Most of us think of sand dollars as round, flat seashells, but that's completely inaccurate. In fact, they aren't shells at all! During their average lifespan of about 10 years, a sand dollar is actually a living organism, and is a cousin of sorts to other echinoderms like sea cucumbers, sea stars (also known as starfish) and sea urchins. "Just like their more recognizable sea star cousins, sand dollars typically have five-part radial symmetry which means that their body could be split into five identical 'slices,'" explains Jessica Brasher, husbandry manager at the Ocean Institute in Dana Point, California in an email.
But that symmetry isn't what lands it in the echinoderm (Greek for "spiny skin") group. Indeed, live sand dollars (unlike their dead, relatively smooth counterparts) feature an endoskeleton that's covered by a layer of spiny skin, according to Leah Biery, director of communications at Sanibel Sea School in Sanibel, Florida. "The skeleton, or test, is composed of bony plates of calcium carbonate called ossicles, held together by connective tissue," she emails. "They have no brain, just a simple nerve ring."
While we're used to living things sporting legs, wings or some other obvious transportation method, sand dollars have a far more subtle way of getting around — a water vascular system. This system not only helps them move, but is also responsible for pumping filtered seawater so that they can eat, Biery says, noting that the preferred sand dollar diet is algae scraped from hard surfaces by their teeth. They also dine on plankton and other food floating freely in the water.
Sand dollars are surprisingly social, preferring to hang out on the ocean floor with lots of other sand dollars. This is especially vital during reproduction season, since being in a large group yields better odds of success. "Sand dollars reproduce by spawning, which means that males and females release eggs and sperm into the water column respectively," Brasher explains. "If fertilized, the sand dollar eggs will hatch into microscopic, free-floating larvae that hardly resemble their parents." The larvae then undergo a series of developmental changes until it grows its test and settles on the ocean floor.