Field Observations of Dendraster excentricus, a Sand Dollar of Western North America

Abstract

Observations were made on behavior, distribution, and biotic relationships of the sand dollar Dendraster excentricus along the Pacific coast of California and Baja California, Mexico. Populations occur on sandy bottom in bays, tidal channels, and along the outer coast. Orientation and feeding behavior, which are related to water movement, vary between populations in the different habitats. In sheltered bays, where there is relatively little water movement, sand dollars remain in shallow water, frequently move, and lie flat on the bottom where they feed on deposited material. In tidal channelsand protected areas of the outer coast, habitats with moderate water movement, adults are more stationary, usually are in an inclined position, and feed primarily on suspended material. Density estimates and aggregation values are greatest in these populations. In outer-coast areas consistently exposed to heavy seas, where water movement is greatest, sand dollars are usually buried, and presumably feed on deposited material. Two types of distribution patterns occur in populations along the protected outer coast. In one type, the populations run parallel to shore as dense bands, and maintain a characteristic pattern that is reformed when disrupted by occasional heavy seas. Usually juveniles are most abundant shoreward, and move seaward with age. The outer margin of these populations is well-defined in 4-12 m of water, and here the largest individuals and greatest densities occur. Most of our observations are of these populations. The other type of distribution is similar, except that it extends into deeper water, and below about 10-15 m; individuals become progressively smaller with depth. Various other organisms coexist with the sand dollars, many of them using the inclined sand dollars as shelter or a hard substrate. Known predators include fishes, crabs and sea stars. Fouling by the barnacle Balanus pacificus also may contribute to sand-dollar mortality. At present, separate taxa for the ecologically different populations of D. excentricus are not justified.