Direct Development in the Sea Urchin Phyllacanthus parvispinus (Cidaroidea): Phylogenetic History and Functional Modification

¹ Institute for Molecular and Cellular Biology, and Department of Biology, Indiana University, Bloomington, Indiana 47405

Development in the Australian sea urchin Phyllacanthus parvispinus (Echinoidea: Cidaroidea) is of interest because it has a highly modified, lecithotrophic larva, and because it belongs to an echinoid group whose development has been little studied. This study documents early development and metamorphosis in P. parvispinus and considers the evolution of features unusual in echinoid ontogeny. Some features, such as lack of a vestibule, occur in other cidaroids, and are likely a product of ancestry. Other unusual features, such as larger gametes, an equal fourth cleavage, a wrinkled blastula, and accelerated development of the adult rudiment, are characteristic of other direct developing echinoids, and are probably functional modifications for altered developmental mode. Since the Cidaroidea form the sister group to the more derived Euechinoidea, cidaroid development is critical in assessing the phylogeny of ontogeny among echinoids. The distribution of developmental features among extant echinoids suggests that the extinct ancestor of cidaroids and euechinoids had planktotrophic larvae that lacked a vestibule during formation of the juvenile rudiment.

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