CYCLOPS PREDATION ON CILIATES - SPECIES-SPECIFIC DIFFERENCES AND FUNCTIONAL-RESPONSES

Experiments were conducted to measure to what extent cyclopoid copepod s ingest ciliated protists. Five freshwater ciliate species, ranging i n size from 22 to 120 mu m diameter, were tested with two species of c yclopoids: Cyclops abyssorum and Cyclops kolensis. Ingestion rates wer e measured by radiolabeling ciliates with C-14, and from these, functi onal 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, t here 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 mov e in rapid jumps elicit Holling type 3 functional responses, with very little change in ingestion rates at low ciliate densities. Thus, whil e cyclopoids are capable of having a very considerable impact on cilia te populations, some ciliate species appear to have behavioral, morpho logical or chemical defenses to reduce their vulnerability. This calls into question the practice of considering ciliates a homogeneous grou p when constructing food web models.