STRATHMANN, RR*; GR�NBAUM, D; Univ. of Washington, Friday Harbor; Univ. of Washingston, Seattle: Good Eaters, Poor Swimmers: Compromises in Larval Form

Ciliated larvae in six phyla achieve high clearance rates (the volume of water cleared of food per time) by development of long ciliary bands extended on arms, tentacles, or lobes. In evolution of pelagic larvae, these body extensions are lost when feeding is lost. Also, many feeding larvae develop bands that are not on arms or lobes when they become competent to settle, and these changes increase swimming speed. These evolutionary and developmental transformations suggest trade-offs between capacity for feeding and capacity for swimming. A hydromechanical model of swimming by ciliated bands on arms suggests a hypothesis for these trade-offs. The model indicates that morphologies with high performance in swimming speed and weight-carrying ability in still water differ from morphologies conferring high stability to external disturbances such as shear flows. According to this model, at least one aspect of performance is compromised when swimming is by ciliary bands on arms or lobes. Observations of echinoplutei in vertical shear and comparisons among ophioplutei are consistent with predictions of the model. Feeding-swimming trade-offs appear to occur in other kinds of larvae as well. For example, comparisons among annelids and mollusks with opposed band feeding mechanisms suggest that longer prototrochal cilia enhance swimming more effectively than feeding.