Sea Stars: Asteroidea

Flexible bodies and suckered tube feet keep sea stars firmly in place so they can withstand the force of crashing waves. Their flexibility also allows the stars to assume a variety of positions to capture and handle prey and closely follow irregular surfaces in search of food. The flexibility also helps sea stars upright themselves if they are overturned.

Sea stars use two different feeding methods. In one method the star takes the prey into its stomach alive. In the other the star uses its tube feet and arms to pull apart the shells of a prey animal. Then it turns its own stomach inside out and pushes it out through its mouth. Digestion begins when the stomach comes in contact with the soft body of the opened animal.

Sea stars swarm in large numbers at certain times of the year, usually for releasing eggs and sperm, feeding frenzies, or group travel to deeper water offshore. Some species of sea stars are active at dawn and dusk. Others are active during high and low tide, when the water is quiet enough for success in finding food.

Most sea stars have separate sexes with no visible differences between them. Each arm contains a pair of organs that fill with eggs or sperm. Most species release eggs and sperm into the water, where fertilization (FUR-teh-lih-ZAY-shun), or the joining of egg and sperm to start development, takes place. To increase the chances of fertilization, sea stars swarm when they are ready to release eggs and sperm. Fertilized (FUR-teh-lyzed) eggs rapidly develop into drifting larvae (LAR-vee), or animals in an early stage that change form before becoming adults. These larvae transform into another type of larvae, which transform into young sea stars that settle on the bottom and grow into adults.

In some species of sea stars, females hold their fertilized eggs in a space under an arm, in the stomach, or in the reproductive organs. After development in the stomach or reproductive organs, the young escape through the small openings in the female's body wall.

Some sea stars use asexual reproduction by splitting into two new stars or regrowing an entire animal from part of an arm. Asexual (ay-SEK-shuh-wuhl) means without the uniting of egg and sperm for the transfer of DNA from two parents.