live sand dollarsThe sand dollar is a remarkable organism that's part of the class of marine animals known as Echinoids, spiny skinned creatures that include the star fish. The animals are commonly found in clusters on or just beneath the surface of the sand in shallow water, feeding on plankton and organic material.

sand dollar sizesThe word echinoid literally means "like a hedgehog"
in Latin, which is very fitting when you see a live specimen. Most people have only encountered the smooth white empty skeletons commonly found on the beach, called "tests." But when a sand dollar is alive, it has a fuzzy green-gray-lavender to deep maroon outer layer. Its entire "shell" (technically not a shell as they have skins) is covered in very fine, moveable spines that allow the animals to tunnel under the sand. A sand dollar can completely bury itself in 10 minutes. I've found "tests" with diameters as large as 4 1/2" and as small as 1/4." Live sand dollars are strictly protected so be sure to take only the skeletons when you're beachcombing.sand dollar burrowing

sand dollar front and back

Like its close relative the sea urchin, the sand dollar has 5 long rows of petal-like pore pairs on the "aboral" (top) side of the body, where specialized tube feet perform gas exchange. At the center is the star-shaped "madreporite"- a perforated plate-like structure that passes sea water into its water-vascular system, enabling the animal to move.

When small organic particles and organisms flow across the top surface, the fine, hair-like cilia on the spines form little eddies where particles get trapped by mucus secreted on the spines. The spines and cilia direct the particles to the edge of the sand dollar and around to the flat bottom "oral" side, where radiating food grooves lined with additional tube-feet move the food in mucus streams to its central "peristome" or mouth. Inside the mouth that crush sand and plankton before they reach the sand dollar's stomach. The tiny hole at the edge is the "periproct" where wasted is excreted.

To reproduce, the female discharges already ripe eggs through her gonopores, 5 tiny holes in the center of the flower, and they're fertilized externally by the male. She can produce over 350,000 eggs per year. Within 2-4 days, four-armed larvae develop and can swim through the water. Over the course of their development the larvae grow additional arms. Eventually they pick up a chemical cue that is produced by the adult, and settle on the seabed within or adjacent to an existing sand dollar bed. There they undergo metamorphosis and begin to grow their adult sand dollar form. sand dollar larvae

Sand dollars have few natural enemies due to their hard skeletons and small edible parts, but heavy storms can destroy  whole beds of them. The sand dollars diverged from other echinoids about 450 million years ago, and their life span can range from 1 to 15 years.

In 2008, scientists made the fascinating discovery that sand dollar larvae can clone themselves as a mechanism of self defense. Larvae exposed to mucus from predatory fish cloned themselves, effectively halving their size. The smaller larvae are believed to better escape detection from fish predators.