Larval Development (with Observations on Spawning) of the Pencil Urchin Phyllacanthus imperialis: a New Intermediate Larval Form?

Abstract

Information for understanding the evolutionary shift from feeding to nonfeeding 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 pencil urchin Phyllacanthus imperialis spawned synchronously with the mass spawning of scleractinian corals at Lizard Island, Australia, in two consecutive years. Their large yolky eggs (507 μm diameter) developed 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 present, with skeletal rods extending the length of the arms. Five primary podia of the juvenile rudiment appeared at 2 days of age. Metamorphosis of the larvae and settlement began 4 days after fertilization. Histological 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 rudiment in older larvae. Like other cidaroid larvae, this species showed no evidence of an amniotic invagination. The larva of P. imperialis appears to be a transitional form between the morphology of feeding and nonfeeding echinoid larvae. Traces of the ciliary band in the oral region and the presence of arms typical of the echinopluteus larva indicate its evolutionary past, whereas the large egg size and absence of a mouth hint at its future. This larval form provides insights into developmental changes that occur during the shift from planktotrophy to lecithotrophy in echinoid larvae.