#dendrasteridae

Another Day, Another Pacific Sand Dollar

The eccentric sand dollar, aka the sea-cake, biscuit-urchin, western sand dollar, or Pacific sand dollar (Dendraster excentricus), are found in the intertidal zone and near-shore sandy bottoms from Alaska, US to Baja California, Mexico.They are the only sand dollars endemic to the Pacific Northwest, though they share the rest of their range with other species. Live individuals are seen either partially buried upright or lying flat on the ocean floor, depending on the strength of the current. To prevent themselves from being swept away, juveniles will also ingest sand to weigh themselves down. Although they are not social, they can form large colonies with as many as 6 sand dollars in a square m (1 sq yd).

Pacific sand dollars are named for their resemblance to silver dollars, especially the bleached exoskeletons that commonly wash up on beaches. Most adults average about 8 cm (3 in) across, though individuals as big as 10 cm (4 in) have been found. The body is a flat disc coated in small, purple tube-like feet and sensory organelles called cilia. The feet are used both for moving across the ocean floor and for pulling oxygen from the water. The mouth and anus-- a single opening-- are located on the sand dollar’s underside. Inside the mouth are five teeth and jaw plates known as doves; together they form a structure known as Aristotle’s lantern, which is unique to echinoderms like sand dollars and sea stars.

D. excentricus is a suspension feeder, using its feet and cilia to pull food from the water or direct it along special groves on the body’s underside. Their main prey are microscopic larvae, copepods, diatoms, algae, plankton, and detritus. The sea-cake is predated upon by a number of sea stars and fish, as well as crabs and sea gulls. To avoid being eaten, adults bury themselves in the sand and larvae will duplicate themselves via a process known as budding and fission, which creates smaller individuals that can distract potential predators.

Although western sand dollars have seperate sexes, they are broadcast spawners. In late spring or early summer, males and females congregate and release gametes into the water where they become fertilized. Larvae, also known as prisms, hatch just a day later. This larvae floats freely through the water, growing arms and metamorphosing into a echinopluteus larva. Once they reach 8 arms, the larva begins to develop an exoskeleton or echinus, and resembles a small adult. The final stage of growth is triggered by chemical cues released by other adults; after this, individuals become sexually mature and settle on the ocean with other sand dollars. In the wild, adults can live up to 13 years.

Conservation status: Although the IUCN has not evaluated the Pacific sand dollar, they are regularly threatened by ocean acidification, warming, and bottom trawling.

If you like what I do, consider leaving a tip or buying me a ko-fi!

Photos

Chan Siuman

Brian Starzomski