Locomotion, Drag, and the Rigid Skeleton of Larval Echinoderms

A model for the forces generated by a swimming pluteus larva is constructed by characterizing flow over the ciliated band and experimentally determining drag on different larval shapes. These and other mechanical calculations are used to examine a hypothesis on mechanical variation of the larval skeleton. Drag on a larva increases with arm number and arm length and with increasingly perpendicular orientation of arms to flow. Drag on arms may reduce sinking rate. Of the two types of skeletal rods found in plutei, fenestrated arm spicules have a flexural stiffness three to five times greater than simple arm spicules. Fenestrated spicules are associated with longer arms in plutei, but they may not be necessary to prevent deflection of arms under forces produced by ciliary locomotion. Other alternatives which may explain fenestration are discussed.