A biomechanical model for the morphogenesis of regular echinoid tests

Experiments were conducted to test the hypothesis that biomechanical constraints determine the morphology of regular echinoids. Hard-bottom-dwelling Tripneustes gratilla elatensis were transferred to an artifical sandy habitat to evaluate whether the change in substrate affects their height to diameter ratio (H/D). Within 1-2 months their H/D ratio increased significantly. This change was shown to be reversible to some extent. Surgical damage to the ambulacral system of one ray caused inactivation of tubefeet and atrophy of injured ambulacra. Test shape was also affected: the damaged ray was lower, and the nondamaged ambulacra deflected toward the treated one, producing bilateral symmetry as in recorded cases of teratology. Study of T. g. elatensis tetramers showed that while "perfect" tetramery was apparently associated with genetic aberration, "imperfect" tetramery results from mechanical injury at an early on- togenetic stage. Micromorphological study shows that in the longitudinal sutures, normally under tension, long and slender trabeculae develop, associated with long and well-aligned collagenous sutural fibers, while the latitudinal trabeculae and fibers are short and less organized. A mechanical effect is suggested by the oval cross-section of the fiber-anchoring trabeculae. Further, echinoid plates interact like soap bubbles, whereas the entire test behaves like a balloon, fastened to the substrate by the ambulacral tubefeet. All these observations support earlier hypotheses on the biomechanical control of echinoid test growth. A model is proposed in which the expansion of the inner mass, counteracted by the mechanical activity of the ambulacral tubefeet, mesenterial threads, and lantern muscles, affects sutural growth, thus controlling echinoid morphogenesis. A morphometric survey among regular echinoids reveals an inverse relationship between ambulacral width and relative ambital height. Although both increase of ambulacral width and lowering of ambitus- line are evolutionary trends, it is suggested that they are a response to a mechanical effect. H/D ratio was not related to ambulacral width on the phylogenetic level. It is therefore suggested that the latter correlation is ontogenetically controlled. Aspects of irregular echonoid evolution, such as bilateral symmetry, flattening, and formation of the ambulacral petaloid, also are explained by this model.