VARIATIONS IN THE APICAL DISC OF A MODERN SEA URCHIN

The Extraxial-Axial Theory (EAT) delineates two skeletal domains in echinoderms. The axial domain of plates is derived in accordance with the Ocular Plate Rule (OPR) and thus obligate, and the extraxial domain defined as plates that can be added anywhere and thus facultative. The echinoid skeleton comprises the corona and ocular plates, which are axial, and the genital and periproctal plates which are considered extraxial. The value of the EAT is dependent on the axial and extraxial domains being homologous across all echinoderms. The echinoid apical disc is associated with non-skeletal soft tissue comprising the gonoducts, the apical complex, and the anal opening of the alimentary tract. A neontological approach considers how the skeletal elements relate to soft tissue development. If the apical disc plates are derived from soft tissue, they are obligate (analogous to derivation of axial plates from the water vascular system) and thus not homologous to extraxial plates in other echinoderm classes.

A suite of several hundred specimens of the modern sea urchin Strongylocentrotus droebachiensis were analyzed for plating patterns in the apical region of the skeleton using planar and directed graphs. Relative sizes of apical disc plates are highly conserved in the normal form (5 ocular plates, 5 genital plates, oculars OI and OV insert). Genital plate topology expresses bilateral symmetry identical to that of exocyclic echinoids while preserving a high-degree of circularity to the disc. A single periproctal or anal plate at metamorphosis is associated with genital plate G3, followed in early growth by a distinct order in periproctal plate addition and differentiation of plate types and persistence of the anal plate. Presence of this plate may be synapomorphic for camarodont echinoids, but consistent overall patterning in periproctal plate addition and growth is obligate. Supernumerary genital plates are associated with G3, but are distinct from the anal plate and not homologous to the suranal plate of saleniid echinoids. Variations in number and location of genital pores and hydropores suggest that their relationship to genital plates is facultative to some degree. These results strongly suggest that neither genital nor periproctal plates of echinoids are homologous with classic extraxial plates of other echinoderms.