Common starfish

Despite its name, this star is not a fish. It has no fins, backbone or gills. Its relatives include sea urchins, sea cucumbers, brittle stars and sand dollars. They form a spiny-skinned group. This star has hard plates under its skin and often a rough or spiny surface. To move, it walks on hundreds of water-powered tube feet under its arms. If it loses an arm, it can grow a new one. A cut-off arm cannot grow a new animal unless it keeps part of the middle disc.

Seawater pressure in canals through the body moves each foot. Sucker discs on the tips help it grip. The body is often about as wide as a tea saucer to a dinner plate. It usually has five arms that are broad at the base, with short white spines often in rows.

Its rough upper side is usually orange, yellow or brown with a pale underside. It lives on rocks, sand, mud and mussel beds, from rockpools near the shore down to several hundred metres deep.