Hydrodynamic analysis of feeding in sand dollars

Subtidal sand dollars, Dendraster excentricus, assume an inclined posture under conditions of moderate water flow (10 cm s^-1 to 2 m s^-1). In this posture, when the test is in the usual position parallel to the water flow, the test acts as a lifting body. Analysis of the hydrodynamic characteristics of the sand dollar test was accomplished by slender body theory. Streamline curvature due to lift moves food particles in the direction of the feeding surface and enhances feeding efficiency. The position parallel to water flow is most advantageous to sand dollars in reversing flows, such as wave surge, whereas sand dollars in unidirectional flows can place the test at a slight angle to the water flow to increase lift. The high camber of sand dollars from a bay habitat characterized by low water velocities was interpreted as an adaptation allowing efficient lift production in low velocity flows; sand dollars from a protected outer coast location characterized by higher water velocities were significantly less cambered.Within sand dollar beds, the animals alter their spacing as a function of the water velocity. This movement is construed as a method for maintaining the optimal gap between sand dollars. Sand dollar tests induce mutual interference effects which vary with the configuration of the animals. In the prevalent configuration, sand dollars are able to take advantage of the streamline curvature induced by neighboring animals. The optimal gap increases as the square of the water velocity. At distances grater than or less than the optimal gap, feeding efficiency was shown to decrease from that at the optimal gap.