Contrary to popular belief, a sand dollar is not a shell
Last weekend my grandson spent the afternoon with me at the beach. One of our favorite pastimes is snorkeling at low tide to observe sea life, a laudable adventure for a middle school student headed for a career in marine biology. We didn’t even need to put on our equipment or place our faces in the water to identify something of interest. When we reached a depth of about four feet Adam made his first discovery. Wiggling his feet in the sand he found dozens of large live sand dollars.
Sand dollars are usually crowded together in a small area. As many as 625 sand dollars can live in one square yard. Reproduction is accomplished through external fertilization. The eggs and sperm are discharged into the water. The free-swimming larvae metamorphose through several stages before the “test” begins to form.
Of course in compliance with local regulations, after enjoying this wonderful learning experience, our specimens were gently placed back on the sea floor.
One of the most common misnomers is that a sand dollar is a shell. The sand dollar is from a group of marine creatures known as echinoderms, a phylum comprised of spiny skinned creatures like sea urchins, sea stars, and sea cucumbers.
This invertebrate is really the skeleton or “test” of a marine mammal. It is circular, flat, thin, hard and fragile. Usually sand dollars are about three inches in diameter. They got their name because they resemble a one-dollar coin. There are many species of sand dollars but the one found most commonly on Sanibel is Mellita quinquiesperforata (five- keyhole sand dollar).
Writers have compared echinoderms to living, moving castles. This comparison has been made because castles are made of interlocking blocks, with a single main entrance and lots of slit windows for ventilation and defense. Echinoderm skeletons are made up of interlocking calcium carbonate plates and spines. Between the skeletal plates, a number of very special structures protrude. These structures are used for breathing, moving and defense.
All echinoderms have a water-vascular system. There is a set of water-filled canals that branch from a ring canal that surrounds the gut. The canals lead to tube-feet, which are sucker-like appendages that can be used to move, hold on to things or manipulate objects. The pressure in the water-vascular system causes the tube feet to be extended or retracted.
The sand dollar’s mouth has a jaw with five teeth-like sections. Sometimes a sand dollar chews its food for 15 minutes before swallowing. It can take two days for the food to be digested.
The exoskeleton (test) of the sand dollar that is found on a beach is white. The top of the sand dollar has numerous tiny perforations that form a five-rayed pattern of pores arranged in a symmetrical petal-like design, illustrating their close relationship to sea stars.
But, the live sand dollar looks quite different. It is a maroon or deep violet-brown color. The live sand dollar is covered with small brown spines covered with tiny hair-like cilia that feel like fur, give them a fuzzy appearance, and are used to ferry food particles to the mouth which is on the underside of the creature. The spines and cilia of the live sand dollar actually hide the star design found on those that are no longer living.
The sand dollar also uses the spines to move along the sand. When the waters are quiet, they stand on end and are only partially buried in the sand but when the sea becomes rough; they either lie flat or burrow under the sand.
Algae and other organic particles found on the bottom of the sea provide a hardy diet for the sand dollars. Sand dollars are rather crunchy and therefore not attractive to a number of predators. However, helmet snails, fig snails, flounders, birds, and otters feast on sand dollars.
Scientists can determine the age of a sand dollar by counting the growth rings on the plates of the exoskeleton. Sand dollars usually live for six to 10 years. Like other echinoderms, sand dollars can repair minor damage but if a large part of the sand dollar is broken, it will probably die.
If you break a sand dollar apart, you will discover that it is hollow, but there are “shelly” bars or struts. Also, when you break the sand dollar open, you will find five teeth in the shape of doves.
It is interesting to note that sand dollar eggs have been extensively studied to better understand cell division and some diseases such as cancer, which is associated with uncontrolled cell growth.