Good eaters, poor swimmers: compromises in larval form

Compromises between swimming and feeding affect larval formand behavior. Two hypotheses, with supporting examples, illustratethese feeding-swimming trade-offs. (1) Extension of ciliatedbands into long loops increases maximum clearance rates in feedingbut can decrease stability of swimming in shear flows. A hydromechanicalmodel of swimming by ciliated bands on arms indicates that morphologieswith high performance in swimming speed and weight-carryingability in still water differ from morphologies conferring highstability to external disturbances such as shear flows. Instabilityincludes movement across flow lines from upwelling to downwellingwater in vertical shear. Thus a hypothesis for the high armelevation angles of sea urchin larvae, which reduce speed instill water, is that they reduce a downward bias imposed bythe vertical shear in turbulence. Observations of sea urchinlarvae in vertical shear and comparisons among brittle starlarvae are consistent with the performance trade-offs predictedby the model. (2) Structures and behaviors that reduce swimmingspeed can enhance filtering for feeding. In the opposed-bandfeeding mechanisms of veligers and many trochophores, ciliapush water to swim but movement of cilia relative to the wateroccurs when cilia overtake and capture particles. Features thatmay increase clearance rates at the expense of speed and weightcapacity include structures that increase drag or body weightand a ciliary band that beats in opposition to the feeding-swimmingcurrent. Larval feeding mechanisms inherited from distant ancestorsresult in different swimming-feeding trade-offs. The differenttrade-offs further diversify larval form and behavior.