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Abstract

Experiments were conducted to measure to what extent cyclopoid copepods ingest ciliated protists. Five freshwater ciliate species, ranging in size from 22 to 120 mu m diameter, were tested with two species of cyclopoids: Cyclops abyssorum and Cyclops kolensis. Ingestion rates were measured by radiolabeling ciliates with C-14, and from these, functional response curves (the change in ingestion rate with changing cell densities) were constructed. Cyclopoids ingest ciliates with very high estimated maximal rates of >200 cells cyclopoid(-1) h(-1). However, there are large differences in ingestion rates that are not predictable by the size of predator or prey. One ciliate species of intermediate size, Coleps hirtus, is nearly immune from cyclopoid predation at all measured ciliate densities. Three other small ciliate species that move in rapid jumps elicit Holling type 3 functional responses, with very little change in ingestion rates at low ciliate densities. Thus, while cyclopoids are capable of having a very considerable impact on ciliate populations, some ciliate species appear to have behavioral, morphological or chemical defenses to reduce their vulnerability. This calls into question the practice of considering ciliates a homogeneous group when constructing food web models