9 Things You Didn't Know About Sand Dollars

They're purple, hairy, and too tough for most predators to mess with.

Sand Dollars in the shallow Pacific Ocean waters
Kirk Wester / Getty Images

Sand dollars' star-stamped skeletons are widely sought-after beach finds, but not many know what the bottom-dwelling creatures are like when they're alive. Truth is, they look almost nothing like what you find in the sand after high tide. The sand dollar — or "sea biscuit," or "sand cake," in other parts of the world — is purple and hairy in its prime. It belongs to the order Clypeastroida and resides in tropical and temperate waters throughout the Northern Hemisphere. From their many nicknames to the fascinating way in which they eat, here are nine things you may not know about sand dollars.

1. Sand Dollars Aren't White When They're Alive

Sand dollars (Echinodermata), Monterey, California, USA
Stuart Westmorland / Getty Images

Most people see sand dollars only after they've deceased. Those white "shells" found along the beach are their skeletons; when the marine animal is alive, its pigment can vary from a rich reddish-brown to a vibrant shade of purple. Contrary to the porcelain-like texture of their gift shop-popular skeletons, living sand dollars are covered in flexible bristles — known as spines — that hide their star design. When it dies, its skeleton (the "test") becomes bleached by the sun, turning it white, and the small spines fade away.

2. Live Sand Dollars Can't Survive for Long Out of Water

Removing live sand dollars from the beach is illegal in most states, but the laws vary when it comes to dead organisms. It's best to never take a sand dollar if you're unsure whether it's alive or dead. When alive, they can only survive out of the ocean for a brief few minutes. Sand dollars breathe through their signature "petals" — officially called petaloids — a series of holes from which tube-like, breathing feet emerge.

3. They're Related to Sea Stars and Sea Urchins

A sand dollar and a sea star, or starfish
sailn1 / Flickr / CC BY 2.0

Sand dollars are flat and burrowing invertebrates included in the class of marine animals known as echinoids, or spiny-skinned creatures. They are commonly referred to as "irregular" sea urchins and share much of their anatomy with their globular cousins. They are also related to similar radially symmetrical animals, like sea lilies, sea cucumbers, and sea stars (aka starfish) — although the latter falls into a different class.

4. They Have Many Nicknames

In the U.S., the common name for the Echinarachnius parma species is "eccentric sand dollar," or simply "sand dollar" for short. The name derives from the animal's resemblance to dollar coins, of course; however, it also goes by "sand cake," "sea biscuit," and "cake urchin," or, in New Zealand, "sea cookie" and "snapper biscuit." In South Africa, it's often called a "pansy shell" for its flower-like pattern.

5. They Use Their Spines To Eat

live sand dollar with sand
bcampbell65 / Shutterstock

According to the Monterey Bay Aquarium, these sand-sweeping critters live on crustacean larvae, small copepods, debris, diatoms, and microscopic algae. They use their spines, covered in tiny and flexible bristles called cilia, to move food particles through the sand, along their prickly body surfaces, and into their central mouths, located on their bottom sides. The Monterey Bay Aquarium says a "tiny, teepee-shaped cone of spines" is where the sand dollar keeps amphipods and crab larvae before dining on them. The animal's mouth features a jaw with five teeth-like sections for grinding, which it may do for up to 15 minutes before swallowing. It can take two whole days for food to digest.

6. Their Holes Serve a Purpose

What is seen on a sand dollar test is always that distinctive flower-like design — really just five sets of gas- and water-processing pores that happen to be arranged in a pretty pattern — and sometimes the same number of oblong holes or slits. These perforations, like its body art, serve an important purpose when the echinoid is living. They're called lunules, and according to the Natural History Museum in London, they "act as pressure drainage channels," preventing the sand dollar from being washed away in waves. They can also be used for food harvesting.

When the water is still, sand dollars may stand upright with one end buried in the sand. When the water gets rough, they tend to lie flat or burrow under the sand to hold their ground. They've adopted other tricks for staying put, too, like growing heavier skeletons or swallowing sand to weigh them down.

7. Their Living Spaces Are Crowded

Eccentric sand dollars, Puget Sound, Washington state
dschreiber29 / Getty Images

Sand dollars are not picky about their living arrangements. Even though they have entire oceans at their (virtual) fingertips, they tend to stick together in packed crowds. The Monterey Bay Aquarium says as many as 625 can reside in a single square yard (or .8 of a square meter). This likely has something to do with their mode of reproduction. Sand dollars practice "broadcast" or "group" spawning, meaning both sexes release eggs and sperm into the water. The more there are, the higher the success rate.

8. They Have Few Predators

Because sand dollars have hard skeletons and very few edible parts, they don't have many predators. A few creatures will accept the challenge of ingesting them, though, such as ocean pout (eel-like fish with wide, fleshy mouths), California sheepheads, starry flounders, and large pink sea stars. (So, yes, they're even at risk of being preyed upon by their own.)

9. You Can Tell a Sand Dollar's Age by Its Rings

Similar to the way rings on a tree stump symbolize every year of life, so do the growth rings on the plates of a sand dollar's test. The number of rings increases with body size, meaning the bigger the sand dollar, the older it must be. According to the Monterey Bay Aquarium, the disk-like, shell-resembling ocean dwellers can live for six to 10 years.