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Typescript, etc. / Digitized by Kansas State University Libraries

Typescript etc. / Digitized by Kansas State University Libraries

In the present study, the incidence and effects of predation and parasitism on three species of lake-dwelling leech, viz. Erpobdella octoculata, Glossinhonia complanata and Helobdella stagnalis Were investigated. A literature survey was carried out, and produced a wide range of studies which reported leeches in the diet of predators. The majority of such records were for fish from lakes and rivers, but a few records from waterfowl were also reported. Simple laboratory experiments demonstrated that a wide range of invertebrate and vertebrate predators may feed on both adult and young leeches. Field collections of these predators were taken and examined, using visual and serological techniques to establish what had been eaten. Positive evidence for predation in the field was provided by this work, but the incidence and intensity of predation was found to be very low. Field experiments, using predator exclusion methods in the stony littoral of a eutrophic lake, were carried out but failed to show any marked changes in leech mortality or population dynamics. The parasites of leeches collected from fifteen lakes in England and North Wales were surveyed and five species recorded. These parasites were the microsporidians, Nosema herpobdellae and Nosema glossiphoniae, and the trematodes, Apatemon gracilis, Cotylurus cornutus and Cyathocotyle opaca. Only A. gracilis and C. cornutus were previously reported from British leeches. The microsporidian species were re-described, using electron microscopy. Detailed studies on the incidence of the parasites were carried out in leeches from a eutrophic lake, and a limited amount of evidence for parasite-induced host mortality and a reduction in the fecundity of infected leeches was presented.The findings of the present work are discussed in relation to previous studies on leech populations, and to other mechanisms that may serve to control and regulate these populations. It is concluded that, whilst causing some mortality, predation and parasitism play a minor role in the control and regulation of the leech populations in the stony littoral of eutrophic lakes. it is suggested that the availability of food, particularly to young leeches, may be a more important factor, and certainly one that deserves future attention.

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