Starfish or sea stars are a class of echinoderms generally shaped like a star polygon. Starfish are also known as asteroids because they form the taxonomic class Asteroidea. About 1,900 species of starfish live on the seabed, and are found in all the world's oceans, from warm, tropical zones to frigid, polar regions. They can occur from the intertidal zone down to abyssal depths, at 6,000 m (20,000 ft) below the surface.
Starfish typically have a central disc and usually five arms, though some species have a larger number of arms. The aboral or upper surface may be smooth, granular or spiny, and is covered with overlapping plates. Many species are brightly coloured in various shades of red or orange, while others are blue, grey or brown. Starfish have tube feet operated by a hydraulic system and a mouth at the centre of the oral or lower surface. They are opportunistic feeders and are mostly predators on benthic invertebrates. Several species have specialized feeding behaviours including eversion of their stomachs and suspension feeding. They have complex life cycles and can reproduce both sexually and asexually. Most can regenerate damaged parts or lost arms and they can shed arms as a means of defense.
Most starfish have five arms that radiate from a central disc, but the number varies with the group. Some species have six or seven arms and others have 10–15 arms. In Antarctic Labidiaster annulatus, the number of arms can exceed fifty. Evidence from gene expression finds that the starfish body corresponds to a head externally (with lips attached to the tube feet) and a torso internally.
Most species are generalist predators, eating microalgae, sponges, bivalves, snails and other small animals. The crown-of-thorns starfish consumes coral polyps, while other species are detritivores, feeding on decomposing organic material and faecal matter. A few are suspension feeders, gathering in phytoplankton; Henricia and Echinaster often feed with sponges, taking advantage of the water current they produce. Various species can absorb organic nutrients from the surrounding water, and this may form a significant portion of their diet.
Starfish live in marine waters around the world including both tropical and polar waters. They are mainly benthic animals, living in sandy, muddy and rocky substrates. They range from shallow, intertidal waters to the deep-sea floor down to at least 6,000 m (20,000 ft). Starfish are most common along the coast.
Most species of starfish are gonochorous, with there being separate male and female individuals. Some species are simultaneous hermaphrodites, producing eggs and sperm at the same time, and in a few of these the same gonad, called an ovotestis, produces both eggs and sperm. Other starfish are sequential hermaphrodites. Protandrous individuals of species like Asterina gibbosa start life as males before changing sex into females as they grow older. In some species such as Nepanthia belcheri, a large female can split in half and the resulting offspring are males. When these grow large enough they change back into females.
Starfish may be preyed on by conspecifics, other starfish species, tritons, crabs, fish, gulls, and sea otters. Their first lines of defence are the saponins present in their body walls, which have unpleasant flavours. Some starfish such as Astropecten polyacanthus also include powerful toxins such as tetrodotoxin, while the slime star can ooze out large quantities of repellent mucus. The crown-of-thorns starfish possesses sharp spines, toxins and bright warning colours.
Several species sometimes suffer from a wasting condition caused by Vibrio bacteria. A more widespread sea star wasting disease sporadically causes mass mortalities. The results of a 2025 study of starfish off the coast of central British Columbia suggest that those living in the fjords can better survive outbreaks of the disease due to the lower temperatures and higher salinity of their environment.
Top-rated dive sites where divers have spotted Starfish.
