Discrimination between algae and artificial particles by freshwater and marine copepods1

Chemically mediated food selection was studied with combinations of living algae and algal‐flavored and untreated polystyrene spheres. The freshwater calanoid Eudiaptomus exhibited a strong preference for flavored spheres over untreated spheres when algae were scarce, but ingested few spheres of either type when algae were abundant. Experiments using a “zooplankton sandwich technique” revealed a distinct ranking of preference: 6‐ µ m Chlamydomonas >6‐ µ m Chlamydomonas ‐flavored spheres >6‐ µ m untreated spheres. Consistent with foraging theory, Eudiaptomus very strongly selected algae over flavored spheres when algae were abundant but was less selective when algae were scarce. Sandwich experiments revealed differences in selectivity among individuals and between species of marine and freshwater zooplankton. In experiments with 12‐ µ m algae and untreated 12‐ µ m spheres, Eudiaptomus was highly selective, ingesting about 1,000 × more algae than spheres, whereas the cladocerans Daphnia and Sida fed nonselectively. The marine copepod Temora longicornis was less selective than Pseudocalanus sp., and both marine copepods were less selective than Eudiaptomus. These complex foraging patterns emphasize the need for carefully designed experiments with mixtures of natural particles of differing nutritional quality.