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 eight 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
You're forgiven if you thought sand dollars are white, as that's often how we see them in gift shops. When they're alive, they're actually a purple color and covered in tiny flexible bristles. 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.
2. Star fish and sea urchins are close relatives
The starfish is a relative of the sand dollar, which is pictured here covered in green algae. (Photo: sailn1/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 star fish and the sea urchin, according to the Gulf of Maine Research Institute.
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. Their pores propel them
"Like its close relative the sea urchin, the sand dollar has five sets of pores arranged in a petal pattern. The pores are used to move sea water into its internal water-vascular system which allows for movement," the Gulf of Maine Research Institute says.
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.
5. Their living spaces are crowded
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.
6. They have few predators
The eel-like ocean pout is one of the sand dollar's few predators. (Photo: Vejlenser/Wikimedia Commons)
"Due to their diminutive edible parts and relatively hard skeleton, few animals bother sand dollars," the Gulf of Maine Research Institute reports. 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.
7. You can tell their age by their rings
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.
8. How they got their name
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.