Most people know the sand dollar as the dried, white "shell" found in craft stores and gift shops. What you are seeing is simply the test, or skeleton, of a once living animal. Sand dollars are related to sea stars and sea urchins, and share many of those other animals’ characteristics, including a ventral mouth, tube feet and spines.
Sand dollars belong to a group of echinoderms called echinoids. The word echinoid literally means "like a hedgehog" in Latin. And it fits this animal pretty well when you look at a live specimen. The entire body is covered by thousands of tiny brown spines.
Sand dollars are found mainly on sandy bottoms. The body is divided into two hemispheres. The top part is called the aboral surface and the bottom is called the oral surface. The mouth is in the center of this bottom oral sphere. We’ll get to the unusual feeding behavior in a minute.
The test of this animal is made up of hundreds of calcareous plates that are all fused together to form a solid protective covering for the internal organs. You can learn more about a echinoid’s test in this diary.
Like urchins, sand dollars have an opening on the ventral surface called a peristome and a dorsal opening called a periproct, although in sand dollars the periproct is located on the margin of the animal. The periproct releases wastes and the peristome contains the mouth. Here’s an excellent diagram labeling the parts of a sand dollar.
Although these animals don’t resemble their relatives the sea stars, you can see the star-shaped "petaloids" on the top surface. While sea stars are predators and sea urchins are herbivores, sand dollars are detritus feeders. Detritus is simply the organic material, including dead plankton, bacteria and animal wastes that accumulate on the ocean bottom.
As you can see in the above photo, sand dollars travel just below the surface of the sand. As they move the tiny spines and tube feet (see how tube feet work here) pick up detritus and move it towards the edges of the body. The food is then transferred to the food grooves on the ventral surface. Tube feet and ciliated spines, called clavules, move the food along the food grooves and into the mouth.
Many species of sand dollars live in rough surf, and the burrowing behavior is an adaptation to this environment. In addition to burrowing, the sand dollars will align themselves parallel to the current flow. When a large number of them are found together they look like they’ve been planted, all of them arranged in rows facing the same way.
Righting is a behavior we take for granted. This is simply the method an animal uses to turn itself over if tipped upside down. For animals covered in shells, exoskeletons or, like echinoderms, a rigid internal skeleton, righting itself isn’t an easy task. Sea stars simply bend their arms under the body to flip themselves over, and sea urchins use the tube feet that extend past the spines. For the sand dollar this is much more difficult and may take up to a half hour. An overturned sand dollar uses the spines to dig its anterior end of the body into the sand. This continues until the animal is rotated and turned rightside up. On a solid surface, like on the bottom of an aquarium, a sand dollar is unable to right itself and will die within days.
Above you see an example of a scam that can be found in many tourist traps and Christian shops, especially in the coastal south. It’s called the Legend of the Sand Dollar and supposedly is some kind of divine sign of the birth and death of Jesus. The legend goes like this:
There’s a lovely little legend
that I would like to tell,
of the birth and death of Jesus,
found in this lowly shell.
If you examine closely,
You’ll see that you find here,
four nail holes and fifth one,
made by a Roman’s spear.
On one side the Easter lily,
its center is the star,
that appeared unto the shepherds
and led them from afar
The Christmas poinsettia
etched on the other side,
reminds us of his birthday,
our happy Christmastide.
Now break the center open,
and here you will release,
the five white doves awaiting,
to spread good will and peace.
this simple little symbol,
Christ left for you and me,
to help us spread his gospel,
through all eternity.
This is really pretty silly. The four nail holes refer to the notches in a southern species of sand dollar called the keyhole urchin. Same with the hole caused by a Roman spear. The easter lily, star and poinsettia designs are simply the patterns of the ambulacral areas (holes where the tube feet protrude through the skeleton) found on the aboral and oral surfaces. The pattern isn’t really unique since it can be seen on sea stars and urchins as well. The white doves may be the most intriguing part of the tale, but these are just the mouthparts that become separated from the skeleton once the musculature of the dead animal has decomposed. I described these mouthparts in August 2006 in a diary about the Aristotle's Lantern.
Other diaries in this series can be found here.