Swimming performance as a constraint on larval morphology in plutei

Understanding evolutionary mechanisms constraining larval morphology is challenging, both because morphology has many potential functional consequences and because the impacts of morphological variation on most functions are difficult to quantify. We assessed the potential importance of swimming as a factor driving larval morphology by hypothesizing that larvae of the sand dollar Dendraster excentricus are at or near a local maximum in swimming performance as a function of their morphological characteristics. To test this hypothesis, we used a hydrodynamic model parameterized to represent the 4-armed larval stage to predict the consequences of morphological changes on larval movement in still water and shear flows. We created 3 morphological families, consisting of series in which either larval body skeleton volume, relative arm length, or arm elevation angle were incrementally altered from observed morphologies, and used upward swimming velocity as a metric of performance. For each morphological family, a wide subset of alternate morphologies performed better than the observed morphology in still water. However, in shear flows, the observed morphology consistently fell within the narrow morphological range that minimized rapid downward velocities. These results suggest that larval swimming performance, especially performance in moving water, is a driving factor in the evolution of D. excentricus larval morphology. Further, our results suggest that small morphological changes resulting from environmental shifts, such as those predicted to result from ocean acidification, may have potentially large deleterious ecological consequences.