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M. A. Learner, D. W. B. Potter, Life-history and Production of the Leech Helobdella stagnalis (L.) (Hirudinea) in a Shallow Eutrophic Reservoir in South Wales, Journal of Animal Ecology, Vol. 43, No. 1 (Feb., 1974), pp. 199-208
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Learner et al. (1974) studied this question.
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