Fossil of the Week

Sea stars are echinoderms, related to sea urchins, brittle stars, sea lilies, and sea cucumbers. Sea stars are sometimes also called starfish, but this term includes the closely related brittlestars (Class Ophiuroidea). Sea stars are members of the Class Asteroidea, and are radially symmetrical (see our earlier Fossils of the Week on crinoids and sea urchins), typically with five arms. They first evolved in the Ordovician Period (about 460 million years ago), but their fossil record is poor because of their relatively soft bodies that easily fall apart after death. Living sea stars move using a water vascular system - a network of blood-vessel-like tubes and hundreds of tiny "tube feet" with suckers to carry them along. They also use the tube feet to open clams, one of their primary food sources. Another unique feature of a sea star is its two stomachs, one for digestion, and another that can be everted out of the mouth to engulf prey, allowing the sea star to hunt prey that wouldn't fit through its mouth. The arms of sea stars can be regenerated if lost or cut off. The planktonic larvae of sea stars and other echinoderms look nothing like the adults until they settle and metamorphose; echinoderms and their larvae are frequent subjects of developmental research. There are between 1,600 and 1,800 species of living sea stars, all of which are marine. They are important members of marine ecosystems, from both good and bad viewpoints - the Crown of Thorns sea star on the Great Barrier Reef off of Australia is a major predator on living corals and can cause death of a coral reef if present in large numbers.

Text by Paula Mikkelsen, Photo courtesy J. Casciano