LARVAL DEVELOPMENT (WITH OBSERVATIONS ON SPAWNING) OF THE PENCIL URCHIN PHYLLACANTHUS-IMPERIALIS - A NEW INTERMEDIATE LARVAL FORM

Information for understanding the evolutionary shift from feeding to n onfeeding in echinoderm larvae can be gained from species whose larval development pattern appears to be intermediate between these extremes . In this paper we report the development of one such species. The pen cil urchin Phyllacanthus imperialis spawned synchronously with the mas s spawning of scleractinian corals at Lizard Island, Australia, in two consecutive years. Their large yolky eggs (507 mum diameter) develope d into nonfeeding echinopluteus larvae with two pairs of larval arms. The arms were identified as postoral and posterodorsal, which are the first and third pairs in typical echinoplutei. A larval skeleton was p resent, with skeletal rods extending the length of the arms. Five prim ary podia of the juvenile rudiment appeared at 2 days of age. Metamorp hosis of the larvae and settlement began 4 days after fertilization. H istological examination of 2-day-old larvae revealed the presence of a developing gut, but no mouth opened in what would be the oral region of a typical echinopluteus, or the oral surface of the juvenile rudime nt in older larvae. Like other cidaroid larvae, this species showed no evidence of an amniotic invagination. The larva of P. imperialis appe ars to be a transitional form between the morphology of feeding and no nfeeding echinoid larvae. Traces of the ciliary band in the oral regio n and the presence of arms typical of the echinopluteus larva indicate its evolutionary past, whereas the large egg size and absence of a mo uth hint at its future. This larval form provides insights into develo pmental changes that occur during the shift from planktotrophy to leci thotrophy in echinoid larvae.