Mermaid coins or sea biscuits: Here's the scoop on sand dollars

There's much folklore surrounding the small, fragile creatures known as sand dollars, but what is their life like before they wash up on the shore? 

Jeff Clements, an aquatic biologist with Fisheries and Oceans Canada, says the critter's coin-like appearance has led to many myths about the marine animal.

"Coins to humans are quite valuable, and so sand dollars are often captured in cultural folklore," said Clements. "For example, some people believe that dead sand dollars that you find on the beach are the coins lost by mermaids or the people of Atlantis."

Sand dollars look much different when they're alive in the water compared to when they're dried out in the sand.

When alive, they are reddish or purple in colour. In the water, they look more like small, flattened cookies rather than coins.

That's why they're sometimes called sea cookies or sea biscuits.

The creatures have thousands of short spines that move in all directions, Clements said.

Dead sand dollars are typically chalky white or tan, and have no spines. These dried-up sand dollars have a flower pattern on them.

Clements said that flower will always have five petals. The reason why they have this pattern? Sand dollars are echinoderms, a group of marine invertebrates that includes sea urchins and starfish.

Echinoderm bodies are radially symmetrical about a central point, just like a four-leaf clover — except that sand dollars have a five-fold symmetry. 

There are many different species of sand dollars, but the scientific name for the ones found in the Maritimes is Echinarachnius parma.

Dried-up sand dollars often wash up on sandy parts of the beach, but live ones live just beneath the sediment surface on the ocean floor.

When walking on tidal flats at low tide, you may be lucky enough to find a live one. Clements recommends digging just beneath the sediment, usually at the spot where the water meets the emerged sand.

If you're extra lucky, you might catch a glimpse of a sand dollar moving slowly just beneath the surface. Sand dollars feed on little bits of algae or other organic matter in the sediment, Clements said.