Benthic animals are classified according to their size and habitat. Starfish are macrobenthos because they are larger than 0.04 inch in diameter or arm span, according to the Encyclopaedia Britannica. Although the size of a starfish varies according to species, they measure from 4.7 inches to 9.4 inches on average, according to National Geographic. Starfish are also called epibenthos because they live on the top of the seafloor or on rocks and corals, unlike other benthos that live buried in the seafloor and are called infauna.
Starfish have their mouths on their underside and their anus on the upper side. Most sea star species can project their stomachs out from their mouths to envelope and digest the prey at the same time. Once they capture the prey and start digestion, starfish withdraw their stomachs back into their bodies, where digestion is eventually ended. This enables starfish to catch prey larger than their mouths. Although animals are the main diet of most starfish, some species also feed on algae.
Under their arms, starfish have hundreds of minuscule "tube feet," which move through a water vascular system. Depending on species, starfish can have from five to 40 arms, according to National Geographic. Water goes into a canal system, which is connected to the feet. The water expands the feet, which move slowly in a wave; the feet are also used to cling on the substrate when starfish climb corals, rocks and other uneven surfaces.
Starfish live only in salt waters. Although they are found on the bottom of all oceans, the northern Pacific has the largest variety of species. Different species are found at distinct depths. According to Encyclopaedia Britannica, Albatrossaster richardi can live at 19,800 feet, but most species of starfish are often found in shallower waters, where food for them is more abundant. A common species in North America is the bat star (Asterina miniata), which can live at depths of 951 feet, according to Monterey Bay Aquarium.