Adaptive significance of cyclomorphic growth likely centers on reducing predation by allowing continued growth of peripheral transparent structures without enlarging the central portion of the body visible to fish. Small cladocerans that increase size by cyclomorphic growth reduce capture success by invertebrate predators like copepods. Changes in rotifer growth form include elongation in relation to body width, enlargement, reduction in size, and production of lateral spines which reduce predation success. Cyclomorphosis is lacking in copepods, which, by means of rapid, evasive swimming movements, can defend themselves better from invertebrate predators than can most rotifers and cladocerans. Prey are visually se- lected, in most cases, on an individual basis, although the gill rakers of certain fish collect some zooplankton as water passes through the mouth and across the gills.
Planktivorous fish select large zooplankters and can eliminate large cladocerans from lakes.
Assimilation and respiration rates generally increase at higher trophic levels, and production decreases. A general, positive correlation exists between the rates of production of phytoplankton and of zooplankton. The productivity of suspension-feeding zooplankton is higher than that of predaceous zooplankton. Separation of the niche hyperspace with relatively small regions of species overlap minimizes interspecific competition and contribute to the large diversity of population interactions that have evolved to permit coexistence in limnetic zooplankton communities. Although nearly all Protozoa are aerobic, a majority can grow very well even when oxygen concentrations are very low.