Evolution of intragonadal development in the diminutive asterinid sea stars Patiriella vivipara and P-parvivipara with an overview of development in the Asterinidae

The diminutive asterinid sea stars Patiriella vivipara and P. parvivipara incubate their embryos in the gonads to the advanced juvenile stage. Despite the small size of their eggs (135-150 mu m diameter), development is lecithotrophic. Development proceeds through the wrinkled blastula, gastrula, and brachiolaria larval stages. The gastrulae and larvae are uniformly ciliated and swim, propelled by the cilia, in the gonadal fluid. The brachiolaria is pear-shaped and has a vestigial brachiolar attachment complex composed of three small brachia. At no stage in development are the embryos attached to the gonad. Metamorphosis occurs as the larvae swim in the gonadal lumen. Internal development involves formation of one large enterocoel at the anterior end of the archenteron and one small posterior enterocoel on the left side of the archenteron. The archenteron closes to form the rudiment for the adult gut. As a result of the small size of the egg and the nonfeeding mode of development, the larvae of P. vivipara and P. parvivipara are minute, about 270 mu m and 210 mu m in length, respectively. Newly metamorphosed juveniles are about 240 mu m and 310 mu m in diameter, respectively. Postmetamorphic development involves substantial growth of the juveniles, which leave the parent at a diameter between 1.0 and 5.0 mm. The presence of a vestigial brachiolar complex and lecithotrophic development indicates that these species had a free-living lecithotrophic brachiolaria in their ancestry. We suggest that the evolution of viviparity in Patiriella sp. involved retention of a large egg by an ancestor that had a lecithotrophic brachiolaria followed by a secondary reduction in the size of the ovum and simplification of the larva. The range of life histories seen in Patiriella is atypical of asteroid genera and supports the contention that the evolution of viviparity and other modes of modified development in the Asteroidea follows phylogenetic lineages.