PREDATION ON CILIATES BY THE SUSPENSION-FEEDING CALANOID COPEPOD ACANTHODIAPTOMUS-DENTICORNIS

The effects of prey size, hunger, and algal presence on the predation of the suspension-feeding calanoid copepod Acanthodiaptomus denticornis on the ciliates Tetrahymena corlisii (32 mum mean length), Paramecium aurelia (114 mum), P. caudatum (124 mum), Loxodes sp. (greater-than-or-equal-to 130 mum), and Stentor coeruleus (greater-than-or-equal-to 280 mum) and the flagellate Astasia longa (14 mum) were evaluated in laboratory experiments. Adult Acanthodiaptomus consumed all species, but not in proportion to their size. At low concentrations (prey <200 mug C.L-1), mean clearance rates ranged from 80 mL.copepod-1.d-1 (for Paramecium) to 13 mL.copepod-1.d-1 (for Loxodes). Clearance rates declined with increasing concentration of the same prey, as expected. Prey capture and ingestion was determined by cell width and prey escape reactivity. Hunger significantly increased predation rates at high prey levels. The presence of algae enhanced ciliate predation by hungry copepods at low prey levels, but had no other significant effects. Protozoan predation by Acanthodiaptomus generally exceeds its feeding on algae and nanoplankton. Results suggest that ciliates can be a significant part of the diet of some suspension-feeding freshwater copepods in situ, confirming the importance of ciliates as mediators of energy transfer from the microbial loop to higher trophic levels.