Evolutionary Conservation of the Larval Serotonergic Nervous System in a Direct Developing Sea Urchin

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Abstract
Development of the larval serotonergic nervous system is examined by indirect immunofluorescence in two congeneric species of sea urchins that exhibit divergent embryonic and larval development. Heliocidaris tuberculata undergoes indirect planktotrophic development via a pluteus larva, whereas Heliocidaris erythrogramma develops directly, passing through a brief, highly derived lecithotrophic larval stage. We have cleared the opaque embryos of H. erythrogramma and discuss internal features of its development. The serotonergic nervous system of H. tuberculata arises in the apical plate at the end of gastrulation and develops into a bilaterally symmetric ganglion lying between the anterolateral arms in the preoral hood. Putatively homologous neurons appear at the apical end of the modified larva of H. erythrogramma well after the completion of gastrulation, coincident with development of the primary podia of the adult rudiment. The neurons form a bilaterally symmetric ganglion whose orientation relative to the vestibule is conserved with respect to that found in planktotrophic larvae. This allows us to define a left and right side for this larval which lacks external points of asymmetry such as a larval mouth. The alteration in the time of nervous system development in H. erythrogramma relative to that of H. tuberculata, and other indirect developers, implicates heterochronies in cellular differentiation as an important component of the evolution of direct development.