Seasonal changes in the biochemical composition and nutritional state of the immigrant triclad Dugesia polychroa (Platyhelminthes: Turbellaria) in Toronto Harbour, Canada

Abstract

The biochemical components of a population of Dugesia polychroa from Toronto Harbour showed, with the exception of total nitrogen, statistically significant variations over the time period studied (May 1971 through January 1972). The variation in biochemical composition was not totally correlated to changes in percentage biomass (mean individual weight), although the latter was itself indicative of events occurring in the population.Fluctuations in carbohydrate, lipid, and caloric content were highly correlated to changes occurring in percentage biomass during the period of reproduction and subsequent recruitment to the population. During this period a maximum in stored food reserves and energy content was observed. Before the winter these fractions increased to a second peak but at a disproportionately higher rate than that for percentage biomass. In both cases the caloric values were lower than the maximum physiological energy values.The changing nutritional state of Dugesia polychroa strongly supports the thesis that triclad population numbers are controlled by intraspecific competition for food, and, specifically shows that Dugesia polychroa exists, at least in Canada, in a state of relative starvation over the whole year.