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.