Differences in the ecology and distribution of lotic pulmonate and prosobranch gastropods

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We hypothesize that pulmonate gastropods, in comparison to prosobranchs should (1) be better competitors because of finely-toothed radulae, (2) be better adapted to variable habitats because of greater powers of physiological regulation, (3) be better dispersers, but (4) face greater risk of predation. In the Salt River, in western Kentucky, U. S. A., pulmonates were fairly common in headwater streams, and were replaced in larger order tributaries by prosobranch snails. The literature suggests that pulmonates have tolerance and capacity adaptations to variation in physicochemical variables like temperature or dissolved oxygen. Only one of the two pulmonates tested in the laboratory, however, had quicker righting responses and rates of movement, important in recolonizing ephemeral habitats. A competition experiment in an artificial stream indicated that the large-river prosobranch, Lithasia obovata (Say, 1829), suffered less reduction in growth than the pulmonate Helisoma trivolvis (Say, 1817). Both the literature and experiments we have performed suggest that prosobranchs are less vulnerable to predators. We interpret these results to suggest pulmonates are more common in temporary headwater streams, or shallow littoral margins of lotic systems, because of greater physiological adaptation, while prosobranchs are more common in larger rivers (or in spring-fed rivers) because of greater competitive ability, lower risk of predation, or less-variable physicochemical regimes.

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American Malacological Bulletin

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