Consequences of the Calcite Skeletons of Planktonic Echinoderm Larvae for Orientation, Swimming, and Shape.

How the echinoderm larval skeleton is used for support of larval arms, passive orientation, and swimming was examined by experimentally removing the skeletons of plutei and by comparing feeding larvae from four echinoderm classes. All four types of echinoderm larvae oriented with their anterior ends upward in still water, but removing the skeletons of both live and dead four-armed echinoplutei demonstrated that their skeletons enhanced passive vertical orientation with their anterior ends upward. In comparisons of dead four-armed echinoplutei with and without skeletons, the skeleton contributed more than half of the excess density and sinking speed. In comparisons of all four types of feeding echinoderm larvae, larvae with a greater volume of skeleton and a smaller volume of tissues and body cavity were densest. The calculated work necessary to prevent the plutei from sinking was much less than 1% of the total aerobic energy expenditure. Thus calcite skeletons are not essential for passive vertical orientation by echinoderm larvae but enhance it, and the increased density and sinking rates impose little energetic cost in locomotion. The evolution of larval skeletons may have been influenced by the benefits of passive orientation and by the low costs of swimming with a skeleton. Whatever the primary function of skeletons, the position and form of skeletal elements is influenced by the functional requirement for higher mass posteriorly for passive orientation. Features that enhance passive vertical orientation include posterior ossicles and skeletal rods, posterior thickening of skeletal rods, and formation of juvenile parts near the posterior ends of larvae.