Their skeletons are prized by beachcombers, but sand dollars look way different in their lives beneath the waves. Covered in thousands of purple spines, they have a bizarre diet that helps them exploit the turbulent waters of the sandy sea floor.

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DEEP LOOK is a ultra-HD (4K) short video series created by KQED San Francisco and presented by PBS Digital Studios. See the unseen at the very edge of our visible world. Explore big scientific mysteries by going incredibly small.

Pristine white sand dollars have long been the souvenir to commemorate a successful day at the beach. But most people who pick them up don’t realize that they’ve collected the skeleton of an animal, washed up at the end of a long life.

As it turns out, scientists say there’s a lot to be said about a sand dollar’s life. That skeleton -- also known as a test -- is really a tool, a remarkable feat of engineering that allows sand dollars to thrive on the shifting bottom of the sandy seafloor, an environment that most other sea creatures find inhospitable.

“They've done something really amazing and different,” said Rich Mooi, a researcher with the California Academy of Sciences in San Francisco. “They’re a pile of novelties, and they’ve gone way off the deep end in modifying their bodies to adapt to where they live.”

Mooi studies echinoderms, a word that roughly translates to “hedgehog skin.” It’s an aptly-fitting name for a group that includes sea urchins, sand dollars, sea stars and sea cucumbers. But Mooi says sand dollars really have his heart, in part because of their incredible adaptations.

--- What are sand dollars?
Sand dollars belong to a group of animals called Echinoderms that includes some more familiar animals like starfish and sea urchins. Sand dollars are actually a type of flattened sea urchin with miniaturized spines and tube feet more suited to sandy seafloors.

--- What do sand dollars eat?
Sand dollars consume sand but they get actual nutrition from the layer of algae and bacteria that coat the grains, not the sand itself.

--- Are sand dollars alive? Why do they Turn White?
When sand dollars are alive, they are covered in tiny tube feet and spines that make them appear like fuzzy discs. When they die, they lose their spines and tube feet exposing their white skeleton that scientists call a test. That skeleton is typically what people find on the beach.

---+ Read the entire article on KQED Science:

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---+ For more information:

Learn more about Chris Lowe’s work with plankton including sand dollars and their relatives
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Rich Mooi’s research into sand dollars for California Academy of Sciences
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Funding for Deep Look is provided in part by PBS Digital Studios. Deep Look is a project of KQED Science, which is supported by the Templeton Religion Trust and the Templeton World Charity Foundation, the S. D. Bechtel, Jr. Foundation, the Dirk and Charlene Kabcenell Foundation, the Vadasz Family Foundation, the Gordon and Betty Moore Foundation, the Fuhs Family Foundation Fund and the members of KQED.

---+ SHOUT OUTS

Here are the winners from our episode image quiz posted in our channel Community Tab:

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🏆#1: Tektyx
Was the first to correctly ID the creature in our episode was a sand dollar.

🏆#2: tichu7
Was the first to ID what kind of sand dollar it was, the Pacific sand dollar.

🏆#3: Miguel Gomez
Also posted what kind of sand dollar it was was, but by another name: Eccentric sand dollar.

🏆#4: Gir Gremlin
The first viewer to identify the sand dollar by its scientific name: Dendraster excentricus!

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