The Distribution, Abundance, Feeding Habits, and Population Biology of the Immigrant Triclad Dugesia polychroa (Platyhelminthes: turbellaria) in Toronto Harbour, Canada

(1) The feeding habits, distribution, abundance and population biology of the immigrant triclad Dugesia polychroa have been studied over a period of 20 months. (2) Using serological techniques, it was found that the food sources of D. polychroa were the tubificid oligochaetes (84.4%) the gastropods (12.0%) and the isopods and amphipods (3.6%). (3) The distribution of D. polychroa in Toronto Harbour was limited, not by food but by gross pollution, to the highly enriched areas around the islands of the harbour complex. (4) The abundance of D. polychroa was estimated at 2220/m2. (5) The weight and length D. polychroa followed a typical allometric growth curve. (6) The triclad population exhibited a yearly maximum and minimum size. The maxima, in August, were characterized by a maximum mean individual length, maximum biomass percentage and a maximum proportion of adults. This was followed by a rapid decline to minima which were characterized by a minimum mean length, and biomass percentage. The minima were followed by a period of growth that was initially slow, halted over the winter period and then became very rapid from spring through to the maxima. The length of the breeding period was 10 weeks. (7) In Toronto D. Polychroa exhibited different feeding habits, related to both the differing relative abundance of the food source and different inter-specific competitors, from those of D. polychroa in Britain. (8) The population abundance was consistent with the levels of food sources, which were considerably higher than those found in Britain. (9) The fecundity estimates and the characteristic maxima and minima of the Toronto population were similar to those exhibited by D. polychroa in Britain, but differed in the rate of growth and reproduction, the rate of decline from the maxima to the minima of size, the length of the breeding period and the maximum, but not the total, observed young. The magnitude of these differences, which were in the order of X2.5-3, showed a relationship to temperature. (10) The population biology of D. polychroa in the two differing environments was remarkably similar although temperature-dependent, thus supporting the concept of self-regulatory dynamics.