Ciliated band structure in planktotrophic and lecithotrophic larvae of Heliocidaris species (Echinodermata : Echinoidea): a demonstration of conservation and change

The evolution of lecithotrophic (non-feeding) development in sea urchins is associated with reduction or loss of structures found in the planktotrophi c (feeding) echinopluteus larvae. Reductions or losses of larval feeding st ructures include pluteal arms, their supporting skeleton and the ciliated b and that borders them. The barrel-shaped lecithotrophic larva of Heliocidar is erythrogramma has, at its posterior end, two or three ciliated band segm ents comprised of densely packed, elongate cilia. These cilia may be expres sions of the epaulettes that would have been present in an ancestral larval form, represented today by the feeding echinopluteus of H. tuberculata. We compared the development and cellular organization of the larval ciliary s tructures of both Heliocidaris species to assess whether the ciliary bands of H. erythrogramma are expressions of the feeding ciliated band or epaulet tes of an echinopluteus. Epaulette development in feeding larvae of H. tube rculata involves separation of specific parts of the ciliated band from the rest of the feeding ciliated band, hyperplastic addition of ciliated cells and hypertrophic growth of the cilia. Like epaulettes, the ciliated bands of H. erythrogramma are composed of long spindle-shaped cells arranged in a cup-shaped collection that bulges into the blastocoel; and these cells hav e elongated cilia. In their developmental origin and topological arrangemen t however, the ciliated bands of H. erythrogramma correspond more closely w ith parts of the pluteal feeding ciliated band than with epaulettes. The la rvae of this echinoid appear to develop epaulette-like bands from parts of the original (but reduced) feeding ciliated band. The evolution of developm ent in H. erythrogramma has thus involved both conservation and change in e chinopluteal ciliary structures.