A method is described for calculating the volumes of closed valves of Margaritifera margaritifera. Dorsal arching of the shell is measured by Arching Index, a ratio expressing the proportionate rise of the dorsal margin above a line drawn from the umbo to the shell margin at the posterior termination of the ligament. Correlation between Weight/Volume (Specific Gravity) of the living bivalves and Arching Index is good. The heavier varieties of the species, length for length, have shells with strongly arched dorsal margins, with a trend towards straight or reflected ventral margins: they characterize waters subject periodically to a higher degree of turbulence than lighter varieties of this species, which have relatively straight ‘hinge lines’, stronger ligaments and shells with curved ventral margins. Washing away of young bivalves through turbulence during times of flood can locally limit the distribution of M. margaritfera. In stations near the margin of this physical limit individuals with higher Specific Gravities predominate, probably through selection, whereas in stations of lower water velocities the mode trends towards more active bivalves with a predominance of oval shells. The retention of a wide range of variation in shape of shell of the species constitutes a means by which its survival is ensured within the widely variable environments of fresh water, where there is continual re-distribution of populations by fish hosts. Evidence from the fossil record suggests that essentially the same pattern of variation provided similar ‘insurance’ in shells of Cenozoic and Mesozoic Unionidae, and in Carbonicola, of the non-marine Family Anthracosiidae of Carboniferous (late Palaeozoic) time.