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Natural populations of the North American stream limpet, F. rivularis, were studied in upstate New York [USA], in a set of localities whose waters have a 15-fold range of dissolved Ca (4.6-67.6 mg/liter) and also range from oligotrophy to eutrophy. Shell component analyses (CaCO3, total organic C and total N) are reported as component mass-fractions (mg/g or .mu.g/g dry weight) and as values for a standard limpet shell of 35 mm aperture length (AL). More than 2-fold differences occur between populations in all 3 components, with relatively little variation occurring within each population. Expressed per standard limpet, CaCO3 values for different populations range from 0.8-1.97 mg with no direct relationship to environmental dissolved Ca. Nominal concentration ratios of body Ca to environmental Ca range from 1953:1-29,130:1. Values for total organic C (9.13-21.0 .mu.g) and total N (2.7-6.69 .mu.g) in the shells parallel each other, all C:N ratios being relatively uniform (3.0:1-3.4:1), and indicating that the non-calcareous components are largely proteinaceous. Although alternative hypotheses predict an inverse or a direct relationship between the organic and the calcareous components, neither is shown by these populations. Genetic controls of shell secretion for the 2-major components apparently are independent, and chance dispersal has resulted in some rather inappropriate shells in certain habitats. This irregular variation in Ferrissia is 1st discussed in relation to other patterns of shell component relationships known for other freshwater mollusk, including direct relationship of the mass of shell CaCO3 to the dissolved Ca available as in Lymnaea peregra and Laevapex fuscus and the apparent regulation producing standard shell weights in L. palustris and Physa gyrina. The results are then discussed in relation to assessment of radionuclide pollution using molluscan shells from fresh waters and in their more general relationship to modes and rates of evolutionary change in freshwater faunas.

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Institute of Malacology



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