Why clams have the shape they have: an experimental analysis of burrowing

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Abstract
The prosogyrous shape and flattened lunule of a typical clam shell form a blunt anterior, the function of which is related to the forward-and-back rocking motion of the shell in burrowing. Analysis of movies has revealed that each rocking motion of a morphologically typical clam, Mercenaria mercenaria (Linné), involves purely rotational movement, with no translational component. The clam is able to burrow by “walking” its way downward only because the axis of backward rotation lies to the anterior of the axis of forward rotation. Experiments with burrowing robots show that the blunt anterior serves to shift the axis of backward rotation anteriorly, thus aiding in downward progress. The prosogyrous condition and the rotational mechanism of burrowing are fundamental adaptations of burrowing clams and were apparently present in the ancestral bivalves of the Cambrian.