Marine Animal Encyclopedia
Sand Dollar Echinodiscus auritus
Sand dollars are a type of invertebrate related to the sea urchins, sea stars, and sea cucumbers – together known as the echinoderms. Echinoderms are the closest living invertebrate relatives of the vertebrates, so our closest invertebrate cousin might just be a sand dollar. Anatomically, a sand dollar is much like a flattened sea urchin with smaller, hair-like spines.
The Eccentric Sand Dollar is a relatively small species (reaching sizes of only a few inches in diameter) that lives off the west coast of North America, from Alaska to Baja California. Though they are totally flat, these sand dollars are often observed buried in the sand on an edge with half of their bodies under the sand and half sticking out into the water. Eccentric Sand Dollars, like most sand dollars, are filter feeders, and they use the small spines that cover their bodies to snag crustacean larva, plankton, and other tiny prey. The food is slowly passed from spine to spine until it reaches mouth at the middle of the bottom side of the body. The spines are also used for locomotion, along with a series of tube feet, so called because they are operated by a hydraulic system controlled by the main body. When Eccentric Sand Dollars move, they lay flat on the bottom with the mouth side facing down. The tube feet are also utilized for obtaining oxygen from the environment.
Eccentric Sand Dollars reproduce through a behavior known as broadcast spawning, where several females release eggs and several males release sperm into the water column above the sand, all at the same time. This method increases the likelihood that eggs will become successfully fertilized and that fertilized eggs will not be eaten by egg predators near the sea floor. In some places, tens of thousands of Eccentric Sand Dollars can be seen carpeting the bottom over large areas.
Eccentric Sand Dollars are almost all skeleton. The soft parts of the body are very small, and there are few, if any, predators that eat these sand dollars. Some individuals are collected and bleached to sell in tourist shops, and some sand dollar species’ populations may be threatened by this practice. Destructive fishing, especially bottom trawling, may also risk sand dollar populations in some areas. Scientists do not have sufficient data to determine population trends in the Eccentric Sand Dollar, but this species is generally assumed to be stable. Population trends of some other sand dollar species are known to be decreasing, rapidly in some cases, and conservation campaigns have been launched in some places in an attempt to reverse these trends.