The size ratio between planktonic predators and their prey

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 raptorialinterception 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. The pathways for flow of organic matter in pelagic food webs are to a wide extent determined by the food selectivity of the pelagic predators. Several criteria may be involved in food selection, including prey size, motility, surface characteristics, biochemical composition, electrostatic forces, etc. (e.g. Poulet and Marsot 1978; Robinson 1983; Gilbert and Bogdan 1984; Van Alstyne 1986). Among these criteria, prey size is generally believed to play a major role, and usually a fixed size ratio of 10 between predator and prey is assumed (Sheldon et al. 1977; Conover and Huntley 1980; Azam et al. 1983). This assumption is in accord with the classification of plankton into logarithmic size fractions (pica, nano, miAcknowledgments We are indebted to T. Fenchel and T. Kiorboe for comments on the manuscript. cro, meso, and macro, sensu Sieburth et al. 1978) and it has facilitated integrative trophodynamic studies of pelagic environments above the population level. This assumption has also been instrumental in the formulation of models of pelagic ecosystems for theoretical and steady state considerations (e.g. Kerr 1974; Azam et al. 1983) as well as for the formulation of carbon budgets and dynamic simulation (e.g. Riemann et al. 1990; Maloney and Field 199 1). Longhurst (1990) asked whether there is a generalized prey : predator ratio within protists, invertebrates, fish, and cetaceans and found a standard deviation of 2.5 times the mean predator : prey ratio. He concluded that including more data only increases scatter around the regression line, and therefore that it is difficult to generalize ratios even within groups that appear to be relatively homogeneous. In this note we re-evaluate the assumption of a fixed size ratio between pelagic predators and their prey on the basis of data now available in the literature. Data were selected from laboratory studies where size selectivity has been assessed with one prey size at a time at a constant initial volume fraction and from in situ studies with tracer particles added to the natural prey composition. In some of these studies selectivity was expressed as an Ivlev electivity index (Ivlev 195 5). The studies cover pelagic predators of 5-1,000 pm (nano-, micro-, and mesozooplankton) from freshwater as well as marine environments. Size selectivity is usually expressed as a ratio of ingestion rates of two or more size classes