Predation on rotifers by the suspension-feeding Calanoid copepod Diaptomus pallidus

Predation on rotifers by the small suspension-feeding Calanoid copepod Diaptomus pallidus was examined in order to quantify the effects of prey density, prey type, and the presence of algal food resources on ingestion rates, and to determine whether ingested rotifer biomass could be utilized to enhance the survival and reproduction of the copepods. Clearance and ingestion rates of D. pallidus on rotifers were 5.5-6.2 times greater than on algae presented at the same concentration and similar to the maximum rates previously reported for more carnivorous cyclopoid copepods preying on rotifers. The survival and reproduction of D. palhdus were substantially enhanced by the addition of rotifers to a threshold algal diet. When presented with a natural assemblage of plankton, D. pallidus preferentially ingested certain rotifer species over others. Predation on rotifers by such diaptomids may form an important trophic link in freshwater planktonic food webs. The importance of rotifers in freshwater planktonic ecosystems has become increasingly apparent over the past several years. Although cladocerans and copepods are the dominant zooplankters in many limnetic environments (Porter 1977), at times rotifers may account for most of the community grazing (Bogdan and Gilbert 1982) and contribute substantially to the biomass and productivity (Makarewicz and Likens 1979) of zooplankton communities. Many species of invertebrate predators prey heavily on rotifers (Williamson 1983) and may subsequently increase the energy available to vertebrate predators as they package these small prey into larger particles (the invertebrate predators themselves) which are more available to the vertebrates (Confer and Blades 1975). The greater size and visibility of the invertebrate predators as food for the vertebrates more than compensates for the energy lost due to the presence of an additional trophic level. The most widespread and abundant invertebrate predators of planktonic rotifers are the copepods (Williamson 1983). Previous research on cyclopoid copepods has demonstrated the intense and selective nature of their predation on natural assemblages of rotifers (Brand1 and Fernando 1979; Williamson 1984). Less information is available on consumption of rotifers by Calanoid copepods, although several in1 Supported by NSF grant BSR 8219860 to C.E.W. vestigators have reported the remains of rotifers in the guts of calanoids (Anderson 1967; Grygierek 197 1; Maly and Maly 1974; Hairston 1979). Recent advances in our understanding of marine Calanoid feeding behavior have demonstrated that large particles are captured actively by coordinated movements of the mouthparts in response to chemical or mechanical signals given off by the particles in the current field near the copepod (Alcaraz et al. 1980; Koehl and Strickler 198 1; Paffenhijfer et al. 1982). This suggests that even some smaller (N 1 mm) freshwater diaptomids previously thought to be totally herbivorous might prey on small, weakswimming animal prey such as rotifers and thus enhance the transfer of energy between trophic levels. The omnivorous nature of many species of marine calanoids has been well documented and supports this hypothesis (Mullin 1966; Robertson and Frost 1977; Landry 198 1; Paffenhijfer and Knowles 1980). We examine here the potential importance of predation by the freshwater suspension-feeding Calanoid Diaptomus pallidus Herrick on several species of sympatric rotifers. The specific questions which we ask are: Does D. pallidus ingest rotifers, and if so, is this ingestion incidental or substantial? Can D. pallidus utilize ingested rotifer biomass to enhance its own survival and reproduction? And is predation by D. pallidus on different rotifer species uniform or selective? We thank R. Stemberger for advice on