With five or more pointed arms, the marine animals called starfish look like drawings of stars. The rest of their name, however, is a bit misleading. Starfish are not a type of fish. A fish has a backbone, but a starfish does not. Sometimes starfish are called sea stars. They belong to the scientific group called Echinodermata, meaning “spiny skinned.”

Where starfish live

Starfish live in all oceans except those of the polar regions. A large number of the approximately 1,800 species, or kinds, of starfish can be found in the northern Pacific Ocean. They can live at sea level or at depths of hundreds of feet.

Physical features

Starfish come in many colors, including brown, red, orange, and pink. While most adult starfish are about 8–12 inches (20–30 centimeters) across, some kinds may reach up to 25 inches (65 centimeters).

The body of a starfish consists of a central disk and a number of arms extending outward. Both the disk and the arms are covered with short spines. In addition, some starfish are covered with pincerlike organs called pedicellariae. On the upper side of a starfish are tiny holes that let water into the body.

Although most species of starfish have five arms, many have more; some can have as many as 50. Many starfish can grow another arm if they lose one. At the tip of the arms are eyespots that are sensitive to light. On the lower side are grooves with rows of flexible tube feet, which may be sucker-tipped. The tube feet help starfish to creep around and cling to rocks and other surfaces.

Behavior

Most starfish feed on such animals as clams, oysters, and snails. Although small prey are usually swallowed whole, many starfish have a unique way of eating larger food. After pulling apart the prey's shell with its suckered feet, a starfish may turn its own stomach out through its mouth. It can then digest the prey mostly outside of the starfish's body.

A few starfish reproduce without a partner. These starfish create another starfish by fission, a process by which a starfish's own body divides. Most starfish, however, reproduce by mating. Some species watch over their eggs before they hatch, but others just release the eggs—sometimes up to 2.5 million at a time—into the water.

Crown-of-thorns starfish

In the 1960s and 1970s scientists were concerned about the crown-of-thorns starfish because it began multiplying at alarming rates. This heavy-spined, many-armed creature feeds on coral, so the worry was that coral reefs and islands might be harmed. Thus, many of these starfish were purposely killed. By the late 1970s, however, new research showed that the increase was part of the animal's natural cycle—similar increases had happened at other times in history and were followed by periods of decline.