Analysis of zooplankton feeding experiments: some methodological considerations

Studies of the feeding ecology of zooplankton are noted morefor the problems associated with their quantitative analysesthan for their conclusions. In order to illustrate one particularlyinsidious problem, I use several more or less traditional methodsto obtain rather different results from experiments with themarine calanoid copepod Eucalanus pileatus and a variety ofphytoplankton. If the probabilities of encountering algal cellsof various taxa are proportional to their environmental frequencies,as is implicit in a feeding mechanism based upon improbableanalogy with a leaky sieve, E. pileatus seems to capture largecells more efficiently than small ones. If these frequenciesare weighted by the average cross-sectional areas of individualcells (i.e., those of spheres of equivalent volume), as wouldbe 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 thanlarge ones. Inasmuch as these conflicting observations resultfrom different conceptions of the feeding mechanism, and sievingis only one of several reasonable alternatives that emerge fromthe relevant fluid dynamics, evidently the mechanism of feedingmust be elucidated before passive selection can be properlycharacterized. However, the feeding mechanism need not be understoodin order to study consumers whose behavior changes with therelative abundance of prey. I find that selectivity by E. pileatusvaries with algal concentration, and that large cells are consumeddisproportionately relative to their frequencies at high concentrations(in accord with results of other investigators), but that smallcells are consumed disproportionately relative to their encounterprobabilities (here a function of size as well as frequency,although other characteristics, such as scent and shape, maybe involved as well) irrespective of concentration. This observationis incompatible with the contemporary paradigm of zooplanktonfeeding, but it does not seem unreasonable considering the sizeand spatial distributions of particles in the ocean. Consequently,these results support the alternative hypothesis that consumersadapt to characteristics of the prey that they encounter mostfrequently.