Forces Generated by the Jaws of Clypeasteroids (Echinodermata: Echinoidea)

Abstract

Aristotle' lantern acts like a five-toothed ‘vice grip.’ Contraction of the interpyramidal muscles creates tangential stresses that are converted to radial forces along the teeth. Two mechanical models are proposed to explain this conversion. In the first, the lantern is regarded as a thick-walled cylinder resisting internal pressure; in the second, it is treated as a cluster of wedges. The two models differ primarily in the allowance of radial forces within the muscle in the cylinder and their exclusion in the wedge model. Maximum muscle stress required for a given force along the teeth depends on the ratio of external to internal lantern radii (ro/ri). Maximal force requires that (ro/ri) should be greater than 2, which is the case in Clypeaster rosaceus (L.). The models allow calculation of a dimensionless number, F, which scales the force exerted by the teeth for changes in lantern size and the number of pyramids. Biting force was measured in C. rosaceus and used to calculate the muscle stress required by the mechanical models. For the thick-walled cylinder, maximum interpyramidal muscle stress was calculated to be 2.8×106N m−2. For the wedge model it was 1.9×105N m−2. The models were supported by comparison of predicted with observed biting forces in another clypeasteroid, Encope michelini L. Agassiz.