Stuff you never think about...

The "sand dollar"! Everyone that has been down to the beach has picked up a few. Gift shops are loaded with items made from them.

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When you're walking on the beach, you may find a sand dollar. What you'll usually find is something called a test, which is the skeleton of a dead sand dollar. The test is usually white or grayish-white, with a star-shaped marking in its center. The name for these animals (yes, they are animals!) came from their likeness to silver dollars.

When they are alive, sand dollars look much different. They are covered with short, velvety spines that may be purple, reddish brown, yellowish, gray, green or black in color.

Here you can learn more about what sand dollars look like, what they eat, where they live and how they reproduce.

What Is a Sand Dollar?

Sand dollars are echinoderms, which means they are related to sea stars, sea cucumbers, and sea urchins. In fact, they are basically flat sea urchins and are in the same class, Echinoidea, as sea urchins. This class is divided into two groups - the regular echinoids (sea urchins and pencil urchins) and irregular echinoids (includes heart urchins, sea biscuits, and sand dollars). The irregular echinoids have a front, a back and basic bilateral symmetry on top of the "normal" pentameral symmetry (5 parts around a center) that regular echinoids possess.
The test of the sand dollar is its endoskeleton - it is called an endoskeleton because it lies underneath the sand dollar's spines and skin. The test is made of fused calcareous plates. This is different than the skeletons of other echinoderms.

Sea stars, basket stars, and brittle stars have smaller plates that are flexible, and the skeleton of sea cucumbers is made up of tiny ossicles buried in the body. The top (aboral) surface of the sand dollar test has a pattern that looks like five petals. There are 5 sets of tube feet that extend from these petals, which the sand dollar uses for respiration.

The sand dollar's anus is located at the rear of the animal. Sand dollars can move by using the spines located on their underside. 

Species and Classification of Sand Dollars

There are many species of sand dollars. Those commonly found in the United States include:

• Echinarachnius parma (Common sand dollar), which may be found in temperate waters of both the Pacific and the Atlantic Ocean. They are 2-4 inches across and have spines that are purple, reddish-purple or brown in color.
• Dendraster excentricus (Excentric, western, or Pacific sand dollar) are found in the Pacific Ocean from Alaska to Baja, California. These sand dollars grow to about 4 inches across and have gray, purple or blackish spines.
• Clypeaster subdepressus (Sand dollar, sea biscuit), which are found in the Atlantic Ocean and the Caribbean Sea, from the Carolinas to Brazil. 
• Mellita sp. (Keyhole sand dollars or keyhole urchins), which are found in tropical waters in the Atlantic, Pacific and the Caribbean Sea. There are approximately 11 species of keyhole sand dollars.

Sand dollars are classified as follows:

• Kingdom: Animalia
• Phylum: Echinodermata
• Class: Clypeasteroida (includes sand dollars and sea biscuits)

Habitat and Distribution

As their name suggests, sand dollars prefer to live in the sand.

They can use their spines to burrow into the sand, where they seek protection and food. They live in relatively shallow waters. 

Feeding and Diet

Sand dollars feed on small food particles in the sand. The particles land on the spines, and then are transported to the sand dollar's mouth by its tube feet, pedicellaria (pincers) and mucous-coated cilia. Some sea urchins rest on their edges in the sand to maximize their ability to catch prey that is floating by. Like other sea urchins, the mouth of a sand dollar is called Aristotle's lantern and is made up of 5 jaws. If you pick up a sand dollar test and shake it gently, you may hear the pieces of the mouth rattling inside.

Reproduction

There are male and female sand dollars, although, from the outside, it is difficult to tell which is which. Reproduction is sexual and accomplished by the sand dollars releasing eggs and sperm into the water.

The fertilized eggs develop into tiny larvae, which feed and move using cilia. After several weeks, the larva settles to the bottom, where it metamorphoses 

Conservation and Human Uses

Visit a shell shop and you may find poems or sand dollars with the Legend of the Sand Dollar, which references Easter, Christmas, and Jesus. Some references say that the 5-pointed "star" in the center of the top of the sand dollar's test is said to represent the Star of Bethlehem that guided the wise men to the baby Jesus. The 5 openings in the test are said to represent Jesus's wounds during his crucifixion - the 4 wounds in his hands and feet and the 5th in his side. On the underside of the sand dollar test, it is said that there is an outline of a Christmas poinsettia. The legend also says that if you break open a sand dollar, you'll find 5 "doves of peace" inside. These doves are actually the 5 jaws of the sand dollar's mouth (Aristotle's lantern).
Dried sand dollar tests are often sold in shops for decorative purposes or souvenirs. In addition to the legend of the sand dollar related to Jesus, other lore about sand dollars references the washed-up tests as mermaid coins or coins from Atlantis.
Sand dollars may be affected by fishing, especially from bottom trawling, ocean acidification, which may affect the ability to form the test; climate change, which might affect available habitat; and collection. (Although you can find plenty of information on how to preserve sand dollars, you should collect only dead sand dollars, never live ones.)
Sand dollars are not eaten by humans, but they can be prey for sea stars, fish, and crabs.

by Jennifer Kennedy

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:o)