If you've strolled along a beach in Maine, you've probably found a sand dollar. But what is a sand dollar? Children have been known to say that sand dollars are pressed sand that has been dried or even the money of mermaids washed-up from the deep. In reality, the fragile disk is the skeleton or "test" of a marine animal. By the time the test washes up on the beach it is missing its velvety covering of minute spines and appears somewhat bleached from the sun. It is hard to believe it was once alive.
Sand dollars are from the class of marine animals known as Echinoids, spiny skinned creatures. Their relations include the sea lily, the sea cucumber, the star fish and the sea urchin. When alive, the local species, Echinarachnius parma is outfitted in a maroon-colored suit of moveable spines that encompass the entire shell. Like its close relative the sea urchin, the sand dollar has five sets of pores arranged petal pattern. The pores are used to move sea water into its internal water-vascular system which allows for movement.
Sand dollars live beyond mean low water on top of or just beneath the surface of sandy or muddy areas. The spines on the somewhat flattened underside of the animal allow it to burrow or to slowly creep through the sand. Fine, hair-like cilia cover the tiny spines. These cilia, in combination with a mucous coating, move food to the mouth opening which is in the center of the star shaped grooves on the underside of the animal. Its food consists of plankters and organic particles that end up in the sandy bottom.
Due to their diminutive edible parts and relatively hard skeleton, few animals bother sand dollars. One animal found to enjoy them on occasion is the thick-lipped, eel-like ocean pout.
On the ocean bottom, sand dollars are frequently found together. This is due in part to their preference of soft bottom areas as well as convenience for reproduction. The sexes are separate and gametes are released into the water column as in most echinoids. The free-swimming larvae metamorphose through several stages before the test begins to form and they become bottom dwellers.
Since the sand dollar lives in sandy locations, anyone who would like to collect their shells should comb beaches as the tide recedes. The very best time for collecting is after a heavy storm, as many of the shells that have died are dredged up by the increased wave action.
Information for this article was taken from:
- Gosner, Kenneth L., Guide to Identification of Marine and Estuarine Invertebrates; 1971 by John Wiley & Sons, Inc.
- Robbins, Sarah Fraser and Clarice Yentsch, The Sea Is All About Us ; 1973 Harcourt Brace Jovanovich, Inc.