Vestigial ophiopluteal structures in the lecithotrophic larvae of Ophionereis schayeri (Ophiuroidea)

Evolution of echinoderm development from a feeding to a non-feeding mode ca n be examined by studying non-feeding larvae with structures that appear to be vestiges derived from a feeding ancestral state. The lecithotrophic lar vae of the Australian brittle star Ophionereis schayeri possess such featur es, and the early development of this species was documented by light and s canning electron microscopy. The embryos undergo irregular cleavage, result ing in the formation of different sized blastomeres, with subsequent develo pment through a wrinkled blastula stage. The lecithotrophic larva of O. sch ayeri possesses several vestigial ophiopluteal structures, including a cont inuous ciliated band, a larval gut, and a larval skeleton. The ciliated ban d is a reduced expression of the continuous ciliated band typical of ophiop lutei. The larval gut is a transiently complete system, but an esophageal p lug and rapid closure of the blastopore renders it nonfunctional. The larva l skeleton, though reduced, consists of four rods corresponding to the body , posterolateral, anterolateral, and postoral rods characteristic of an oph iopluteus. Due to a heterochrony in larval skeletogenesis, the postoral rod s develop early and simultaneously with the other rods. Compared with the l arvae of other lecithotrophic ophiuroids, the larva of O. schayeri is one o f the most reduced ophiopluteal forms reported to date.