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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
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