Starfish, or Sea Star, a bottom-dwelling marine animal. Despite its name, the starfish is not a fish; it is an echinoderm, a type of marine animal that is spiny-skinned. There are about 2,000 species, found in all seas, and most often near rocky shores. Starfish typically have five or more tapering arms radiating from a central point. The mouth is in the center on the underside. Extending from the mouth to the tip of each arm are grooves lined with tubelike feet. In certain species, the feet have suction disks. The starfish uses its feet to crawl along the ocean bottom. When an arm is lost, a new one soon grows in its place. Most starfish are brightly colored; red, orange, yellow, and pink are common colors. They vary in size from less than 1/2 inch (13 mm) across to more than four feet (1.2 m).

StarfishStarfish typically have five or more arms.

Starfish feed mainly on such invertebrates as coral and shellfish, especially clams and oysters. Starfish with suction disks use them to hold and force open the shells of shellfish. Once it has opened a shell, the starfish extends its stomach membranes from its mouth, inserts them into the shell opening, and secretes digestive juices that break down the shellfish's body into a form that can be absorbed into the stomach membranes. Starfish without suction disks swallow their prey whole, and eject the parts that are not digestible. Starfish are not eaten by humans.

What Makes a Sea Star an Echinoderm?

Unlike fish, sea stars don’t have backbones. Instead, sea stars and their relatives have hard plates under their skin. Some of these hard plates have spines. That’s one reason a sea star is an echinoderm.

The sea star’s center is round. Arms grow out from the center—like spokes in a wheel. The body of an adult sea star has several nearly identical sections. It can be divided into similar pieces—like the slices of a pie. Some other echinoderms have other shapes. Some look like balls. Others look like barrels. But they even have bodies with many similar sections.

Like all echinoderms, sea stars have a system of tiny tubes inside their bodies. The tubes extend outside the animal’s body. The closed tips of these tubes are called tube feet. Sea stars have rows upon rows of tube feet—as do most other echinoderms.

How Does a Sea Star Use Its Spines?

A sea star’s spines are sharp. If eaten, they can make for a very painful meal. That’s why many predators avoid sea stars. Still, a few animals—such as king crabs, sea otters, and gulls—eat sea stars. Somehow, they seem to be able to handle the sea star’s spines and bony plates.

The sea star called the crown-of-thorns has spines all over the top of its body. Like other sea stars, it also has shorter spines on its underside. Those spines lie along the rows of tube feet. If the sea star is in danger, it can close the spines together to protect its soft feet.

Sea stars use their spines for protection. Some sea stars also have tiny pinchers in between the spines on top of their bodies. These sea stars use their pinchers to snap at intruders. They can also use their pinchers to clean the sand off their bodies.

Where Is a Sea Star’s Head?

A sea star has no head. It has no brain either. But a sea star doesn’t need a brain to sense what is going on around it. Special cells on the sea star’s skin gather information about its surroundings. These cells then send signals through a network of nerves inside the sea star’s body. These signals trigger the animal to take some kind of action, such as to turn or to crawl.

A sea star also has another kind of network inside its body—a network of tubes. Tiny tubes extend from a sea star’s center to the tip of each of its arms. These tubes carry seawater throughout the animal’s body. A sea star uses the network of tubes to move its tube feet.

A sea star’s mouth is on the underside of the animal’s round center. The mouth leads directly to a large, baglike stomach. The sharp spines found all over the sea star’s body also surround its mouth. These spines help protect a sea star’s soft insides.