Sea Urchins

Echinoid Lovenia woodsii from the Upper Miocene of Beaumaris, Victoria. Source: Museum Victoria.

These animals, together with sea stars and crinoids, belong to a large group of marine animals called echinoderms.

Sea urchins are free-living, either on the ocean floor or in burrows up to 18 cm deep. Their bodies are enclosed in a cushion-shaped skeleton, which is often covered with movable (and sometimes poisonous) spines. Both skeletons and spines are often found on Victorian beaches.

Many sea urchins show a five-fold symmetry, some are more irregular, and others are heart-shaped. Interesting features of some species include brood-pouches in which the females hatch their eggs, and a special mechanism on the underside of the animal known as 'Aristotle's lantern', which is a mouth enclosed by five jaws, each with a single tooth. Tube feet are located on the upper and lower sides of sea urchins, and may be sticky to catch food, or may have suction cups attached to their ends to aid movement of the animal.

There are about 7000 extinct species of sea urchins and about 900 living species. They first appeared in the Ordovician Period (434 to 490 million years ago) and subsequently became very diverse. Sea urchins were quite rare during the Permian and Triassic Periods (298 to 205 million years ago), almost becoming extinct at the end of the Permian Period (about 251 million years ago).

Fossil sea urchins are found at Point Addis and Beaumaris in Victoria.