On a swampy August day, I had the delightful experience of helping a group of kids build a sandcastle. We built it above the tide line and created motes and walls and a system of drainage to hold back the incoming tide. All of this was to protect the precious “treasury” — a collection of small sand dollars that were carefully placed in the interior of the castle.

Sand dollars really do seem like treasures – they are rare to find and are also easily broken. These tiny, perfect ones were lucky finds. It got me thinking about just how delicate and skeletal they are. They have no real meat on them that would make them tasty either to us or to other predators. And the exterior prickly covering they have while still alive doesn’t make them any more enticing. Nevertheless, other sand dwellers like flounder sometimes eat them.

Sand dollars are part of the echinoderm (spiny skinned) family that also includes urchins and sea stars along with the odd-man-out — sea cucumbers. Sea cucumbers have warty skin rather than spines, which still qualifies them as a part of the family. Other commonalities amongst echinoderms are a radial pattern. This sometimes looks a bit like a flower like in the case of a sand dollar that looks like it has five petals on its top side.

The sand dollar species we have in Maine is Echinarachnius parma. The “arachnius” part refers to the spidery flower pattern on its shell. While the sand dollars we found were small, the biggest ones of the species aren’t very big — only 3 inches or so across at the largest. The spines are so tiny that you’d hardly know they were there. They form a felt-like coating that is brownish or sometimes slightly purple or red.

The color can come off and stain your hands or clothes a yellow-ish hue. Not to worry as it isn’t harmful. Once the living part of the creature has died and you are left with the white skeleton, you can see a five-petaled pattern on the surface of the shell (or test). The tiny holes that make up this pattern are where tiny tube-like feet poke through when the animal is alive. These are how they breathe and also how they grip onto sand particles to stay in one place.

I sometimes forget sand dollars when thinking of seashore species because they are found on sandy beaches rather than the rocky ones that dominate our coast. They spend their lives using their tiny spines to shift through the sand and eat tiny plankton through the little opening on their underside. They have a little cone of spines that make up their “mouth” that they can use to grind up their prey. They aren’t exactly voracious eaters, sometimes taking up to two days to digest their food.

Perhaps their slow digestion is responsible in part for their longevity. Sand dollars can live up to ten years, which is impressively long for a skeletal little creature. One neat thing you can look for in a sand dollar test is growth rings akin to those you can see in a tree. These are faint but can be seen if you look closely and even feel lightly with your fingernail.

Sand dollars are just one of the myriad species found along our coast. They, among many others, are included in a recent terrific pocket field guide put out by the Harpswell Heritage Land Trust (HHLT) in collaboration with the Casco Bay Estuary Partnership. The guide, entitled “Seashore Life of Harpswell Maine”, is available for purchase through HHLT.

That day at the beach, we unfortunately left before the tide challenged our castle. So, I don’t know whether our precious dollars survived. But, I am hopeful. At the very least, it was an excuse to learn a little more about these weird creatures that we are sometimes lucky enough to find on one of Maine’s rare sandy beaches.