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 Asteroidea - Sea Star
The name Starfish has been replaced by the less misleading
name "sea star." After all a sea star is not a fish.
On your journey to the tide pools you are bound to see many of
these great creatures.
The class Asteroidea contains about 1,500 living species.
The body of a sea star has radial symentry and allows the creature
to meet the environment 360 degrees.
Bat star - Patiria miniata
Sea stars usually have five arms but it is not uncommon for
you to find one with more or less. If the sea star looses an
arm it immediately begins growing a new one. This is why you
may find some varieties of sea stars with some long arms and
some nubs. However, the ability to reproduce body parts varies
greatly among different species. At the tip of each arm there
is a small red or orange light sensitive eye spot.
Ochre seastar - Pisaster ochraceous
The body is dorsoventrally flattened. At the end of each
arm, on the "oral side," are a series of tubular feet.
The sea star moves by a water vascular system as part of each
of these tiny tube feet.
The water vascular system of echinoderms is a series of fluid
filled canals that utilizes hydraulic pressure for movement.
This fluid in the water vascular system is similar to sea water
but also contains cells, proteins and some unique ions. This
fluid is moved by the action of cilia.
Water enters the water vascular system through the madreporite.
The madreporite serves the dual function of pressure reception
and ensuring that an equal fluid pressure is maintained between
the environment and the inside of the water vascular system.
(Brusca and Brusca, 1990)
The tube feet adhere with tiny suction cups to a surface
and pull itself along. The tiny tube feet of the sea star are
used for location, attachment and respiration. On the "aboral"
top side, the sea star has a series of tiny pedicellaria.
The tiny pedicellaria of a sea star looks like a claw when
viewed under a microscope. It can remove unwanted particles from
the seastar's body. If food happens to land on the sea stars
aboral side, it can also use these pedicellaria to transport
the food to the arms and then into its mouth.
The pedicellaria is composed of three ossicles: one forming
the base and two in the form of jaws. In some, the ossicles forming
the jaws can be crossing each other like a pair of blades in
scissors.These tiny pinchers can help transfer food that lands
on the sea star's back to its' mouth. and also serve to groom
the sea star and keep its' aboral side clean.
If you put the sea star upside down on your arm, it will
immediately use these pedicellaria to adhere to the hair on your
arm. And likewise if you put the sea star right side up on your
arm, you will feel the tiny tube feet as it moves.
On the oral (bottom) side the mouth is located in the center
of a central disk. Five ambulacral grooves also radiate from
the central disk and extend outward through a series of calcareous
plates that make up the endoskeleton. The tube feet bear a small
suction cup at their tips. They serve in locomotion, attachment
and respiration.
The sea star has two stomachs, the pyloric and the cardiac.
The cardiac stomach can be inverted to the outside of the sea
star thus enabling it to get into small openings. The other stomach,
the pyloric, can produce digestive enzymes and store digested
food.
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