Body size and food size in freshwater zooplankton

We used double-label liquid scintillation techniques to measure the efficiencies with which eight different-sized zooplankton species ingested four cell types relative to a standard cell type (Chlamydomonas). Efficiency ratios (ERs: clearance rate on cell type X / clearance rate on Chlamydomonas) on the three ultraplankton (<5 mum in diameter) cells (a coccoid bacterium and the algae Synechococcus and Nannochloris) varied greatly among zooplankton species but were not correlated with zooplankton body length. Variation in ERs on a much larger (17 x 14 mum) algal cell (Cryptomonas) was only partly explained by zooplankton body length. The eight zooplankton species were classified into three functional groups: (i) species having moderate to high ERs on all ultraplankton (0.4 < ER < 1.6) and ERs on Cryptomonas proportional to their body lengths (Conochilus, Diaphanosoma, and probably Keratella cochlearis and Ceriodaphnia); (ii) species having extremely low ERs on bacteria (mean ER < 0.05), higher but still low ERs on ultraphytoplankton (ER generally < 0.4), and ERs on Cryptomonas proportional to their body lengths (Bosmina, Diaptomus copepodites and adults); (iii) species having extremely low ERs on all ultraplankton (mean ER < 0.05) and ERs on Cryptomonas much higher than expected given their body lengths (Keratella crassa, Polyarthra, and Diaptomus nauplii). These functional groups follow neither taxonomic nor body-length groupings. We conclude that zooplankton body length may influence the maximal particle size a species can ingest but has little influence on the ingestion of smaller particles. Two frequently used models relating zooplankton body size and food size are unrealistic.