Body form and patterns of ciliation in nonfeeding larvae of echinoderms: Functional solutions to swimming in the plankton?

Abstract

SYNOPSIS. Nonfeeding larval forms of echinoderms are believed to have evolved repeatedly from feeding larval forms, and these transformations usually result in major shifts in morphogenesis. Current hypotheses on form change invoke relaxation of stabilizing selection on traits that functionin feeding, coupled with selection for rapid development of juvenile traits. However, comparative evidence from 51 species of nonfeeding larvae, representing 19 independent origins, suggests that body form, patterns of ciliation, and possibly buoyancy reflect functional requirements for maintenance of swimming performance. Nonfeeding larvae with body lengths less than 600 μm usually have several transverse ciliated bands, while those with body lengths greater than 800 μm usually have uniform ciliation. A preliminary model which compares estimated drag and buoyancy forces with ciliary propulsive forces predicts that bands of simple cilia do not produce sufficient propulsive forces to permit swimming in larger larvae. For larger larvae, increases in areal coverage of cilia may be required to produce propulsive forces sufficient to oppose drag and buoyancy forces and permit movement. For these larger larvae, estimates of water velocities at the tips of uniform arrays of cilia are well below the upper limits of water movements by cilia 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 from feeding to nonfeeding larval development. ©1994 by the American Society of Zoologists.