Highly Derived Coelomic and Water-Vascular Morphogenesis in a Starfish with Pelagic Direct Development

The coelomic development of the starfish Pteraster tesselatus (order Velatida, family Pterasteridae) is fundamentally different from that reported for all other asteroids. Coeloms arise from seven separate enterocoels that evaginate from different regions of the archenteron. The water-vascular coelomic system develops from the first five enterocoels (homologous to hydrocoel lobes) which extend radially, in a transverse orientation, from the central region of the archenteron. All other coelomic compartments derive from two enterocoels that evaginate later in development from posterior regions of the archenteron. This mode of coelom formation in P. tesselatus leads directly to the adult organization. We hypothesize that this altered pattern of coelomogenesis evolved from the pattern that occurs in the larvae of other spinulosacean asteroids, by a rotation in the site of origin of the anterior enterocoels relative to the archenteron. The altered pattern of coelomogenesis accounts for most of the unusual features of development in P. tesselatus: parallel embryonic and adult axes of symmetry, transverse orientation of the juvenile disk, absence of bilateral symmetry, absence of purely larval structures, and the lack of a metamorphosis. We conclude, contrary to previous interpretations, that P. tesselatus does not have a larval stage and thus represents the only described case of truly direct development in the asteroids.