Usually a coveted prize for beachcombers, large numbers of sand dollars washed ashore on the northern Oregon Coast recently. Thousands of sand dollars - alive and dead - were cast up on beaches near Seaside on the August 16 high tide.
Essentially flat, short-spined sea urchins, sand dollars are related to sea urchins, sea stars, brittle and basket stars, sea cucumbers and crinoids: they’re all echinoderms, the phylum of invertebrates with a (usually) spiny (“echino”) skin (“derm”). In life, a purplish fuzz of very short, blunt spines and very small tube feet covers the hard “test” we’re more familiar with.
Reminiscent of the familiar five-armed sea star, the five-parted design on the top side of a sand dollar is another reminder of that relationship.
That five-parted design on the top, called a “petalidium” for the petal-shape, is outlined by the tiny openings for the animal’s respiratory tube feet. The structure in the center of the petalidium is a madreporite - the same light-colored bald spot you can see on the back of a sea star - and the tiny holes around the madreporite are the genital pores through which the eggs or sperm are released.
In our sand dollars the petalidium is quite a bit off-center and not directly lined up with the mouth - possible reasons for the “eccentric sand dollar’s” scientific name, Dendraster excentricus.
A sand dollar’s mouth is the hole in the middle of the underside. Five thin channels radiate from the mouth, each branching off almost immediately to two each then more branching as the channels make their way toward the edge of the animal. A sand dollar’s anus is also on the underside: it’s the very small hole near the edge.
Sand dollars pass sediment (sand, mostly) with their spines and tube feet along those channels to their mouth. Inside the mouth are five teeth that are together called “Aristotle’s lantern” - just as the five teeth inside a sea urchin’s mouth are. Sand dollars use their teeth to manipulate the bits of sediment that have been passed to the mouth. (It’s the disarticulated Aristotle’s lantern that people sometimes refer to as “doves.”)
Sand dollars actually feed on the algae, tiny animals, detritus, and other organic material between and stuck to the sand grains.
In life, sand dollars are sometimes partially buried in sand, on edge and at a slight angle, usually with the longest petal downward and the anus upwards. They tend to gather in groups in sandy subtidal areas. from the very lowest intertidal to a depth of about 130’. The animals may stack up in close order, one next to another, with just a little space between them.
Sand dollars can also crawl flat along the bottom and can bury - and unbury - themselves. Especially rough waves or strong currents can dislodge even flat-lying sand dollars, however, sometimes even pushing them onto the beach.
Sand dollars, particularly small ones, can help keep themselves in place by sorting through the sand and ingesting the heavy grains for ballast. On our beaches, the heaviest sand grains are the black ones, which is why your empty sand dollar test might have black sand inside.
While the top and bottom are symmetrical on the outside, the entrails inside aren’t as the stomach, followed by the guts, spiral around the animal before looping up to exit at the anus. Newborn sand dollars are a bigger surprise: they’re widely cone-shaped with long spines forming off the open end of the cone; they’re so small they drift in the water as part of the plankton.
Although they might not seem palatable to us, natural predators of sand dollars include sea stars, fish and crabs. Our species ranges from Alaska to Baja and are said to live up to 10 years.
So why did those sand dollars wash ashore? No word as of this writing on what might have caused the “wracking.” Some quirk in local currents leads the list of suspects, but low oxygen or acidic water that made them especially vulnerable to being dislodged are also possibilities.
You can gently return living sand dollars back in the sea, but take home only the dead, fuzz-less and bleached tests.
For a link to a video on sand dollars eating, visit our Facebook page, www.facebook.com/wavecrestdiscoveries,, For information on how you can arrange your own exploration of our fascinating natural history, contact Marty at firstname.lastname@example.org, via Facebook, or by calling 541/267-4027. Questions and comments about local natural history are welcome. www.wavecrestdiscoveries.com