Starfish belong to the phylum Echinodermata, class Asteroidea, which consists of roughly 1,500 species (Boyer and Bearzi, 2003; Shine, 2002). They are not fish at all, and should correctly be called sea stars (Shine, 2002). Sea stars are benthic animals, living in every type of marine environment: shallow bays, lagoons, the back and front of reefs, external reefs and deep reefs, and can be found from the Arctic to the Antarctic oceans (Boyer and Bearzi, 2003; Shine, 2002). Like most Echinodermata, sea stars show radial symmetry, with no head or tail, and have a star-shaped body (Boyer and Bearzi, 2003). The symmetry of sea stars is usually pentaradial, with many of the organs repeated in each of arms. Sea stars have a distinctive shape, even when it is not immediately obvious, as in the armless species Culcita novaeguineae (Boyer and Bearzi, 2003). Once these are looked at from the ventral side, the pentaradial structure is obvious. Brittle stars have long, thin, flexible arms, and a small round or pentagonal central disc, e.g. the class Ophiuroidea.
The number of arms can vary from five in a species such as the northern star Asterias rubens, to as many as 50 in the Labidiaster annulatus (Shine, 2002). The sunstar, Solaster endeca, has long, thin arms, while the cushion sea star, Oreaster reticulatus, has short, wide arms. Sea stars range in size from the blood sea star (Henricia sanguinolenta), which is as small as the thumb, to the Northeastern Pacific star (Pycnopodia helianthoides), which is more than four feet across. They occur in a variety of colors from white to brilliant red to vivid blue.
The body wall of the sea star is reinforced with hard dermic plates, making it quite rigid (Boyer and Bearzi, 2003). The upper surface has a layer of rough dermal gills through which gas exchange takes place (Shine, 2002). Small pincer-like spines protect it from barnacles. Some species, such as the fish-eating Styl...