The Population Biology of Lake-Dwelling Polycelis Species with Special Reference to P. nigra (Mull.) (Turbellaria, Tricladida)

Abstract

Breeding activity and size structure of field populations of Polycelis nigra were observed over 18 months. This species was found to breed all the year except for the coldest periods but a larger spring peak and a smaller autumn peak were discernible. Cyclic changes in the proportion of adults (specimens > 7.0 mm. length) and in size-structure of the population together with low fecundity suggested that a period of acute starvation coincided with the later period of breeding. The ratio of food to population size seemed to be readjusted by mortality. This hypothesis was supported by results of field experiments in which increasing the absolute and relative amounts of food eliminated or delayed symptoms of food shortage. Evidence was also obtained by observing the entire populations of two small habitats that mortality was a readjusting factor. Field experiments with P. tenuis which behaves similarly but with different timing, corroborated these results. Examination of seasonal changes in the reproductive organs of P. nigra showed that cyclic events stemmed from ecological rather than physiological causes with water temperature a modifying factor.