How can you tell if a sand dollar is dead or alive? Sand dollars are flat sea urchins that burrow into the soft sand. When alive, their undersides are full of “velvet-textured spines” covered with cilia, a latin word for eyelashes. When they’re no longer alive, the spines disappear. The test or skeleton of the creature begins to turn white, bleached by sunlight, and they’re more easily recognized by the five petal pattern on their backs.

YouTuber and teacher Sea Something explains the difference in this video from 2012. More info from the Monterey Bay Aquarium:

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. 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. Unlike sea stars that use tube feet for locomotion, sand dollars use their spines to move along the sand, or to drive edgewise into the sand. On the upper half of the sand dollar’s body, spines also serve as gills.

In quiet waters, these flattened animals stand on end, partially buried in the sand. When waters are rough, sand dollars hold their ground by lying flat—or burrowing under. In fast-moving waters, adults also fight the currents by growing heavier skeletons. Young sand dollars swallow heavy sand grains to weigh themselves down.

If you see a sand dollar on the beach, consider its color and look for short spines, cilia, and tube feet. If it has those, be sure to return them to the water after observation. YouTuber April Morris shares another example:

Plus, here’s a time lapse from Monterey Bay Aquarium that shows sand dollars burrowing and moving sand grains: “One second of this video represents about two and half minutes in the life of these animals!”

Next: The weird world of the sea urchin and How Life Begins in the Deep Ocean. Plus: More animals that burrow.

h/t Reddit.

See more videos about...