page_title: Starfish
section_title: Living groups
hierarchical_section_title: Starfish / Evolution / Diversity / Living groups
context_page_description: 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 1,500 species of starfish occur on the seabed in all the world's oceans, from the tropics to frigid polar waters. They are found from the intertidal zone down to abyssal depths, 6,000 m below the surface.
  Starfish are marine invertebrates. They 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.
context_section_description: Brisingida (2 families, 17 genera, 111 species) 
  Species in this order have a small, inflexible disc and 6–20 long, thin arms, which they use for suspension feeding. They have a single series of marginal plates, a fused ring of disc plates, a reduced number of aboral plates, crossed pedicellariae, and several series of long spines on the arms. They live almost exclusively in deep-sea habitats, although a few live in shallow waters in the Antarctic. In some species, the tube feet have rounded tips and lack suckers.
  Forcipulatida (6 families, 63 genera, 269 species) 
  Species in this order have distinctive pedicellariae, consisting of a short stalk with three skeletal ossicles. They tend to have robust bodies and have tube feet with flat-tipped suckers usually arranged in four rows. The order includes well-known species from temperate regions, including the common starfish of North Atlantic coasts and rock pools, as well as cold-water and abyssal species.
  Notomyotida (1 family, 8 genera, 75 species) 
  These starfish are deep-sea dwelling and have particularly flexible arms. The inner dorso-lateral surfaces of the arms contain characteristic longitudinal muscle bands. In some species, the tube feet lack suckers.
  Paxillosida (7 families, 48 genera, 372 species) 
  This is a primitive order and members do not extrude their stomach when feeding, lack an anus and have no suckers on their tube feet. Papulae are plentiful on their aboral surface and they possess marginal plates and paxillae. They mostly inhabit soft-bottomed areas of sand or mud. There is no brachiolaria stage in their larval development. The comb starfish (Astropecten polyacanthus) is a member of this order.
  Spinulosida (1 family, 8 genera, 121 species) 
  Most species in this order lack pedicellariae and all have a delicate skeletal arrangement with small or no marginal plates on the disc and arms. They have numerous groups of short spines on the aboral surface. This group includes the red starfish Echinaster sepositus.
  Valvatida (16 families, 172 genera, 695 species) 
  Most species in this order have five arms and two rows of tube feet with suckers. There are conspicuous marginal plates on the arms and disc. Some species have paxillae and in some, the main pedicellariae are clamp-like and recessed into the skeletal plates. This group includes the cushion stars, the leather star and the sea daisies.
  Velatida (4 families, 16 genera, 138 species) 
  This order of starfish consists mostly of deep-sea and other cold-water starfish often with a global distribution. The shape is pentagonal or star-shaped with five to fifteen arms. They mostly have poorly developed skeletons with papulae widely distributed on the aboral surface and often spiny pedicellariae. This group includes the slime star.
caption: Red-knobbed starfish, a member of Valvatida
exif: {}