The Anatomy of a Horseshoe Crab
A horseshoe crab has a helmet shaped shell which helps
prevent the horseshoe crab from flipping over. The shell is made out of
a tough horny material called chitin. It is similar to the substance in
animal hooves, horns, and claws. The shell is not brittle, but flexible,
like a very thick fingernail. Young horseshoe crabs are often green brown,
and older ones are usually dark brown.
phot permission: BioWhittaker
Horseshoe crabs have three main body parts. The front part
is called the cephalothorax. It is shaped like a horse’s hoof. The second
part, the abdomen, is smaller and is shaped somewhat like a triangle. These
two parts are connected by a hinge that enables the crab to bend in the
middle. With this hinge the horseshoe crab is able to do a kind of bend-flip
swimming motion. The third body part is a sharp pointed tail. It is called
a telson. It looks like a spike, but it is not dangerous. The tail
is used as a rudder and it helps move the crab though the sand and mud.
Also, if the crab gets flipped upside down, it bends its abdomen where
it joins the main shell and digs into the sand with its tail to support
itself while it turns over.
When you turn over a horseshoe crab, the first
small pincers you see on the cephalothorax are called chelicerae. Some
people call them feeding grippers because they are used to probe the muck
for food. Next there are five pairs of walking legs. The back pair of legs
is extra long. They have fanlike structures on their tips to help them
burrow in the sand and mud. There are small pincers on the last pair that
are used for cleaning the gills in the abdomen. Also, when the male crab
becomes an adult, the first pair of legs changes into “boxing glove” shaped
claspers, so it can hold on to the female during mating.
A horseshoe crab’s mouth is located between its
legs. It is a slit, and it does not have any jaws or teeth.
photo permission: Assateague Naturalist
Behind the legs and mouth, on the underside of the
abdomen are five pairs of gills. Each of the ten gills holds a stack of
about one hundred sheets of tissue that are broad and flat and look like
the pages of a book. If the crab is out of the water, the gills must
be kept moist for the crab to live.
A horseshoe crab has ten eyes, but not like our eyes
or the eyes of other animals we might know. There are two large compound
eyes on either side of its shell, two simple eyes in the center, five light
sensing organs under its shell, and cells in its tail that react to light.
The two eyes on the underside and five of the eyes on top probably can’t
see images, but can tell light from dark. With the two large compound eyes
on either side of the cephalothorax the horseshoe crab can see images,
but they are blurry and are black and white.
permission granted by Enchanted
The heart of an adult horseshoe crab is about the size
and shape of a long, thin link of breakfast sausage. The nerve that connects
the heart to the brain lies outside of the heart. This has made it easy
for scientists to examine a horseshoe crab’s heart, and they have learned
important facts about human hearts by studying the hearts of horseshoe
crabs. Also, scientists discovered that when horseshoe crab blood is exposed
to air it is bright blue. It turns blue because horseshoe crabs have copper
in their blood. (Human blood looks red because it contains iron.)
photo permission: BioWhittaker
Adult female horseshoe crabs are sometimes
twice as big as males. Adult females are about two feet long, including
the tail, and the biggest and oldest females may weigh up to ten pounds.
The biggest crab in the picture is the female. Two males are hanging on
to the back of her shell.
How a Horseshoe Crab Eats
How a Horseshoe Crab Breathes
How a Horseshoe Crab Sees
How a Horseshoe Crab Molts
How a Horseshoe Crab Moves
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