Sand Dollars

The Western sand dollar Dendraster excemtricus, an Echinoderm and a close relative of sea urchins and more distantly of starfish, lives in dense beds in the quieter waters on the western side of Netarts Bay. More than a hundred individuals may occupy a square meter. There are also offshore populations of Western sand dollars that live just beyond the waves. While walking the sandy beaches between the Capes, we see many of the creamy-white shells of dead sand dollars, or tests as they are properly called, which are simply the animal's skeletons that have washed ashore.

Live sand dollars are covered with a short, rough "fur" of spines that help the animal move and feed. Individuals are flat on the bottom or "oral" side and domed on the top or "aboral" side where you will find a five-pettaled flower-like pattern of pores, the ambulacra, through which extend appendages called podia that are used in respiration. The spines of the Netarts Bay sand dollars are mostly colored dark gray or black except those on the edge with the longest petal, which are light brown. It is this edge that is usually buried in the sand.

Dendraster feeds on detritus, organic particles suspended in the water or settled on the sand, plus small crustaceans or their larvae, and sometimes their own larvae. They capture and filter the food with their spines, then move it with their spines and miniature pincers called pedicellariae to food grooves on the oral side that lead to the mouth, the hole you see in the center of test. They increase their feeding efficiency by burying their anterior edges into the sand so they stand obliquely with their bodies parallel to the passing current. This enables them to influence the current and collect more food. When the sand dollars are exposed to air at low tide, they bury themselves completely by using their spines to push into the sand like little bulldozers until they are fully covered and are protected from the hot sun and predators, such as birds. Crabs, starfish such as Pycnopodia healianthoide, and finfish also feed on sand dollars. You may see individuals scarred by crabs that have pinched off pieces along their edges. Sand dollars can live to an old age of 15 years or more.

Text and Photographs by Jim Young
Oceanside, Oregon
jsy4990@embarqmail.com

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