THE FUNCTIONAL SIGNIFICANCE OF THE LUNULES IN THE SAND DOLLAR,MELLITA QUINQUIESPERFORATA

Abstract

Experiments indicate that the lunules in Mellita quinquiesperforata function as food gathering devices. Allometric analysis of dead specimens shows that the lunules grow faster than does the body as a whole. This is analogous with the situation in certain fossil brachiopods, in which the lophophore (feeding structure) grows faster than the body. Field experiments involving sand dollars with plugged lunules show that righting and burrowing are not affected. However, animals with plugged lunules cannot feed as well as normal individuals, based on analysis of stomach contents. Thus, the lunules play an important role in feeding, although the exact mechanism has not been conclusively demonstrated. Our study, as well as previous work, suggest that the lunules act as a site for the passage of food particles from the aboral (upper) surface to the oral (lower) surface. Flow tank observations suggest that the lunules may act as devices to bring water, and thus food particles, up from the sand below the animal. In scutellid sand dollars, the fossil record shows an increase in lunule number over time. In Western Atlantic species, there is also an increase in lunule number with decreasing latitude. Perhaps these correlations reflect progressive adaptation by sand dollars to less productive waters.