The Regulation of Interspecific Variations of Shell Shape in Bivalves: An Illustration with the Common “Clams” Fauna along French Coasts

I report an unexpected negative covariance occurring between two major parameters governing shell growth in marine bivalves, especially within the order Veneroida. This relationship is highlighted, here, considering a set of forty, rather common species of clams collected from French coasts. Interestingly, this negative covariance has two (geometrically related) consequences on the pattern of variation of shell shape at the inter-specific level:

(i) An extended range of variation of shell elongation ‘E’ is made compatible with.

(ii) A severely restricted range of variation of the ventral convexity ‘K’ of the shell contour.

I suggest that:

(i) The extended range of interspecific variation of the shell elongation ‘E’ results from a trend towards larger differentiation between species according to this functionally important parameter E, while, in contrast,

(ii) The strongly restricted range of variation of the ventral convexity ‘K’ of the shell contour might arguably result from a common need for improved shell resistance, face to mechanical solicitations from the environment, either biotic or abiotic.

Accordingly, the negative covariance reported between these two growth parameters is understood as the indirect consequence of the selective pressures applying primarily upon the functionally-relevant shell shape parameters E and K.