Distribution of aquatic gastropods across an interface between the Canadian Shield and limestone formations

This quantitative study gives a convincing demonstration in situ of the direct relationship which exists between the distribution and density of aquatic gastropods and the calcium concentration of their natural waters. The area chosen, in the Gravenhurst–Barrie district of southern Ontario, north of Lake Simcoe, provided a well-demarcated natural gradient across the interface between limestone formations and the calcium-poor rocks of the Pre-Cambrian Shield. Monthly quantitative samples over a 14-month period at 25 stations showed that most pulmonates, notably Physa gyrina and Helisoma anceps, had high densities in hard and medium water, whereas the prosobranchs Amnicola limosa and Campeloma decisum showed a reverse pattern, with highest densities in soft and medium water and very low densities in hard. Most pulmonates showed a slower individual growth rate in soft water and a lower proportion of large individuals.