Sea Star Line Art

Sea stars (Asteroidea; derived from ancient Greek ἀστήρ astḗr "star" and εἶδος eídos "form, shape") are a class of Eleutherozoa within the phylum Echinoderms. About 1,600 recent species of sea stars are known worldwide, making them the third largest group of animals within the echinoderms (Echinodermata) after the brittle stars (Ophiuroida) and sea cucumbers (Holothuroidea).

The basic body shape is a usually five-armed star, with the arms becoming uniformly more slender toward the tips. In many species the interradii (area between the arms) can become so wide that a pentagon is formed. This may be highly convex and cushion-like (Culcita) or extremely flat (Anseropoda placenta). Also almost spherical forms are possible. Thus, representatives of the genus Podosphaeraster resemble a spineless sea urchin. On the other hand, species of the genera Labidiaster, Brisinga, and Freyella are reminiscent of brittle stars with their rotund, uniformly thin long arms and an offset central body disk.

Sea stars colonize marine benthal worldwide from the intertidal zone to the deep sea. Representatives of the genus Hymenaster have been recorded in the Philippine Trench to depths of 10,000 meters. The largest genetic center of sea stars is found in the shelf seas of the northeast Pacific coast of the Americas from San Francisco through Alaska to the Aleutian Islands, Kuril Islands, and Sakhalin Island. More species occur there than in any of the remaining distribution areas. A second genetic center is in the Indonesian-Philippine-Australian region, with Australia and New Zealand in particular characterized by many endemics. In the polar seas, especially in Antarctica, starfish are the most important group of macrofauna in shallow water.