Eccentric sand dollar (Dendraster excentricus)
This well known inhabitant of sandy beaches can be found in the lower intertidal zone and subtidal area in Puget Sound. It ranges from Alaska to Baja California. The eccentric sand dollar, also called the pacific or west coast sand dollar, is a member of the major group of animals (called Echinoderms) that includes sea stars, brittle stars, sea cucumbers, feather stars, and it's close relative the sea urchin. Think of the sand dollar as being a flattened sea urchin.
If you look closely at the animal's surface you will find short spines and tube feet which give the living animal a dark purple color. Most beach goers have seen only the white skeleton, called a test, with it's five-pointed star pattern on the upper surface. This star pattern reflects the underlying radial symmetry of this sea star relative. The little holes that outline the star pattern are for the respiratory tube feet. The species name, eccentric, refers to the off center star pattern.
The sand dollar can grow to four inches in size (the one in the photo is three inches) and live to be 13 years old. They are eaten by starry flounder and the pink short-spined sea star. You may find the living animal buried during low tide, reappearing as the tide comes in. Subtidal individuals will lie flat on the surface in exposed areas, but when sheltered they will partially burrow and tip on their sides. Tube feet help grab detritus and diatoms which are moved along by cilia to the mouth. Sand particles come along for the ride but are discarded by the adult. However, juveniles retain heavy sand grains in their stomachs as a sort of weight belt to keep them from being washed away.
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David W. Jamison is a marine biologist and Boston Harbor resident.