You can find them in most Lowcountry gift shops — those sun-bleached, disc-like “shells” of sand dollars, popular as decorations and souvenirs.
Each cookie-shaped, calcium carbonate skeleton once protected the soft body parts of a small invertebrate that spent most of its life burrowing into the ocean floor.
Sand dollars are echinoderms, related to starfish and sea urchins, and they’re common in the intertidal zones of the Atlantic and Pacific coasts.
The outer skeleton, or “test,” of a living sand dollar looks quite different from the gift shop version, as it’s covered by a thin, velvety skin and numerous short spines. The spines help the animal creep along the sand or bury itself in sediments.
On the top surface of the test is a flower-like pattern of pores radiating around a central, perforated plate.
Seawater flows through this plate into an internal hydraulic canal system that ends in many tiny projections, called tube feet, which emerge from the pores in the “petals.” It’s through the thin walls of the tube feet that the sand dollar absorbs oxygen from its surroundings.
On the bottom side of the test are still more tube feet, which help funnel minute organic particles along a network of mucus-lined grooves towards the mouth in the disc’s center. Here, food and debris are strained and chewed via a complex feeding apparatus of muscles, calcareous plates, and sharp teeth.
Sand dollars may pack together in huge aggregations, sometimes standing up on their edges, in shallow water. Females release eggs and males emit sperm through tiny pores on the top surface of the test.
Fertilization occurs in the surrounding seawater.
A sand dollar begins life as a tiny, free-swimming larva that looks nothing like the adult. Eventually it assumes more sand-dollar- like shape, forming its own test, unless it’s eaten first by fish or other predators.
Some kinds of sand dollars have slit-like holes, called lunules, in the test. The possible functions of these holes have intrigued biologists for decades. They may facilitate feeding, according to one hypothesis — and/or they may have a hydraulic function, reducing the risk of being dislodged from the sand by strong waves.
A common species in our area is the keyhole sand dollar (Mellita quinquiesperforata), which has five lunules and is widely distributed along the East coast.
You can tell if a sand dollar is alive by its reddish or brownish color and the presence of all those tiny spines, which are lost after it dies. You may also see the tiny, waving tube feet.
Don’t bring a living sand dollar home — it will die if removed from its seashore habitat.
Also, it’s illegal to collect live sand dollars from Hilton Head beaches. But you can keep the dried, sun-bleached tests of nonliving animals, which make nice souvenirs.
Vicky McMillan, a retired biologist formerly at Colgate University,lives on Hilton Head Island. She can be reached at email@example.com.