number of authors. Mattice (1970) and Fretter and Graham (1962) discuss the literature describing filter-feeding in B. tentaculata. Early workers merely demonstrated that filtering of material from the pallial water currents occurred and that this filtered material was concentrated in the ciliated groove which traverses the floor of the mantle cavity. At least some of this material was observed to be subsequently ingested (Lilly, 1953; Starmuhlner, 1952). Various other viewpoints have been expressed about importance of filter-feeding in individuals of B. tentaculata. These range from the contention that filter-feeding provides only a supplement to grazing (Fretter and Graham, 1962; Harman, 1968a, b) to the idea that in some populations all or most food is obtained by the filtering mode (Russell-Hunter, 1957; cf., Mattice, 1970). This study has examined filter-feeding in specimens of Bithynia tentaculata from a population in Oneida Lake (New York, U.S.A.). The primary focus of this work is to assess ingestion rates and subsequent bioenergetic partitioning of ingested protein and carbon. Information from feeding and assimilation studies was combined with respiratory and excretory measurements on the same individual. Suspensions of 'Present address: Department of Biology, Kenyon College, Gambier, Ohio 43022.