Abstract

Sea urchin larvae near metamorphosis form an adult rudiment that is a complex of the juvenile structures. However, the details of the mechanisms that form the adult rudiment are unknown. The temnopleurid sea urchins Temnopleurus toreumaticus and Temnopleurus reevesii occur in Japan, but the development of their juvenile morphology has not been described. In this study, we observed their development by light and scanning electron microscopy to follow the adult rudiment formation and to consider the mechanisms of evolution of juvenile morphology in sea urchins. The prism embryos of both species formed two primary pore canals that elongated from the left and right coelomic sacs; the left canal connected the presumptive water vascular system to the hydropore. These organs were formed bilaterally and symmetrically in T. toreumaticus and with left-right asymmetry in T. reevesii. The right canal of both species had degenerated by the four-armed larval stage. At the prism stage, about six cells from the left oral ectoderm located between the left post-oral arm and the oral lobe formed a cell mass. The cell mass grew in diameter stepwise in T. toreumaticus by cell migration and by the formation of an epithelial pouch during the four- to six-armed larval stages and more rapidly in T. reevesii by the formation of a thin epithelium during the six-armed larval stage. In both species, the adult rudiment was formed by attachment of the cell mass to the hydrocoel. The larvae of T. toreumaticus metamorphosed from a tiny hole on the left ectoderm between the post-oral and postero-dorsal arms. These findings suggest that the developmental process involving the formation of two primary pore canals and a cell mass may have been acquired and conserved as common traits in the early development of indirect-developing temnopleurid species during evolutionary divergence from the Cidaroida.

Highlights

  • Sea urchin larvae near metamorphosis form an adult rudiment that is a complex of the juvenile structures

  • We describe the development of T. toreumaticus and T. reevesii, with respect to the formation of the juvenile morphology, and we discuss these mechanisms in relation to sea urchin evolution

  • Development of T. toreumaticus after the blastula stage Morphogenesis of T. toreumaticus embryos after the blastula stage was observed by microscopy

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Summary

Introduction

Sea urchin larvae near metamorphosis form an adult rudiment that is a complex of the juvenile structures. We observed their development by light and scanning electron microscopy to follow the adult rudiment formation and to consider the mechanisms of evolution of juvenile morphology in sea urchins. Sea urchins may display indirect development involving a planktotrophic pluteus larval stage or direct development in which the feeding pluteus larva is absent. Both modes of development have been studied for more than a century. The juvenile oral structures, including the primary podia, are formed within the adult rudiment, which consists of the hydrocoel and ectoderm on the larval left side (Okazaki 1975; MacBride 1914; Dan 1957). In the formation of the hydrocoel, the left coelomic sac, which develops from the tip of the archenteron at the prism stage, divides along the anterior-posterior axis into two parts, the anterior coelomic sac and the somatocoel

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