The Sea Star

We usually call sea stars starfish.  But they're not really fish at all.  They are invertebrates belonging to the phylum Echinodermata, the same phylum as sea lilies, feather stars, brittle stars, sea daisies, sea urchins, sand dollars, and sea cucumbers.

Name: Sea star
Scientific name: Asterias
Range: Oceans, seas, and other salt water areas.
Habitat: Found in tidewater coastal regions and in deeper parts of oceans.
Status: Abundant in wild (5 orders and 1500 living species known)
Diet in the wild: Feed on ocean floor deposits, small marine animals (eaten whole), larger prey (eaten by extruding stomachs and digesting prey outside body).
Diet in the zoo: Crustaceans, small fish, algae.
Location in the zoo:  James R. Record Aquarium (exhibit closed)


Body plan: The star-shaped body consists of a central disc and five arms or "rays."  Each ray contains an extension of the body cavity and its organs.  Sea stars range in size from 1-2 cm (.4-.8 in.) to 65 cm (26 in.) in diameter.  They are flattened and stiff-bodied. Sea stars are secondarily radially symmetrical -- i.e. the adults have five-fold radial symmetry. However the larvae are bilaterally symmetrical.

Tube feet: Each arm contains a groove consisting of rows of tiny, flexible tube feet .  Sea stars can move in any direction by gripping with some of its tube feet and pulling itself forward.  Think of them as an animal with no head, four legs and a tail -- except the five arms get to take turns being the tail. The tube feet have suction discs that attach to a substrate and allow the sea star to remain stationary or move around in turbulent waters.

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Madreporite plate: The madreporite plate extends from the central disc and is made of calcium carbonate.  It lets water seep into the sea star's intricate water vascular system, which aids in locomotion. 

Mouth: The mouth is located in the center of one side of the central disc.  It usually faces downward and is surrounded by movable spines.

Anus: The anus is not always present in the sea star, but when it is, it is on the body's upper surface, in the middle of the central disc.

Digestive glands: A sea star's digestive glands branch from secretory and digestive ducts called the pyloric ceca.  Two pyloric ceca extend into each arm of the sea star, and are connected to the stomach.  (See Digestion for more information.)

Feeding: When eating bivalves, a sea star will wrap its arms around the animal, grips it with its tube feet, and pries the valves apart.  Once the valves are open, the sea star pushes slips its stomach between them and partially digests what's inside.

Digestion:The sea star digests food using juices secreted from the pyloric ceca.  Partially digested food goes to the oral stomach first, and then goes to the pyloric stomach.  After going through the intestine, indigestible material goes through the rectal ceca and out the anus.

Reproduction: Most sea stars have separate sexes and spawn their gametes into the water for fertilization.  External fertilization must occur in the right light and temperature to be successful.  To ensure this success, a sea star will release pheromones with its sperm or eggs to lure others to the area to spawn.
A sea star can also reproduce asexually by dividing the central disc in two and regenerating each half.

Development: Embryos feed on plankton and are ciliated.  After gastrulation, the embryo becomes a larva, which forms arms and grows into a juvenile sea star.  The juvenile eventually matures into an adult.

Regeneration: Many sea stars can regenerate body parts.  If a detached portion of a sea star contains part of the central disc, that portion can grow into a brand new star.  Regeneration usually takes up to one year to complete. Not a bad survival trait to have, if you ask me.

Nervous system: A nerve ring encircles the mouth.  Radial nerves extend into each arm and coordinate the functions of the tube feet.  A nerve net in the body wall coordinates other parts of the sea star's brainless nervous system.

Sensory structures: Sea stars sense light with red eyespots on each arm.  Sensory tentacles at the end of each arm sense chemicals and vibrations in the water.

Comments about sea stars at the Fort Worth Zoo: The Fort Worth Zoo has two bat sea stars (shown here) on display.  Their surroundings are pretty sparse--I think they could use some more plants in their tank, along with more fish and crustaceans to feed on.

Personal observations:  The only sea star I knew for a long time was the dried up one on top of my TV at home.  But these animals are more than decoration. They are living creatures precious to the ocean and freshwater ecosystems.

Sources cited:

Hickman, Cleveland P. and Frances M. Hickman. Laboratory Studies in Animal Diversity. Boston: McGraw-Hill, 1995.

Miller, Stephen A. and John B. Harley.  Zoology.  Boston: McGraw-Hill, 1999.

"Sea star." The Columbia Encyclopedia.  New York: Columbia UP, 1993.

"Starfish." Microsoft Encarta Online.


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Julie King's Sea Star Page - Texas Wesleyan University - 1999