- Norwegian: sjøstjerner
Most sea stars or starfish, have five arms connected to a central disk, but there are several exceptions. If an arm is lost, a new grows out. As long as the central disk is intact, there is a god chance the sea star will survive. This depends on food access while waiting for the new arm to grow. The sea stars feed on mussels, snails, many bristled worms and even other echinoderms. The mouth is positioned under the disk. On the tip of each arm there are light sensitive organs acting almost as eyes. Every arm includes a hydraulic system using water to extract the arms and muscles to contract them again. Sea stars have a rather complex nervous system, but lack a central brain.
The class of sea stars form the subphylum Asterozoa, together with the brittle stars (Ophiuroidea). The seven orders of sea stars include more than 2000 species worldwide, 43 of them are registered in Norway.