Differences in Larval Arm Movements Correlate with the Complexity of Musculature in Two Phylogenetically Distant Echinoids, Eucidaris tribuloides (Cidaroidea) and Lytechinus variegatus (Euechinoidea).

Within a common body plan, echinoid planktotrophic larvae are morphologically diverse, with variations in overall size, the length, and number of arms and the presence or absence of epidermal structures. In this report, we are interested in variation in larval arm-flexing behavior and correlated differences in larval musculature. Larvae of the cidaroid Eucidaris tribuloides exhibit conspicuous and regular arm-flexing behavior. In contrast, Lytechinus variegatus, a representative of the euechinoid clade, does not exhibit this behavior. We hypothesized that there were differences in musculature that correlated with this behavioral contrast and compared the development and structure of larval muscles between these species. We report substantial differences in some aspects of larval musculature. In addition to previously described oral musculature, both larvae possessed polygon-shaped musculature at the basal end of the larva. However, larval musculature in E. tribuloides was larger and contained additional muscles not observed in larvae of L. variegatus. Therefore, a conspicuous larval behavior consisting of repeated flexing of the postoral and posterodorsal larval arms was correlated with a larger, more complex musculature. This simple contrast indicates that larval musculature not associated with endoderm evolves in a manner that relates to differences in larval behavior and that additional comparisons are warranted.

Neural development in Eucidaris tribuloides and the evolutionary history of the echinoid larval nervous system.

The structure and development of the larval nervous systems of all classes of echinoderms have been described and details of embryonic signaling mechanisms patterning neurogenesis have been revealed experimentally in sea urchins. Several features of neuroanatomy and neural development indicate that echinoids are the most derived group. Here we describe the development and organization of the nervous system of a cidaroid, Eucidaris tribuloides. The cidaroids are one of two major clades of echinoids, and are considered to have features of anatomy and development that represent the common ancestor to all echinoids. The embryos of E. tribuloides lack a thickened animal plate and serotonergic neurons arise laterally, associated with the ciliary band. Although lacking a discrete apical organ, plutei have serotonergic neurons associated with the pre-oral ciliary band joined by a few diffusely arranged connecting axons. Chordin and Hnf6, early markers for oral ectoderm and ciliary band, are expressed in similar patterns to euechinoids. However, an animal pole domain marker, Nk2.1, is expressed in a broader region of anterior ectoderm than in euechinoids. Six3, a proneural marker that is restricted to the animal plate of euechinoids, is expressed laterally in the preoral ciliary band at the same location as the serotonergic neurons. We conclude that the organization and development of the larval nervous system of E. tribuloides retains features shared with other echinoderm larvae, but not with euechinoids. These data support a model in which several distinctive features of euechinoid neural organization are derived, having arisen after the divergence of the two clades of echinoids about 265 million years ago. We hypothesize that differences in the developmental mechanisms that restrict neurogenesis to the animal pole forms the basis for the distinctive neuroanatomy of euechinoids.