SEA STARS: Asteroidea

PHYSICAL CHARACTERISTICS

Sea stars have spiny skin and at least five arms large enough to hold digestive and reproductive organs. Some species of sea stars have as many as thirty arms. Sea stars are 0.4 inch (1 centimeter) to almost 3 feet (91 centimeters) across. The bottom of each arm is covered with rows of tube feet along a groove. Depending on the species, tube feet have suckers that help the sea star stick to hard surfaces or assist in prying open the shells of its prey.

The skeleton of sea stars consists of small plates that act as a firm but flexible skeleton. The upper and lower body surfaces also are covered with pinchers that range from simple spines to hooks. The upper surface is covered with many small, clear sacs used for exchanging oxygen, that is, for breathing. Sea stars have a nerve net but no brain. Even so, they are advanced enough to change on the basis of previous experiences and to stop behaviors, usually feeding behaviors, that continue to be unsuccessful.

GEOGRAPHIC RANGE

Sea stars live in the Pacific, Atlantic, and Indian oceans, including the Antarctic regions.

HABITAT

Sea stars live in the sea from the shore to the deepest ocean. They live in sea grass beds, under rock rubble, on coral reefs and rocky underwater cliffs, and in sand and mud.

DIET

Sea stars eat anything that is too slow to escape, such as mollusks, crustaceans, sponges, worms, and even other sea stars and their relatives. Mollusks (MAH-lusks) are animals with a soft, unsegmented body that may or may not have a shell. Crustaceans (krus-TAY-shuns) are water-dwelling animals that have jointed legs and a hard shell but no backbone.

BEHAVIOR AND REPRODUCTION

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.

That's a Lot of Shellfish

Sand stars can swallow hundreds of live young mollusks in one feeding trip.

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.

SEA STARS AND PEOPLE

Sea star swarms eat so many mollusks, they harm the livelihood or earnings of people who catch and sell shellfish. Coral reefs also often fall victim to the destructive feeding power of sea stars. However, sea stars do have some benefit to people. In some places, sea stars are used as an ingredient in fish meal, which is fed to poultry. Some companies collect sea stars for biological supplies to schools and collectors. The dried bodies of sea stars also are important to people who make a living selling souvenirs.

YOU THOUGHT YOU WERE HUNGRY?

Sea stars can go for months without food. One species can survive for eighteen months without eating.

CONSERVATION STATUS

Sea stars are not considered threatened or endangered.

SAND STAR (Astropecten irregularis): SPECIES ACCOUNTS

Physical characteristics: Most sand stars are 2 to 4 inches (5 to 10 centimeters) across, but those in deep water can be as large as 8 inches (20 centimeters) across. Sand stars are pale violet to yellowish and have five arms that form stiff angles. These stars have upper and lower plates fringed with small spines. The tube feet are pointed and have no suckers.

Geographic range: Sand stars live in the western part of the Atlantic Ocean from Norway to Morocco.

Habitat: Sand stars live in water 16 to 3,280 feet (5 to 1,000 meters) deep. They live on bottoms ranging from coarse gravel to fine mud, although they usually live on sand.

Diet: Sand stars are greedy predators of mollusks, worms, crustaceans, and other sea stars and their relatives. They dig up their prey and swallow it whole.

Behavior and reproduction: Sand stars live partially or completely buried. They travel into deeper water during the winter and swarm closer to shore when the seawater warms between May and July. They then release their eggs and sperm into the water, where fertilization takes place.

Sand stars and people: Arm damage to sand stars is used as an indicator of damage caused by fishing boats that drag their nets along the bottom.

Conservation status: Sand stars are not considered threatened or endangered. ∎

CROWN-OF-THORNS (Acanthaster planci): SPECIES ACCOUNTS

Physical characteristics: Crowns-of-thorns are about 16 inches (40 centimeters) across and have ten to thirty arms covered in thorn-like spines, which are venomous. These sea stars are red and green, and the spines have reddish tips. Young crowns-of-thorns have camouflage coloring.

Geographic range: Crowns-of-thorns live in the Pacific and Indian oceans.

Habitat: Adult crowns-of-thorns live on open sand and feed among coral. The young hide among the coral, under rocks, and in coral rubble.

Diet: Crowns-of-thorns are greedy predators of coral.

