Leech predation on juvenile freshwater snails: effects of size, species and substrate

Summary. The leech Glossiphonia complanata does not appear to have substantial impact on snail populations, but this may be due to most studies focusing on adult snails rather than juvenile snails. In this study I investigated how predation rates of G. complanata feeding on newly-hatched and juvenile snails was affected by snail species, snail size, snail density and substrate, in a laboratory experiment. Number of snails eaten increased with increasing density resulting in a type II functional response curve. Predation rates were higher when leeches were feeding on Lymnaea emarginata than on Physa 9yrina, whereas there was no significant difference in predation rates when they were feeding on L. emarginata and Helisoma aneeps. Sandy substrates and greater snail size resulted in decreased predation rates. Sand reduced movement speed of G. complanata, which probably reduced encounter rates. Thus, there was a comparatively large effect of leech predation on newly-hatched snails, due to a high probability of encounter and high predation rates, but spatial and temporal refuges probably reduce the importance of leech predation as a structuring force in freshwater snail assemblages.

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