9 Things You Don't Know About Sand Dollars

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

a live sand dollar, purple in color, with a pink barnacle growing on it
Live sand dollars are purple, like this one with a pink barnacle growing on it. Melissa E Dockstader/Shutterstock

We've all seen sand dollars wash up on the beach, a pretty white shell with a stunning star shape stamped on one side. But what's up with those five oval holes? And what are they like in their ocean homes, where humans see them much less often?

We answer those questions and more with these surprising facts about sand dollars, which can be found in tropical and temperate waters throughout the Northern Hemisphere.

1. When They're Alive, They Aren't White

Sand dollar alive
This live sand dollar was found on a Costa Rican beach. Gerhard H / Wikimedia Commons / CC BY-SA 3.0

You're forgiven if you thought sand dollars are always white, as that's often how we see them on beaches or in gift shops. When sand dollars are alive, however, they're a purplish color. Their fuzzy spines are covered in tiny flexible bristles called cilia, which they use to move food along to a central mouth. When a sand dollar dies, its skeleton (known as a "test") is bleached by the sun, turning it white, and the small spines fade away. The familiar star pattern on the shell is more hidden when the animal is alive.

The name "sand dollar" comes from their resemblance to dollar coins.

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

If you find a live sand dollar, don't take it home with you. They can only survive out of the ocean for a few minutes, so the most humane thing to do with a live sand dollar is to place it gently back in the water near where you found it. In many places, including Florida, it's illegal to take a live sand dollar away from its habitat.

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

A sand dollar and a sea star, or starfish
Sand dollars, like this one covered with green algae, are close relatives of sea stars. sailn1 / Flickr / CC BY 2.0

Sand dollars are invertebrates in the class of marine animals known as echinoids, or spiny-skinned creatures. Their cousins include sea lilies, sea cucumbers, sea stars (commonly known as "starfish"), and sea urchins.

4. They Eat With Their Hair

A sand dollar relies on its tiny moveable spines and cilia to eat.

"In their sandy seafloor habitat, sand dollars use their fuzzy spines, aided by tiny hairs (cilia), to ferry food particles along their bodies to a central mouth on their bottom side," according to the Monterey Bay Aquarium. "They capture plankton with spines and pincers (pedicellariae) on their body surfaces. A tiny teepee-shaped cone of spines bunched up on a sand dollar's body marks a spot where captive amphipods or crab larvae are being held for transport to the mouth."

A sand dollar's mouth has a jaw with five teeth-like sections to grind up food, which it may do for up to 15 minutes before swallowing. It can take two days for food to digest.

5. They Have a Diverse Diet

live sand dollar with sand
Sand dollars eat algae and bits of other animals in the sand. bcampbell65 / Shutterstock

When sand dollars are using their spines and cilia to move food particles through the sand for dinner, those morsels can be a mix of various things. They are usually microscopic algae, but a sand dollar's diet can also include copepods, crustacean larvae, and other tiny animals.

6. Their Pores Propel Them

Sand dollar washed up on a beach
See those five holes in this bleached sand dollar? Those are pores, and sand dollars pass water through them to move. Mariette Ho-Sam-Sooi / Shutterstock

Similar to sea urchins, sand dollars have five sets of pores arranged in a pattern like a flower's petals. The pores move water and gas in and out and allow movement.

When the water is still, sand dollars may stand on one end with the other end buried in the sand. But when the water gets rough, they lie flat or burrow under the sand to hold their ground.

Sand dollars have evolved other tricks for staying put, as well. Adults can grow heavier skeletons and young sand dollars swallow grains of sand to weigh them down.

7. Their Living Spaces Are Crowded

Sand dollar colony
A sea slug maneuvers across a bed of sand dollars. KGrif / Shutterstock

You thought sharing a bedroom with a sibling as a kid was hard? Imagine hundreds of family members packed into your living space. That's life for sand dollars — as many as 625 can live in one square yard (0.8 square meter), according to the Monterey Bay Aquarium.

8. They Have Few Predators

Ocean pout
The eel-like ocean pout is one of the sand dollar's few predators. Vejlenser / Wikimedia Commons / CC BY-SA 4.0

Since sand dollars have a hard skeleton and few edible parts, not many animals bother them. A few creatures will take up the challenge for an occasional snack, however, including ocean pout (an eel-like fish), California sheepheads, starry flounders, and large pink sea stars.

9. You Can Tell Their Age By Their Rings

Sand dollar rings
See the darker spots in a circular pattern around the shell? Those are growth rings that indicate age. elena moiseeva / Shutterstock

Similar to counting rings on a tree stump, you can count the growth rings on the plates of a sand dollar's test to see how old the animal is, according to the Monterey Bay Aquarium. They usually live for six to 10 years.