Journal du guide nature: Dollars des sables | Local News
Usually a coveted prize for beachcombers, a large number of Sands Dollars have recently washed up on the North Oregon Coast. Thousands of dollars from the sands – alive or dead – were dumped on the beaches near Seaside at high tide on August 16.
Predominantly flat sea urchins with short spines, sand dollars are related to sea urchins, starfish, brittle stars and brittle stars, sea cucumbers and crinoids: these are all echinoderms, the phylum of (usually) spiny invertebrates (“Echino”) skin (“derm”). In life, a purplish down of very short, blunt spines and very small tubular feet covers the difficult “test” with which we are most familiar.
Reminiscent of the familiar five-armed starfish, the five-part design on top of a sand dollar is another reminder of that relationship.
This five-part design on the top, called a “petalidium” for the shape of the petals, is highlighted by the tiny openings for the feet of the animal’s breathing tubes. The structure in the center of the petalidium is a madreporite – the same light-colored bald spot that you can see on the back of a starfish – and the tiny holes around the madreporite are the genital pores through which the ova or sperm are released.
In our sand dollars, the petalidium is a bit off-center and not directly aligned with the mouth – possible reasons for the scientific name of “eccentric sand dollar”, Eccentric dendraster.
The sand dollar mouth is the hole in the middle of the underside. Five thin canals radiate out from the mouth, each branching almost immediately to two each, then branching out more as the canals move towards the edge of the animal. The anus of a sand dollar is also on the underside: it is the tiny hole near the edge.
Sand dollars carry sediment (mostly sand) with their thorns and tubular feet along these channels to their mouths. Inside the mouth are five teeth which are together called “Aristotle’s Lantern” – just like the five teeth inside a sea urchin’s mouth. Sand dollars use their teeth to manipulate the pieces of sediment that have been passed to the mouth. (It’s Aristotle’s disarticulated lantern that people sometimes call “doves”.)
Sand dollars actually feed on algae, small animals, trash, and other organic material that gets in and stuck to the grains of sand.
In life, sand dollars are sometimes partially buried in the sand, on the edge and at a slight angle, usually with the longest petal facing down and the anus facing up. They tend to congregate in groups in sandy infralittoral areas. from the lowest intertidal zone to a depth of about 130 ‘. Animals can stack in close order, next to each other, with just a little space between them.
Sand dollars can also crawl flat along the bottom and can burrow – and loosen – on their own. Particularly choppy waves or strong currents can dislodge even flat sand dollars, however, sometimes even pushing them onto the beach.
Sand dollars, especially the smaller ones, can help hold themselves in place by sorting sand and ingesting heavy grains for ballast. On our beaches, the heaviest grains of sand are black, which is why your empty sand dollar test may contain black sand.
While the top and bottom are symmetrical on the outside, the innards on the inside are not, as the stomach, followed by the guts, wrap around the animal before curling up to exit through the ‘anus. The newborn sand dollars are a bigger surprise: they are broadly cone-shaped with long spines forming the open end of the cone; they are so small that they drift through the water as part of plankton.
While they may not be acceptable to us, the Sand Dollar’s natural predators include starfish, fish, and crabs. Our species extends from Alaska to Baja and is said to live up to 10 years.
So why did those sand dollars wash up on the shore? No word at the time of this writing on what could have caused the “devastation”. Some quirk in local currents leads the list of suspects, but low oxygen or acidic water that makes them particularly vulnerable to dislodgement are also possibilities.
You can gently put the dollars from the living sands back into the sea, but only bring home the dead, fuzz-free, and bleached tests.