Domes, arches and urchins: The skeletal architecture of echinoids (Echinodermata)

Summary

A combination of simple membrane theory and statical analysis has been used to determine how stresses are carried in echinoid skeletons. Sutures oriented circumferentially are subject principally to compression. Those forming radial zig-zags are subject to compression near the apex and tension near the ambitus. Radial and circumferential sutures in Eucidaris are equally bound with collagen fibers but in Diadema, Tripneustes, Psammechinus, Arbacia and other regular echinoids, most radial sutures are more heavily bound, and thus stronger in tension. Psammechinus, Tripneustes and several other echinoids have radial sutures thickened by ribs which increase the area of interlocking trabeculae. Ribs also increase flexural stiffness and carry a greater proportion of the stress. Further, ribs effectively draw stress from weaker areas pierced by podial pores, and increase the total load which can be sustained.

Allometry indicates that regular echinoids become relatively higher at the apex as size increases, thus reducing ambital stresses. Some spatangoids with very high domes (eg Agassizia) maintain isometry, but others (eg Meoma) become flatter with size. Both holectypoids (Echinoneus) and cassiduloids (Apatopygus) maintain a constant height to diameter relationship. Flattening, and consequently ambital tensile stress, is greatest in the clypeasteroids. In this group the formation of internal buttresses which preferentially carry stress, reaches maximum development. A notable exception, however, is the high domed Clypeaster rosaceus.

In this analysis it was assumed that local buckling or bending does not occur. The test of some echinoids (e.g. Diadematoida) have relatively wide sutures swathed in collagen, which allows local deformation. Others (e.g. Arbacia) have rigid sutures with reduced collagen. In Psammechinus and other members of the Order Echinoida, in addition to rib formation, inner and outer surface trabeculae are thickened so that the individual plates are stiffened. Some spatangoids (Meoma, Paleopneustes) have extensive sutural collagen, but the cassiduloid Apatopygus has collagen confined to junctions of sutures, and elsewhere the joints are strengthened and stiffened by fusion of trabeculae. Fusion of surface trabeculae is almost complete in the holectypoid, Echinoneus, and the sutures are obscured.