ADAPTIVE SIGNIFICANCE OF POLYMORPHIC LIFE-CYCLES IN PROTOZOA - RESPONSES TO STARVATION AND REFEEDING IN 2 SPECIES OF MARINE CILIATES

The life cycles of two related marine ciliates, Uronema marinum and Pseudocohnilembus pusillus were studied. Both species are adapted to exploit patches with high densities of bacteria such as decaying particulate matter. When growing cells of Uronema are deprived of food, the cells undergo from one to three cell divisions and form motile swarmer cells. These have a high motility and a long survival, but when refed they need a long lag time before they resume cell divisions. This reflects cell physiological change and reduction in metabolic rate during starvation and so the cost of long term survival. This ciliate is particularly adapted to exploit food resources with a patchy spatial distribution. Pseudocohnilembus cells do not divide when starved; instead they form motile swarmer cells and these can resume cell divisions quickly when food is available, but their survival time is only ≈ 1 3 of that of Uronema cells. However, about half of the Pseudocohnilembus swarmers are capable of forming resting cysts 10-30 h after the onset of starvation. These cysts remain viable for several months and depend on episodic food supply; the delay in cyst formation and the fact that only some cells produce cysts underscores the adaptive significance of dispersal in this species as well. The findings are discussed in terms of a generalised ciliate life cycle; traits found in individual species reflect the degree of spatial and temporal heterogeneity of their habitats and their resources. © 1990.