EMBRYOGENESIS AND LARVAL DEVELOPMENT OF THE ASTEROID PATIRIELLA-REGULARIS VIEWED BY LIGHT AND SCANNING ELECTRON-MICROSCOPY

The sea star Patiriella regularis (Verrill, 1867) has indirect development through bipinnaria and brachiolaria larvae. Development of this species is typical of asteroids with planktotrophic larvae and takes 9-10 weeks. The embryos develop through a wrinkled blastula and hatch as early gastrulae. In contrast to most asteroids, a third enterocoel forms on the left side of the stomach of the bipinnaria. This structure gives rise to the left posterior coelom; its significance is discussed. We suggest that this coelom is homologous to the trunk coelom in enteropneust embryology. The surface features of the larvae were examined by scanning electron microscopy. Newly hatched gastrulae are covered by cilia, and the bipinnaria have bands of cilia that follow the contours of the larval processes. A previously undescribed plug-like structure positioned on the post-oral surface appears to function as a seal for the mouth. Brachiolaria larvae have three brachiolar arms and a centrally located adhesive disc. Each arm is covered by adhesive papillae. Raised epithelial cells that dot the surface of the papillae and adhesive disc may be batteries of secretory cells. The brachiolar arms have an extracellular coat that may serve as a protective cover for the adhesive surfaces. Competent brachiolaria swim along the substratum and exhibit searching behavior with flexure of the median brachium. They settle on the undersides of natural shell substrata and do not respond to a primary algal film. Shade appears to be an important factor in settlement and metamorphosis in P. regularis. Metamorphosis takes 5-6 days, and the post-larvae take up a free existence at a diameter of 450-500-mu-m. The indirect development of P. regularis contrasts with the lecithotrophic and viviparous modes of development of other Patiriella species and provides the comparative basis to determine the ontogenic changes involved with evolution of direct development in the genus. The use of the divergent life histories of Patiriella as a model system for the study of evolutionary change in development is discussed.