MORPHOLOGY AND DEVELOPMENT OF A UNIQUE TYPE OF PELAGIC LARVA IN THE STARFISH PTERASTER-TESSELATUS (ECHINODERMATA, ASTEROIDEA)

Several unusual features characterize the morphology of the pelagic larva of the starfish Pteraster tesselatus and its metamorphosis into the juvenile stage: (1) morphogenesis of the supradorsal membrane during metamorphosis by fusion of 15 lobes on the aboral region of the body; (2) absence of brachiolar arms and attachment disk; (3) heterochronic acceleration of development in the water vascular system, and use of podia for attachment to the substratum at settlement; (4) radial (rather than bilateral) symmetry of the larva; and (5) congruent larval and adult axes of symmetry, and a transverse orientation of the adult rudiment within the larva. Collectively, these features demonstrate that P. tesselatus has a highly derived mode of development and a larva that is unique among the asteroid echinoderms. In contrast to the current interpretation of this larva as a modified pelagic brachiolaria, I suggest that the unusual larva of Pteraster represents an example of an apparently rare evolutionary transition in animal development: the re-evolution of pelagic larval development from benthic brooding.