Size selectivity spectra of 28 planktonic predators from 18 studies in the literature are compared. The linear size ratio between predators and their optimal prey is 1 : 1 for a dinoflagellate, 3 : 1 for other flagellates, 8 : 1 for ciliates, 18 : 1 for rotifers and copepods, and ∼50: 1 for cladocerans and meroplankton larvae. These size ratios seem consistent within groups, and their validity is supported by quantitative information from the literature. However, a difference between filter feeders and raptorial‐interception feeders, preferring relatively smaller and larger prey respectively, is evident across the taxonomic groups. A classification of planktonic predators into functional groups is therefore crucial for the construction of models of pelagic food webs.

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In order to study the size dependency of grazing and growth rates in zooplankton, data wcrc collected from laboratory studies in the literature, covering both limnic and marinc organisms. Data were obtained from about 60 species of nano-, micro-, and mesozooplankton, representing flagellates, ciliates, rotifers, meroplankton larvae, copepods, and cladocerans. Estimates of maximum ingestion and clearance were extracted from functional responses (ingestion rates as a function of food density) cstablishcd from laboratory experiments. Maximum specific rates were expressed as a function of predator body volume. Maximum specific clearance and ingestion rates decreased with predator volume within each group of zooplankton, with a-common exponent (scaling factor) of -0.23 (SE = 20.12) in accordance with previous findings. However, significant differences wcrc found between groups. In particular, among the protists, ciliates display maximum ingestion, growth, and clearance rates that exceed those of dinoflagellates by a factor of 2-4. Among the metazooplankton, Calanoid copepods have maximum clearance rates that cxcccd those of filter-feeding cladocerans and meroplankton larvae by a factor of 10. Because of these differenccs between the groups, the entire set of observations could not be fitted by an overall regression.

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