Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea. Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish are also known as asteroids due to being in the class Asteroidea. About 1,900 species of starfish live on the seabed in all the world's oceans, from warm, tropical zones to frigid, polar regions. They are found from the intertidal zone down to abyssal depths, at 6,000 m (20,000 ft) below the surface.
The Arcida is an extant order of bivalve molluscs. This order dates back to the lower Ordovician period. They are distinguished from related groups, such as the mussels, by having a straight hinge to the shells, and the adductor muscles being of equal size. The duplivincular ligament, taxodont dentition, and a shell microstructure consisting of the outer crossed lamellar and inner complex crossed lamellar layers are defining characters of this order.
The Valvatida are an order of starfish in the class Asteroidea, which contains 695 species in 172 genera in 17 families.
Xanthoidea is a superfamily of crabs, comprising the three families Xanthidae, Panopeidae and Pseudorhombilidae. Formerly, a number of other families were included in Xanthoidea, but many of these have since been removed to other superfamilies. These include Carpilioidea, Eriphioidea, Hexapodoidea, Pilumnoidea and Trapezioidea. Even in this reduced state, Xanthoidea remains one of the most species-rich superfamilies of crabs.
The Echinasteridae are a family of starfish in the monotypic order Spinulosida. The family includes eight genera and about 133 species found on the seabed in various habitats around the world.
Goniasteridae constitute the largest family of sea stars, included in the order Valvatida. They are mostly deep-dwelling species, but the family also include several colorful shallow tropical species.
The Forcipulatida are an order of sea stars, containing three families and 49 genera.
The Velatida are an order of sea stars containing about 200 species in five families. These sea stars normally have thick bodies with large discs.
The Brisingids are deep-sea-dwelling starfish in the order Brisingida.
Elasipodida is an order of sea cucumbers. They have numerous appendages, including conical papillae and leaf-like tentacles. Although many species are benthic, a number are pelagic, and may have their appendages modified to form sails or fins. Most members of the order inhabit deep-sea environments, such as the species of the genus Enypniastes.
Trapezioidea is a superfamily of crabs. Its members live symbiotically with corals, and have a fossil record stretching back to the Eocene.
Odontohenricia is a genus of starfish in the family Echinasteridae in the order Spinulosida.
Archasteridae is a family of starfish found in shallow waters in the tropical Indo-Pacific region. The genus Astropus, previously included in this family, is now included in the genus Archaster with the single species, Astropus longipes, being accepted as Archaster lorioli Sukarno & Jangoux, 1977.
Pterasteridae is a family of sea stars in the order Velatida, consisting of eight genera.
Metrodira is a genus of starfish in the family Echinasteridae in the order Spinulosida.
Aleutihenricia is a genus of starfish in the family Echinasteridae in the order Spinulosida.
Dictyaster is a small genus of starfish in the family Echinasteridae in the order Spinulosida.
Rhopiella is a genus of starfish in the family Echinasteridae in the order Spinulosida.
The Stichasteridae are a small family of Asteroidea in the order Forcipulatida. Genera were formerly unassigned, or in the family Asteriidae.
