There are more than 6,000 species today, however at their peak there were as many as 20,000 species. Fossil records date Sea Urchins as being 600,000,000+ years old.

A sea urchin has a round or flat case of 5 close-fitting, rigid plates called a test. this test is covered with a thin skin with rows of swivelling spines. Long, thin tube feet with suckers protrude from channels between the plates. Spines can be long, short, fine or thick.

Sea Urchins are free moving bottom dwellers. The tube feet are used for movement, helped by leverage from the spines on the underside of the body. These feet are also used for breathing, touch, taste, digging and passing body waste.

They eat algae, encrusting organisms such as sponges and bryozoans. The mouth is set in the middle of the underside of the body and contains a set of jaws and horny teeth which was named "Aristotle's lantern" over 2000 years ago.

Also Sea Urchins don't have a brain as such, instead they have a nerve ring near the hydraulic system used to power the tube feet.

The primary food of many urchin species is seaweed. Sea urchins have also been known to eat meat, including each other.

Usually found on hard or soft bottoms, in rock pools and below the low tide water mark between rocks and coral. They tend to wedge themselves in hollows during the day and are active at night. They range from 5-25 cm in diameter.

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