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Wild Thing: Sand Dollar

Scour the beach for these intricate skeletal remains.

By Sheryl Smith-Rodgers

Stroll along a Texas beach this summer and you'll likely find sand dollars (Mellita quinquiesperforata), delicate beauties bleached white by the sun and treasured by beachcombers for their symmetrical, flat shape.

Often mistaken simply for a shell, the sand dollars you find washed ashore are actually skeletal remains of sea animals that burrow into the sandy ocean floor. Related to sea urchins, heart urchins and sea biscuits, sand dollars – also called keyhole urchins – lack arms and use spines to move around.

Those fuzzy, gray spines cover both sides of a live sand dollar's endoskeleton (called a test). Topside, five symmetrical petalloids resemble their namesake – flower petals – and serve as gills. As for the five oval holes, called lunules, researchers believe they're used to pass algae, bacteria and other planktonic food to an urchin's mouth, located on the bottom side.

If you break open a brittle sand dollar, five tiny "doves" will fall out. These pieces, arranged in a circle that's called "Aristotle's lantern," work as jaws to crush and chew food.

Sand dollars live in dense colonies and scavenge for food by night. They breed in late spring and summer by external fertilization. This occurs when females and males respectively release their eggs and sperm into the water. The young, easily swept away by tides, swallow sand to help anchor them in place. Generally, sand dollars live approximately four years before they die and end up in some beachcomber's bucket.

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