Sea Stars

Sea star Eoactis stachi, from the Upper Silurian in excavations for the Melbourne Underground Rail Loop. Source: Museum Victoria.

These animals are echinoderms, and so are related to crinoids, sea urchins and sea cucumbers. They are also known as starfish or asteroids and are often found along the coast of Victoria.

Sea stars usually have five arms, although these may be further subdivided. The length of the arms varies, depending on the species, but are usually flexible enough to enable the animal to climb on rocky surfaces. Each arm has tube feet located on its underside, and respiratory organs on its upper side. The animals are fairly active, and are the main predators of bivalves (like clams). Sea stars are able to prise open the shells of bivalves using their suckered feet, after which they extend their stomach through their mouth and into the opening in the bivalve shell. Some digestion actually takes place within the bivalve shell, after which the stomach is retracted back into the sea star.

Sea stars are first known in the fossil record in the late Ordovician and early Silurian Periods (about 434 million years ago), and are thought to have evolved from crinoids.

Their skeletons break up fairly easily, and so fossils are not found frequently, although they are known to have occurred at times in vast numbers.

Fossil sea stars are found in various parts of Victoria, in particular east of Kilmore.