Asterias forbesi

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Geographic Range

Asterias forbesi are commonly found in intertidal areas and shallow waters of the Atlantic Ocean on the North American Coast from the Gulf of Maine to the Gulf of Mexico (Dale, 2000; Pfeffer, 1989).

Habitat

Asterias forbesi is found in the littoral zones of the North American Atlantic. They may be found in abundance, but they don't form colonies. They like rocks, boulders, and oyster/clam/scallop/mussel beds. Rocks are important to help prevent washing away and oyster beds have plenty of food within range (Moore, 1997).

Physical Description

Most A. forbesi range from 7-15 cm in diameter. They are tan, brown, or olive with tomes of orange, red, or pink. Like all sea stars, A. forbesi have "spiny skin" (a thin layer of skin covering spiny ossicles) covering their skeleton. The skeleton is made of plates called ossicles and bound by connective tissue so that they move like flexible joints. The small spines are surrounded at the base by pedicellariae which are little jaws that keep the body free of debris and maybe catch a little food, too. The spines of A. forbesi are large in diameter and are an obvious feature of the external appearance. There are little tufts of skin on the surface that serve as gills. The mouth is on the ventral (oral) side and the anus is on the dorsal (aboral) side. A. forbesi along with the rest of the echinoderms are radially symmetrical. They have five arms that are capable of regenerating. The ventral sides of the sea star's arms are covered with tube feet. These tube feet have suctions at the bottom that suck in water and channel it to canals that run through the body. There is a small hard spot on the dorsal surface of the body called the madreporite or sieve plate. In A. forbesi, the madreporite is pink in colors. The madreporite has pores that allow it to filter the water before it enters the sea star's water vascular system. The madreporite feeds into the ring canal in the middle of the sea star. Attached to the ring canal are radial canals that branch off. Each radial canal runs down an arm. The radial canals are surrounded by ampullae and tube feet on each side. Ampullae are the bulbs at the top of the tube feet. (Amaral, 2000; Amos and Amos, 1985; Bertin, 1967; Dale, 2000; Fox, 1999; Raven, 1999; Vodopich, 1999).

Reproduction

Sea stars in general have separate sexes. There are gonads in each arm on the ventral side. There is external fertilization because sperm and eggs are shed into the water. Females can release up to 2.5 million eggs. When one female sheds her eggs, other females in the area are stimulated to shed their eggs and then males are stimulated to shed their milt. The eggs develop into bipinnaria larvae, which lasts for about 3 weeks before settling and metamorphosing. The larvae are free-swimming and bilaterally symmetrical. They develop into sessile radial adults (Bertin, 1967; Raven, 1999).

Behavior

A. forbesi can move at a rate of 15-20 cm per minute in unthreatened ocean water, but when under attack, can speed up to 25-35 cm per minute. Sea stars move via their water vascular system by changing the water pressure and moving the tube feet. A. forbesi relies heavily on chemoreception to orient themselves and get around. A. forbesi responds to chemosensory stimuli and can locate the source of odors. They use different orientation paths for different odors. When there is prey around, A. forbesi walks faster and at more direct angles to the food. If a sea star is turned on its back, it turns one arm so that it grips the ground surface with its tube feet. The arm turns until the body has completed a slow somersault into its normal position. Sea stars do not move like wheels. They move in straight lines with one arm in advance when they are moving their fastest. During times of great wave action in shallow water, sea stars cling to rocks, flattening themselves against the rocks with all the power their tube feet can exert (Bertin, 1967; Dale, 2000; Grzimek, 1972).

Food Habits

Sea stars are carnivorous and like to eat other invertebrates, marine worms, crustaceans, gastropods, sea urchins, and molluscan bivalves like mussels, clams, and oysters. A. forbesi feeds chiefly on bivalve molluscs. They grasp the mollusk and use their tube feet to suction and pull the shells apart enough to extend their stomachs out through their mouth into the mollusk. Digestion (via possible toxic juices) occurs inside the shell, turning the mollusk into liquid that is guided into the sea star's mouth by the cilia on its arms. The sea star will win in the battle against the bivalve due to muscle endurance and its ability to insert its stomach through thin openings (Amaral, 2000; Amos and Amos 1985; Bertin, 1967; Dale, 2000; Pfeffer, 1989).

Economic Importance for Humans: Positive

Another species of starfish in the genus Pisaster is a keystone predator in the rocky intertidal zone off the Pacific Coast. It maintained diversity in the tidal region by keeping the strongly competitive bivalves at a low enough population level that they could not monopolize all the resources and form a monoculture. Although not studied, it is conceivable that A. forbesi plays a similar role on the Atlantic and Gulf Coast. (Raven and Johnson 1999)

Economic Importance for Humans: Negative

Asterias forbesi can get into mollusk beds and compete with the farmers and fishermen for food. If there is an overpopulation of sea stars, they are hard to get rid of because if they break, they will regenerate, and then there will be many more.

Conservation Status

Asterias forbesi is doing fine. Currently, there is no special need to save the starfish. The population is thriving without human help.

Other Comments

Sea stars aren't highly preyed upon due to their tough, non-nutritious calcified skin. There are some kinds of crustaceans and fish that will eat them, though. A. forbesi is also known as Forbes' Sea Star and as the common sea star. It is one of the most common starfish species on the North American Atlantic coast (Dale, 2000).

Sea stars can completely regenerate as long as they have one fifth of the central disk and one arm. There is one eyespot at the end of each arm.

Contributors

Kim Chau (author), Southwestern University, Stephanie Fabritius (editor), Southwestern University.

Glossary

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

World Map

coastal

the nearshore aquatic habitats near a coast, or shoreline.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

native range

the area in which the animal is naturally found, the region in which it is endemic.

References

Amaral, K. 2/1/95. "Sea stars" (On-line). Accessed February 19, 2000 at http://www.umassd.edu/Public/People/Kamaral/thesis/SeaStar.html.

Amos, W., S. Amos. 1985. The Audubon Society Nature Guides: Atlantic and Gulf Coasts. New York: Alfred A. Knopf.

Bertin, L. 1967. The Larousse Encyclopedia of Animal Life. Verona: McGraw-Hill.

Dale, J. 01/26/2000. "Madreporite Nexus" (On-line). Accessed February 20, 2000 at http://www.vsf.cape.com/~jdale/.

Fox, R. 04/04/99. "Asterias" (On-line). Accessed February 28, 2000 at http://www.science.lander.edu/rsfox/asterias.html.

Grzimek, B. 1972. Grzimek's Animal Life Encyclopedia, Volume 3: Mollusks and Echinoderms. New York: Van Nostrand Reinhold Company.

Moore, P., D. Lepper. June 1997. Role of chemical signals in the orientation behavior of the sea star Asterias forbesi. Biological Bulletin, 192: 410-417.

Pfeffer, P. 1989. Predators and Predation: The Struggle for Life in the Animal World. New York: Facts On File.

Raven, P., G. Johnson. 1999. Biology (5th ed.). Boston: McGraw-Hill.

Vodopich, D., R. Moore. 1999. Biology Laboratory Manual (5th ed.). Boston: McGraw-Hill.