You’ve probably found one. Most people have. You're walking along the shoreline, the tide is pulling back, and there it is—a bleached, white disk resting in the wet sand. It looks like a heavy coin or a flattened stone, etched with a flower pattern that feels too perfect to be natural. Naturally, you pick it up. If it's already dead and dried out, it rattles.
That rattle is the secret.
People love to break them open to find the "doves" inside. There's this old poem—it's been around forever—that claims inside a sand dollar you'll find five white birds representing peace. It’s a nice sentiment, honestly. But the reality is actually way more fascinating from a biological standpoint. Those little white shards aren't birds at all. They are the teeth.
The living machine under the shell
Before we get into the "doves," we have to talk about what this thing actually is. A sand dollar isn't a rock. It's an animal. Specifically, it's an echinoderm, which puts it in the same family tree as sea urchins and starfish. When they’re alive, they aren’t white. They’re kind of a moody purple or reddish-brown, covered in a carpet of tiny, velvet-like spines.
These spines aren't just for show. They move.
If you were to shrink down and stand on the ocean floor, you’d see these disks crawling—slowly—through the sediment. They use those spines to breathe, to move, and to push microscopic bits of food toward the center of their body. That's where the mouth is. If you flip a dead sand dollar over, you’ll see a small hole right in the middle.
This is where things get weird.
Aristotle’s Lantern and the Five Doves
When you crack open a dried test (that’s the technical name for the "shell"), five V-shaped pieces fall out. In the world of marine biology, this structure is called Aristotle’s Lantern. Why? Because the philosopher Aristotle thought the mouth structure of a sea urchin looked like an ancient horn lantern. Since sand dollars are basically flattened urchins, they have a modified version of this same complex hardware.
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Each "dove" is a tooth.
Think about that for a second. The sand dollar has five individual jaws that work together to grind up crustacean larvae and algae. It’s a grinding mill. These teeth are incredibly strong and made of calcium carbonate. When the animal dies and the soft tissue decays, the teeth become detached from the internal structure. They just bounce around inside the hollowed-out skeleton. That’s what creates the rattle.
The engineering of a flat body
The holes you see in the shell? They aren't random. They’re called lunules.
Imagine you’re a flat disk living in a world of heavy surf and crashing waves. You’d get tossed around like a frisbee. The lunules act like pressure relief valves. They allow water to pass through the body so the sand dollar doesn’t get swept away by the current. It’s high-level hydrodynamics.
Also, they have a "weight belt."
Young sand dollars are light. To keep from being washed out to sea, they actually swallow heavy grains of sand. They store this sand in their digestive tract to add ballast. It’s literally a biological scuba belt. As they grow older and heavier, they don't need to do it as much, but for a juvenile, it’s the difference between staying in the feeding grounds or being lost in the deep ocean.
How they actually eat
Most people assume they just sit there. They don't.
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When the tide is right, a sand dollar will often stand on its edge, burying one side in the sand and letting the other catch the current. This is "filter feeding" at its most basic. They catch passing plankton in their spines. Then, tiny hairs called cilia act like a conveyor belt, moving the food along "food grooves" toward the mouth.
It’s a very organized system.
Once the food reaches the mouth, the five teeth (the doves) go to work. It can take a sand dollar up to fifteen minutes to swallow a single meal, and then it can stay in their gut for two days before it's fully digested. They aren't in a rush. Life on the bottom of the sea moves at its own pace.
Why the color matters
If you find a sand dollar that is still fuzzy or dark in color, put it back.
It’s alive.
The white ones you find at the gift shop or high up on the beach have been bleached by the sun. When a sand dollar dies, its spines fall off almost immediately. The skin decays, and the sun turns the calcium carbonate white. If you pick up a live one, it might leave a yellow stain on your hand. That’s not pee—it’s a harmless pigment called echinochrome.
In some places, it's actually illegal to take live ones. Check the local laws in places like Florida or California; the fines are surprisingly steep because these guys are vital to the ecosystem. They turn over the sand, oxygenating it so other little creatures can live there.
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The growth rings of the ocean floor
You can tell how old a sand dollar is by looking at the rings on its shell. It’s just like a tree.
Wait. Not the flower pattern on top.
If you look really closely at the plates that make up the test, you can see growth lines. Most sand dollars live about seven to ten years. In that decade of life, they survive predators like sea stars and certain types of fish (like the sheephead) that have teeth strong enough to crush through that "stone" exterior.
What to do with your finds
If you’ve found a dead, white sand dollar and you want to keep it, you have to treat it right. They are incredibly fragile.
- Rinse it gently. Fresh water is fine, but don't use a high-pressure hose or you'll shatter the thing.
- Soak in a bleach solution. Use a mix of one part bleach to three parts water. Don't leave it in for more than a few minutes or the bleach will actually start to dissolve the calcium and make it crumble.
- Sun dry it. This finishes the whitening process naturally.
- Harden it. Most collectors use a 50/50 mix of white glue and water. Paint it on with a brush. This soaks into the pores and makes the shell much less likely to snap when someone touches it.
Honestly, though, there's something to be said for just leaving them. When you crack one open to see the doves, you’re destroying a piece of natural architecture that took years to build. If you want to see the teeth, find one that's already broken. They're usually scattered all over the high-tide line if you look closely enough.
It’s easy to look at a beach and see a bunch of sand and water. But when you realize that every little rattle in a dried shell represents a complex feeding system and a decade of survival, the beach gets a lot more interesting. The "doves" are a beautiful myth, but the reality of a biological grinding machine is arguably much cooler.
Next time you're at the shore, look for the "petals" on the shell. Those are actually sets of pores that the animal uses to move its tube feet—not for walking, but for breathing. It's an entire respiratory system shaped like a flower. Nature doesn't usually waste space on aesthetics, so every line you see has a job to do.
If you’re planning on collecting, bring a small padded box. A sandwich bag won't cut it; they'll be dust by the time you get to the parking lot. Respect the ones that are still purple and hairy, and enjoy the engineering of the ones that aren't.