Analysis of zooplankton feeding experiments: some methodological considerations

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Abstract
Studies of the feeding ecology of zooplankton are noted more for the problems associated with their quantitative analyses than for their conclusions. Several more or less traditional methods were used to obtain rather different results from experiments with the marine calanoid copepod Eucalanus pileatus and a variety of phytoplankton. If the probabilities of encountering algal cells of various taxa are proportional to their environmental frequencies, as is implicit in a feeding mechanism based upon improbable analogy with a leaky sieve, E. pileatus seems to capture large cells more efficiently than small ones. If these frequencies are weighted by the average cross-sectional areas of individual cells (i.e., those of spheres of equivalent volume), as would be appropriate if they were intercepted (i.e, encountered) by single fibers (e.g., maxillary setae or setules), then E. pileatus seems to capture small cells more efficiently than large ones. Since these conflicting observations result from different conceptions of the feeding mechanism, and sieving in only 1 of several reasonable alternatives that emerge from the relevant fluid dynamics, evidently the mechanism of feeding must be elucidated before passive selection can be properly characterized. The feeding mechanism need not be understood in order to study consumers whose behavior changes with the relative abundance of prey. Selectivity by E. pileatus varies with algal concentration and large cells are consumed disproportionately relative to their frequencies at high concentrations (in accord with results of other investigatiors), but small cells are consumed disproportionately relative to their encounter probabilities (here a function of size as well as frequency, although other characteristics, such as scent and shape, may be involved as well) irrespective of concentration. This observation is incompatible with the contemporary paradigm of zooplankton feeding, but it does not seem unreasonable considering the size and spatial distributions of particles in the ocean. These results support the alternative hypothesis that consumers adapt to characteristics of the prey that they encounter most frequently.

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