BODY FORM AND PATTERNS OF CILIATION IN NONFEEDING LARVAE OF ECHINODERMS - FUNCTIONAL SOLUTIONS TO SWIMMING IN THE PLANKTON

Nonfeeding larval forms of echinoderms are believed to have evolved re peatedly from feeding larval forms, and these transformations usually result in major shifts in morphogenesis. Current hypotheses on form ch ange invoke relaxation of stabilizing selection on traits that functio n in feeding, coupled with selection for rapid development of juvenile traits. However, comparative evidence from 51 species of nonfeeding l arvae, representing 19 independent origins, suggests that body form, p atterns of ciliation, and possibly buoyancy reflect functional require ments for maintenance of swimming performance. Nonfeeding larvae with body lengths less than 600 mu m usually have several transverse ciliat ed bands, while those with body lengths greater than 800 mu m usually have uniform ciliation. A preliminary model which compares estimated d rag and buoyancy forces with ciliary propulsive forces predicts that b ands of simple cilia do not produce sufficient propulsive forces to pe rmit swimming in larger larvae. For larger larvae, increases in areal coverage of cilia may be required to produce propulsive forces suffici ent to oppose drag and buoyancy forces and permit movement. For these larger larvae, estimates of water velocities at the tips of uniform ar rays of cilia are well below the upper limits of water movements by ci lia of echinoderms. Functional constraints on nonfeeding larval forms should be considered, along with (above mentioned) current hypotheses, in explanations of morphogenetic changes associated with transition f rom feeding to nonfeeding larval development.