Guest Post by Jocie Brooks
As soon as we set foot on the beach, the kids take off like rockets. The expanse of sand and fresh air makes them giddy with excitement, and they have huge smiles as they bolt away from us. My son likes to dodge between the geysers of water that shoot up from the siphons of horse clams. My daughter has a beachcomber’s collecting urge and likes to load shells and sand dollars into her red pail. The sand dollar shells, or “tests” are always a favourite, and there are large colonies along the edges of the sand bars and in the pool-like depressions between the bars. Here they live alongside eel-grass, crabs and small fish.
Living sand dollars are a rich purple-black colour, covered in tiny spines that give them a velvety texture. I’ve taught the kids to tell these living dollars from the lavender-grey and tan-coloured dead ones. Sand dollars are about the size of a palm, and are more or less circular and flattened, like oversized loonies. On the upper side, which is slightly domed toward the middle, there is an attractive flower-like pattern formed by five loops of tube feet. The underside has grooves resembling rivers that lead to the central mouth.
Sand dollars are found on the sandy bottoms of sheltered bays, as well as the open coast. They are often found in quite shallow waters, but also live in waters 40 or more metres deep. They can form dense populations, with as many as 625 dollars in an area less that a square metre. There is only one species in our area, known as the Eccentric Sand Dollar (Dendraster excentricus), which ranges along the coast from Alaska to Baja California.
The Keyhole Sand Dollar, in the Genus Mellita, has several distinctive keyhole shaped slots in the shell. The keyhole’s range includes the Caribbean Islands and the southern coast of the Atlantic U.S, and extends south to Brazil. They are also found along the Pacific coast of Mexico and Costa Rica. Unfortunately, the keyhole sand dollar is often erroneously adopted as a logo for many of Vancouver Island’s beach themed developments and accommodations- too bad they didn’t do their homework!
Sand dollars have many features in common with sea urchins and sea stars. These creatures are classified in the phylum Echinodermata, which literally means “spiny skin.”
Echinoderms have three shared characteristics: a hydraulic system of tubes and valves that operate rows of tube feet, five rows of tube feet that radiate from the center (seen as the 5-petaled flower design on the sand dollar), and a calcareous shell made up of calcite.
When feeding, the dollars are tilted up half buried in the sand. They move slowly, and the spines transfer food particles, such as microscopic diatoms, plankton and other bits of detritus, to the river-like food tracts. Specialized hairs move these particles down slippery food tracts to the mouth. There, a structure known as the Aristotle’s lantern uses muscles and hardened teeth to grind food into smaller particles. The anus is visible as a small hole on the underside near the margin.
To reproduce, eggs and sperm are released directly into the water. A fertilized egg develops into a free-swimming larva, which eventually settles on the sea floor and metamorphoses into an adult.
There are many fine beaches for viewing sand dollars in our area. Any beach with sand bars that are exposed at low tide is sure to have them. Stories Beach, Miracle Beach and William’s Beach are just a few examples.
Every year, I see kids playing Frisbee with live sand dollars. These creatures are vulnerable to this sort of abuse and can easily break if thrown. Once, I was surprised to see a man picking up live sand dollars with barbecue tongs and placing them in a large bucket. I imagine he was going to use them as some sort of garden fertilizer.
Surely, we can all enjoy the beach and be respectful of the creatures that live there. Let’s work on that!