Starfish are a group of invertebrates belonging to the echinoderms. Starfish live on the bottoms of all oceans; from tidal areas to the deep sea, but only in salt water. They form one of the largest groups of echinoderms, with about 1900 species.
Starfish have a star-shaped body with a central disc and five or more elongated lobes called arms. The central disc encloses the stomach, with the mouth opening at the bottom. In case the species has an anus, it lies at the top. The appearance varies from species to species. Species are known with several dozens of arms and besides the many brown-grey also red, blue and yellow species. Some species have spines, others are smooth. On the underside of the arms there are tube feet with sticky cups. Furthermore, each arm houses parts of the gastrointestinal system and genitals.
Starfish do not mate but release their gametes into the seawater. Some species have a form of breeding care. The larvae of most starfish are free-swimming and look like transparent, shrimp-like creatures. There are also starfish in which the larvae develop in a breeding pouch, and do not have a free swimming stage. The larvae go through different stages of development before turning into a small starfish. Starfish can regenerate lost or damaged parts of the body. Most species are predators; some can consume many bivalves in a short period of time.
The most common species along the Belgian and Dutch coast is the common starfish (Asterias rubens), which can reach a diameter of up to 50 centimeters. Starfish are often found alive on the beach and in tidal pools.