Larval Form and Metamorphosis of a "Primitive" Sea Urchin, Eucidaris thouarsi (Echinodermata: Echinoidea: Cidaroida), with Implications for Developmental and Phylogenetic Studies.

The order Cidaroida (Echinodermata, Echinoidea) is universally recognized as an ancient (∼230 mya) lineage and is thought to be the sister group to the more modern euechinoids. The present study on Eucidaris thouarsi corroborates earlier findings that cidaroids have a characteristic larval form that is different from that of euechinoids and gives the first detailed description of juvenile rudiment formation and metamorphosis in a cidaroid. Larvae of E. thouarsi lack an amniotic invagination (vestibule), have many (∼-20) juvenile spines on the larval epidermis and do not histolyze the entire larval epidermis at metamorphosis. Consequently, metamorphosis of cidaroid larvae is simple when compared to that of euechinoids. In larvae of E. thouarsi, epithelial cells appear to grow over the epidermis that becomes radial nerve tissue, but this process is not visible externally and may occur by a different mechanism than that reported for euechinoids. Typical development and metamorphosis of the class Echinoidea is usually represented by the euechinoids of the family Echinidae. The present study shows that feeding larvae of echinoids have greater variability than previously recognized in developmental patterns and processes, including differences in the fates of larval epidermal tissues and the timing of production of adult spines. The growth of podia exposed on the left side of the larval body is strikingly similar between cidaroid and asteroid larvae and is an example of probable convergence of characters among the echinoderms. The absence of a vestibule in cidaroids also raises uncertainties about the homology of this structure across the phylum Echinodermata.