Physiologic and environmental factors influencing the calcium-to-tissue ratio in populations of three species of freshwater pulmonate snails

Abstract

A study of three species of freshwater pulmonate snails, Physa gyrina (11 populations), Physa integra (17 populations), and Helisoma anceps (18 populations) was carried out from 1973 to 1976, primarily in Michigan. The ratio of whole animal calcium content to tissue dry weight (Ca: tissue ratio) changes with age, but these changes are largely restricted to early growth and do not significantly affect individuals over 40 mg total dry weight, regardless of species. Parasitization by larval trematodes was found to have no significant effect on the Ca: tissue ratio in the populations examined regardless of species. Ca: tissue ratio of P. gyrina did not vary greatly between populations and showed a narrow range over a wide range of environmental calcium concentrations. Shell mass in P. gyrina is therefore either physiologically regulated in all populations (at a species specific ‘constant’ level) or interpopulation variation is minimized by moderately high levels of gene flow between populations. There was no relationship between Ca: tissue ratio and environmental calcium for either P. integra or H. anceps over a wide range of snail and environmental calcium values. In both of the foregoing species, shell calcium content varied greatly between populations. This variation did not conform to any apparent geographic cline. In 7 of the population sites studied, P. integra and H. anceps coexist, and when these sites are listed from highest to lowest Ca: tissue ratios, the rank orders for the two species are nearly identical. This suggests that there is some environmental factor (possibly trophic conditions) at these localities that influences shell mass. In 8 other species of pulmonates briefly examined, differences in Ca: tissue ratio between populations of the same species were often as great as or greater than differences between species. While the patterns of shell mass and environmental calcium for these species are not unlike those previously reported, the proposed causal factors for these patterns are clearly distinct from those suggested for other freshwater pulmonates.