Behavior and reproduction: Crowns-of-thorns feed at night. Large swarms appear suddenly, feed on coral, and then disappear. These sea stars turn their stomach inside out over coral, releasing an enzyme that turns the coral tissue to liquid, and then absorb the liquid. Crowns-of-thorns can survive without food for six months and feed on about 3 square miles (8 square kilometers) of coral per year. Crowns-of-thorns can regrow broken arms to form another star. These animals have separate sexes and reproduce by releasing eggs and sperm into the water, where fertilization takes place. There are two stages of larvae.

Crowns-of-thorns and people: Crowns-of-thorns have caused widespread damage to coral reefs. The stings of these sea stars are painful and cause nausea.

Conservation status: Crowns-of-thorns are not considered threatened or endangered. ∎

BLUE STARFISH (Linckia laevigata): SPECIES ACCOUNTS

Physical characteristics: Blue starfish have five arms and can be as large as 12 inches (30 centimeters) across. Adults are brilliant blue. The young are bluish green or purplish with dark spots.

Geographic range: Blue starfish live in the Indian and Pacific oceans.

Habitat: Blue starfish live on coral gravel in direct sunlight, in sand, and under rocks.

Diet: Blue starfish graze on waste and small animals.

Behavior and reproduction: Adult blue starfish hide during the day in coral and rocky crevices. They turn their stomachs inside out to feed on coral. Blue starfish use asexual reproduction, but scientists do not know the details.

Blue starfish and people: Blue starfish are used in home aquariums and are the most commonly imported sea star.

Conservation status: Blue starfish are not considered threatened or endangered. ∎

NORTHERN PACIFIC SEA STAR (Asterias amurensis): SPECIES ACCOUNTS

Physical characteristics: Northern Pacific sea stars are 16 to 20 inches (40 to 50 centimeters) across. They have five arms that are turned up at the tips. These sea stars are rosy brown, yellowish brown, red, or purple. Their underside is very flat. The skin is covered with unevenly arranged spines that have jagged ends.

Geographic range: Northern Pacific sea stars live in the western, northern, and eastern parts of the Pacific Ocean.

Habitat: Northern Pacific sea stars live in shallow water on sheltered coasts in sand and mud and among rocks and algae thickets. Algae (AL-jee) are plantlike growths that live in water and have no true roots, stems, or leaves.

Diet: Northern Pacific sea stars eat scallops, oysters, mussels, shrimp, and other sea stars.

Behavior and reproduction: Northern Pacific sea stars feed by using their tube feet and arms to pull apart the shells of their prey before turning their stomachs inside out. When it is time to release eggs and sperm, the swarms are so dense that the females lift themselves above the ground on their arms and release eggs between the arms while the male sea stars crawl beneath. The females release about twenty million eggs, which when fertilized develop into drifting larvae.

Northern Pacific sea stars and people: Northern Pacific sea stars accidentally introduced into Australia and Tasmania have caused damage to the shellfish business.

Conservation status: Northern Pacific sea stars are not considered threatened or endangered. ∎

VELCRO SEA STAR (Novodinia antillensis): SPECIES ACCOUNTS

Physical characteristics: Velcro sea stars have ten to fourteen arms with rows of spines and teethlike pinchers. The arms are long and thin. These sea stars are brick red.

Geographic range: Velcro sea stars live in the Atlantic Ocean near the West Indies.

Habitat: Velcro sea stars live in water 1,970 to 2,625 feet (600 to 800 meters) deep. They attach themselves to steep rocky surfaces in areas where the current is strong. They often live near large sponges, sea fans, and hard corals.

Diet: Velcro sea stars eat animal plankton, which is microscopic animals drifting in the water.

Behavior and reproduction: Velcro sea stars look like baskets when they extend their arms and curl the tips inward over their mouth. They stay still while waiting for prey but slowly bend their arms to capture prey, which becomes stuck on arm spines and hooks. Velcro sea stars release eggs and sperm into the water, where fertilization takes place.

Velcro sea stars and people: Velcro sea stars have no known importance to people.

Conservation status: Velcro sea stars are not considered threatened or endangered. ∎

FOR MORE INFORMATION

Books:

Carson, Rachel. The Edge of the Sea. 1955. Reprint, Boston: Mariner, 1998.

Niesen, Thomas M. The Marine Biology Coloring Book. 2nd ed. New York: HarperResource, 2000.

Web sites:

"Echionoderms: The Spiny Animals." Oceanic Research Group.http://www.oceanicresearch.org/echinoderm.html (accessed on February 28, 2005).

"Sea Stars." OceanLink.http://oceanlink.island.net/oinfo/biodiversity/seastars.html (accessed on February 28, 2005).