LARVAL SPICULES, CILIA, AND SYMMETRY AS REMNANTS OF INDIRECT DEVELOPMENT IN THE DIRECT-DEVELOPING SEA-URCHIN HELIOCIDARIS-ERYTHROGRAMMA

Nonfeeding larvae of the echinoid Heliocidaris erythrogramma were rais ed in culture and examined for expression of a larval skeleton and for the arrangement of the ciliated band. Opaque larvae were fixed, clear ed, and examined under polarized light for evidence of calcification. By 35 hr after fertilization (at 22 degrees C), a pair of triradiate s picules was present at the posterior end of the larvae. Each member of this pair formed a fenestrated spicule as it grew laterally. This pai r and another pair which formed subsequently, were arranged across a p lane of bilateral symmetry orthagonal to the juvenile oral aboral axis . These paired larval spicules can be identified as reduced expression s of postoral and posterodorsal rods found in plutei, and their expres sion indicates that the juvenile rudiment of H. erythrogramma forms on the left side and that larval body axes are conserved in this modifie d larva. By 44 hr the ciliated band formed as an incomplete transverse loop of three segments at the posterior end and on the dorsal surface of the ovoid larva. Cilia in these segments grew to lengths of 45-50 mu m, longer than other swimming and feeding cilia reported for echino derm larvae. Band segments are interpreted as expressions of epaulette s (specialized swimming bands) rather than the feeding ciliated band o f the pluteus. The ciliated band segments and the larval spicules are both bilaterally symmetrical with respect to the same plane and indica te conserved larval bilateral symmetry despite the major asymmetry of the fates of cells on either side of this plane in their contribution to juvenile development. (C) 1995 Academic Press, Inc.