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 said sand dollars really have his heart, in part because of their incredible adaptations.
Sand dollars are actually a type of sea urchin, one that struck off on its own in an evolutionary pilgrimage to take advantage of a new environment.
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Another compelling trick: While older sand dollars are heavy enough to keep themselves firmly on the bottom, younger dollars have crafted something that may be familiar to any scuba diver or spear fisherman. They use a weight belt.
Sand is made up of small bits of rock and debris. The rocks along the coast determine which minerals will be present in the sand. Magnetite is one such mineral, an iron-rich deposit named for its magnetic properties. In sand, it is often seen as very small, slightly shiny black specks.
Young sand dollars can pick small grains of magnetite from the surrounding sand and store them in specialized chambers of their gut called diverticula. When X-rayed, the magnetite appears as bright patches. They help weigh down the younger dollars, keeping them grounded until they bulk up as adults so they don’t wash away.
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