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Ask the Aquarium: Is a Star Fish Really a Fish?
8 May 1999

 

Ask the Aquarium recently received an e-mail from the parents of a second-grader whose drawing of a sea star was "corrected" by her teacher. The teacher marked through the label on the drawing and wrote, "No, star fish". The family wanted to know which term is correct.

 

Although most of us probably learned to call this invertebrate a star fish, a more modern and accurate term for it is indeed sea star. It is an echinoderm, which means it belongs to a group of animals known for spiny skin. That group also includes sea urchins and sea cucumbers. Sea stars belong to the scientific class called Asteroidea, which means star form.

 

Their cousins, the brittle and basket stars, belong to the Class Ophiuroidea, which means serpent tail in appearance. A sea star is not a fish because, among other things, it does not have a backbone. Fish are vertebrates, as are mammals, including humans.

 

Sea stars normally have five arms, although some species may have more. The arms come together in the middle of the animal, forming a central disc. The arms, in most cases, are triangular in shape. One of the most interesting characteristics of the sea star is its ability to regrow a severed arm.

 

Each arm is lined with tiny feet that are tipped with small suction cups. The sea star uses them to move and to capture prey.

 

While sea stars have delighted many beachcombers along our shores, some fishermen don’t look upon them so kindly. Sea stars can wreck havoc on oyster beds and on other shellfish populations. The sea star has a voracious appetite - - it eats almost continuously -- and has been known to cause serious damage to coral reefs in certain parts of the world.

 

Ironically, the sea star’s mouth is about a quarter of an inch in diameter and as a result, it cannot take in large bites of food. Therefore, it has found unique ways to eat. For instance, the common sea star (Asterias forbesi) eats not with jaws and teeth as many other animals do, but with its stomach. It pushes its stomach out through its mouth and wraps it around its prey. Clams and mussels can be digested alive this way.

 

While the sea star may seem invincible, it is completely defenseless against certain predators. Gulls, ravens and other birds, parasites and some fishes will eat the sea star, whose only protection is its rough, spiny armored skin. Its ability to regenerate limbs certainly comes in handy after an attack by these foes.

 

Sea stars may live in the low tide line close to shore or on sandy or rock bottoms out in the deep ocean. They do not travel fast and as a result, do not migrate far.

 

Several species of sea stars inhabit North Carolina waters, but the three most common are the common sea star, the striped or gray sea star (Luidia clathrata), and the margined or armored sea star (Astropecten articulatus).

 

Additional Resources

Sea Stars: Life in a Massachusetts Tide Pool

Sea Stars: San Antonio Zoo

SoundWaters - Long Island Sound

Secrets of the Ocean Realm Page - PBS