CHANGES IN LARVAL MORPHOLOGY IN THE EVOLUTION OF BENTHIC DEVELOPMENT BY PATIRIELLA-EXIGUA (ASTEROIDEA, ASTERINIDAE), A COMPARISON WITH THE LARVAE OF PATIRIELLA SPECIES WITH PLANKTONIC DEVELOPMENT

Patiriella exigua(lamarck)isa small asterinid sea star that deposits large eggs onto the substratum. Development is lecithotrophic and entirely benthic, proceeding without parental care. The embryos develop to the brachiolaria stage before hatching and there is no trace of a bipinnaria larva. In its early stage the larva of P. exigua resembles a planktonic brachiolaria in having one long central brachium and two short lateral brachia. By hatching, the brachia are equal in length, giving the larva a tripod-like appearance. Comparison of the larva of P. exigua with the brachiolaria of Patiriella species with planktonic development supports the hypothesis that the tripod larval form results from differential growth of the lateral brachia. At hatching, the P. exigua larva has a well-developed attachment complex composed of a large adhesive disk and three muscular brachia; the latter bear a striking resemblance to adult tube feet. This hypertrophic elaboration of the brachiolar complex is an adaptation for permanent benthic attachment. Internally, one large enterocoel forms at the anterior end of the archenteron. The archenteron then closes to form the rudiment for the adult gut. Metamorphosis involves gradual resorption of the brachiolar complex concomitant with formation of the first tube feet. The adhesive disk plays a major role in attachment during late metamorphosis but is gradually reduced to a plug of tissue as the tube feet become functional. Juvenile P. exigua are negatively geotactic and float on the water surface, behavior that may act as a mechanism for dispersal. The similarity of the early larva of P. exigua to planktonic brachiolariae suggests that the evolution of benthic lecithotrophy by this species involved modification of a planktonic larval form. These modifications include elimination and reduction of larval feeding structures, formation of one rather than three enterocoels, and hypertrophy of the brachiolar complex to form a tripod larva. Heterochronies in the ontogeny of P. exigua include the delay in hatching to the brachiolaria stage and the accelerated development of the juvenile form and adult skeleton.