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 The bottom of a sand dollar found on a Costa Rican beach. (Photo: Gerhard H [CC BY-SA 3.0]/Wikimedia Commons)

You're forgiven if you thought sand dollars are white, as that's often how we see them in gift shops. When sand dollars are alive, they're actually a purple color. Their fuzzy spines are covered in tiny flexible bristles called cilia which they use to move food along the ocean flood to a central mouth. When they die, their skeletons get bleached by the sun, turning them white, and the small spines fade away. The familiar star pattern seen clearly on the bleached shell is more hidden when they're alive.

Here's a video of a live sand dollar:

2. Starfish and sea urchins are close relatives

Starfish and sand dollarThe starfish is a relative of the sand dollar, which is pictured here covered in green algae. (Photo: sailn1 [CC BY 2.0]/Flickr)

Sand dollars are invertebrates in the class of marine animals known as echinoids, or spiny-skinned creatures. Their cousins include the sea lily, the sea cucumber, the starfish and the sea urchin.

3. They eat with their hair

The sand dollar's tiny moveable spines, which encompass the entire shell, are what enables it to eat a diet of crustacean larvae, small copepods and algae.

"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."

Their mouth has a jaw with five teeth-like sections to grind up food, which it may do for up to 15 minutes before they swallow. It can take two days for food to digest.

4. They are carnivores

live sand dollar with sand Sand dollars eat algae and bits of other animals in the sand. (Photo: 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, reports Thought Co. They are usually microscopic bits of algae, but can also be tiny fragments of other animals. That's why they are classified as carnivores, according to the World Register of Marine Species.

5. Their pores propel them

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

Similar to its relative, the sea urchin, the sand dollar has five sets of pores that are arranged in a pattern like a flower's petal. The pores move sea 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.

6. Their living spaces are crowded

Sand dollar colony A sea slug maneuvers across a bed of sand dollars. More than 600 of them can fit in a square yard. (Photo: 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 — the Monterey Bay Aquarium says as many as 625 can live in one square yard.

7. They have few predators

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

Due to the fact that they have a hard skeleton and very few edible parts, few animals bother sand dollars. However, a few creatures will take up the challenge for an occasional sand dollar snack, including the ocean pout (an eel-like fish), California sheepheads, starry flounders and large pink sea stars.

8. 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. (Photo: elena moiseeva/Shutterstock)

Just like counting the rings on a tree stump, you can count the growth rings on the plates of the exoskeleton to see how old a sand dollar is, according to the aquarium. They usually live six to 10 years.

9. How they got their name

Inside of a sand dollar Here's what the inside of a sand dollar looks like. Kinda looks like a ravioli with a bite out of it, no? (Photo: By His Design/Shutterstock)

They're not money — you can't use them to buy anything — so why are they called dollars? "People thought the skeletal remains (called the test) resembled silver coin currency, which is how the name 'sand dollar' came about," according to the Sanibel Sea School in Florida.

Editor's note: This story has been updated with new information since it was written in August 2017.

Angela Nelson ( @bostonangela ) is an exhausted mom of two young daughters and two old cats, and a Pulitzer Prize-winning digital editor with more than 15 years of experience delivering news and information to audiences worldwide.

9 things you don't know about sand dollars
Sand dollars are echinoids that live on the ocean floor. You may enjoy collecting their shells, but how much do you really know about these quiet creatures?