Meeting Abstract
Rotulid sand dollars have a uniquely shaped test: instead of the classic round outline with smooth edges, they have small indentations that create finger-like lunules on about a third to a half of the distal margin’s circumference. The function of these lunules, if any, is unknown. In this study, we test the hypothesis that the finger-like lunules provide the same lift-reduction benefits as the hole-like lunules of the Mellita spp. by measuring lift and drag forces and visualizing flow using particle image velocimetry on dynamically scaled physical models of tests of a rotulid sand dollar (Heliophora orbiculus) and a keyhole sand dollar (Mellita quinquiesperforata). Average and peak lift forces, lift-to-drag ratios, and flow visualization on the tests of the rotulid and keyhole sand dollar tests will be presented. Flow visualization shows that the patterns of flow around the rotulid sand dollar resemble a flat plate than the patterns around the keyhole sand dollar. The rotulid tests were also more unstable than those of the keyhole sand dollar, showing higher peak lift forces. It is likely that the finger-like lunules of rotulid sand dollars do not provide the same benefit, or the same extent of benefit, of lift reduction as the hole-like lunules of keyhole sand dollars.
