sea star, also called starfish, any marine invertebrate of the class Asteroidea (phylum Echinodermata) having rays, or arms, surrounding an indistinct central disk. Despite their older common name, they are not fishes.The roughly 1,600 living species of sea stars occur in all oceans the northern Pacific has the greatest variety. Most species are 20 to 30 cm (8 to 12 inches) across some are only 1 cm (0.4 inch), and others are as much as 65 cm (25 inches) across.
Sea star arms—typically five in number—are hollow and, like the disk, covered with short spines and pedicellariae (pincerlike organs); on the lower side are grooves with rows of tube feet (see video of tube foot anatomy and physiology), which may be sucker-tipped or pointed. A sea star can lose one or more arms and grow new ones. Its tube feet enable it to creep in any direction and cling to steep surfaces.Primitive sea stars feed by sweeping organic particles that collect along the arm grooves into the mouth on the underside of the disk. Advanced forms either evert (turn outward) the stomach upon the prey (bivalve mollusks, coral polyps, other echinoderms) for external digestion or swallow the prey whole (see video of sea star preying upon a mussel). The internal skeleton of the sea star consists of limy plates. Respiration usually is through skin structures. Light-sensitive spots occur at the tips of the arms.
