Vulnerability of Dreissena polymorpha Larvae to Predation by Great Lakes Calanoid Copepods: the Importance of the Bivalve Shell

Abstract

Dreissena polymorpha larvae were vulnerable to predation by three different species of calanoid copepods, Diaptomus sicilis, Limnocalanus macrurus, and Epischura lacustris, when presented to these copepods in bottle experiments. The degree of vulnerability was dependent upon the stage of the larva and the type of predator: trochophore larvae (without shells) were much more vulnerable than D-stage larvae (with shells). D. sicilis and L. macrurus were offered algae as alternate food, and each cleared trochophore larvae at a higher rate than algae. However, the clearance rate for D. sicilis feeding on D-stage larvae was not significantly different from zero, suggesting that this suspension-feeding omnivore-herbivore was not able to ingest D-stage larvae. Of the three species, the large cruising predator, L. macrurus, had the highest clearance rate for trochophore larvae (55.8 mL • animal−1 • d−1), but had a significantly lower clearance rate for D-stage larvae, only one eighth of that for trochophores. The smaller predator, E. lacustris, was more adept than L. macrurus or D. sicilis at preying on D-stage larvae: its clearance rate for D-stage larvae (17.9 mL • animal-1 • d−1) was about one half of its clearance rate for trochophore larvae. Since bivalve larvae, including Dreissena, and copepods co-occur in many aquatic environments, our results suggest that copepod predation may have been a selective force for production of a protective shell early in the larval development of bivalves.