FOOD PARTITIONING BY LAKE-DWELLING TRICLADS AND GLOSSIPHONIID LEECHES- FIELD AND LABORATORY EXPERIMENTS

The triclads Polycelis tenuis and Dugesia polychroa and the glossiphon iid leeches Glossiphonia complanata and Helobdella stagnalis are abund ant on the stony shores of productive British lakes. All species are f ood limited and there is considerable overlap in the diets of these tr iclads and leeches. This paper investigates interactions between the t wo groups using field and laboratory experiments to try to identify th e mechanism of their co-existence. Triclad and leech numbers were mani pulated inside experimental enclosures, matched by controls, erected o n the stony shore of an eutrophic English lake. Increasing the numbers of P. tenuis and D. polychroa prior to the reproductive season in spr ing resulted in a significant decrease in the numbers and body size of G. complanata and H. stagnalis compared with control populations in t he summer months, and vice versa. However, increases and decreases wer e temporary with a readjustment of numbers and body size to control le vels in the autumn after reproduction had ceased. It is suggested that increasing the numbers of either group elevated the severity of both intra- and interspecific competition for food. The ''condition'' of pr ey may, in part, determine the strength of competition, and this was e xamined in laboratory experiments in which different densities and rat ios of P. tenuis and H. stagnalis were offered either live of recently crushed Asellus aquaticus. In monospecific controls, growth rates of P. tenuis were greater when fed on crushed than live Asellus, but ther e was no significant difference in the growth of H. stagnalis fed eith er live or crushed prey. In mixed cultures of predators, P. tenuis and H. stagnalis were the superior competitors when fed on crushed and li ve Asellus, respectively. However, when competitive pressure was low, at low densities of predators, the presence of H. stagnalis in mixed c ultures fed on live prey was beneficial to the growth of P. tenuis. Th ese results are explained in terms of the greater ability of triclads to detect damaged prey, leaking body fluids, due to their sophisticate d chemosensory system, and the ability of leeches to capture live prey due to the presence of suckers. It is concluded that co-existence of the two groups in British lakes is assisted by the partitioning of foo d on a live or damaged basis.