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Form, performance and trade-offs in swimming and stability of armed larvae

Diverse larval forms swim and feed with ciliary bands on arms or analogous structures. Armed morphologies are varied: numbers, lengths, and orientations of arms differ among species, change through development, and can be plastic in response to physiological or environmental conditions. A hydromechanical model of idealized equal-armed larvae was used to examine functional consequences of these varied arm arrangements for larval swimming performance. With effects of overall size, ciliary tip speed, and viscosity factored out, the model suggested trade-offs between morphological traits conferring high swimming speed and weight-carrying ability in still water (generally few arms and low arm elevations), and morphologies conferring high stability to external disturbances such as shear flows (generally many arms and high arm elevations). In vertical shear, larvae that were passively stabilized by a center of buoyancy anterior to the center of gravity tilted toward and consequently swam into downwelling flows. Thus, paradoxically, upward swimming by passively stable swimmers in vertical shear resulted in enhanced downward transport. This shear-dependent vertical transport could affect diverse passively stable swimmers, not just armed larvae. Published descriptions of larvae and metamorphosis of 13 ophiuroids suggest that most ophioplutei fall into two groups: those approximating modeled forms with two arms at low elevations, predicted to enhance speed and weight capacity, and those approximating modeled forms with more numerous arms of equal length at high elevations, predicted to enhance stability in shear.