DOES BODY-SIZE DIFFERENCE IN THE LEECHES GLOSSIPHONIA-COMPLANATA (L) AND HELOBDELLA-STAGNALIS (L) CONTRIBUTE TO COEXISTENCE

The effect of predator and prey body size on the feeding success of th e British lake-dwelling leeches Glossiphonia complanata and Helobdella stagnalis was examined in the laboratory, and any involvement of size difference between the leeches in allowing coexistence in the field a ssessed. G. complanata breeds in advance of H. stagnalis and maintains a body size advantage throughout their annual life-cycle. In experime nts, conducted at 14-degrees-C and a photoperiod of 16 hrs L: 8 hrs D, three size classes of leeches of each species were each exposed to ea ch of three size classes of each of five prey species, viz. Tubifex sp ., Chironomus sp., Asellus aquaticus, Lymnaea peregra and Potamopyrgus jenkinsi. For each prey species, three different types of experiments were performed: one leech exposed to four prey individuals; four leec hes of the same species with sixteen prey; and two leeches of each spe cies with sixteen prey. In the first experiment, all sizes of G. compl anata were capable of feeding on all sizes of the prey types offered; the same was true for H. stagnalis with exceptions of feeding on large A. aquaticus and large L. peregra. For both species, but especially f or G. complanata, there was a trend within each size class of leech fo r decreasing proportions of fed leeches with increasing prey size, and within each size class of prey for an increasing proportion of fed le eches with increasing leech size; however there were several exception s to these trends. Both leeches fed extensively on Tubifex sp. but the re were significant differences in the proportions feeding on other pr ey types; G. complanata fed more on A. aquaticus and the two snail spe cies, and less on Chironomus, than H. stagnalis. The effect of increas ing the number of leech individuals from one to four individuals, of t he same or mixed species, had little effect on the proportion of leech es which had fed. It is concluded that large G. complanata will have a ccess to large individuals of certain prey taxa denied H. stagnalis, w hich may lessen the intensity of interspecific competition.