MEROSTOMATA (Horseshoe Crabs)
By Jamie Forysth
MEROSTOMATA IS :
A mostly extinct class of Arthropods (class: arthropoda). They are in the kingdom of animalia and the domain of
Where did they come from? part is the abdomen it is where the
Horseshoe crabs evolved out of the gills are attached. The last part is the
Paleozoic era along with trilobites. telson or tail. It is used to steer in
They have been unchanged for water and ﬂip its self over if needed.
nearly 400 million years. They are T r a n s p o r t a t i o n
distantly related to spiders. They For there external transportation
mature at age nine and are able to most horseshoe crabs have four legs
live for thirty years. that end in pincers. Males have large
W h a t d o t h e y l o o k l i k e ? claws on their two front legs. Their
T h e y a r e p r o t e c t e d b y a n internal transportation is when it
exoskeleton mad up of three parts: eats its food it is broken down in the Above is the underside of a
the Carapace, Abdomen, and the gizzard. Then enzymes are secreted horseshoe crab. You can see the
legs, the mouth, and the start
telson. The Carapace is ﬁrst it into a stomach-intestine by an organ of the book gills.
contains the eyes, legs, pincers, called hepatopancreas.
mouth, brain, and heart. The next
What do they eat? How do they keep living?
Horseshoe crabs eat worms, thin- Horseshoe crabs mate in late spring,
shelled mollusks, crabs, and bits of
early summer on sandy beaches. The
ﬁsh. They have no jaw so they grind
there food with the bristles on there females dig out holes in the sand
legs and in their gizzard. The and lay 200 to 300 eggs in each. The
gizzard contains sand and gravel.
nests are below the high-water
Place in the Food Chain mark. When the females are done
Adult horseshoe crabs eat almost laying the eggs the males cover the
anything. They eat clam, worms, eggs with sperm. The eggs will hatch
scrape algae, and plants off of rocks. within several weeks.
As said before they have no jaw so Responding to stimuli
they grasp their food with their legs T h e r e h a s b e e n b e h a v i o r a l
and use the spines on them to grind experiments on horseshoe crabs to
it up. Other types of animals depend investigate there visual perception.
on the horseshoe crab and their eggs H a b i t u a t i o n ( d e c r e a s e i n
such as: shore birds, ﬁsh, and psychological response/behavioral
turtles. re s p o n se t o s t i m u l i) t o l ig ht
H o w d o t h e y b r e a t h ? occurred. Also males use brightness
Horseshoe crabs have book gills and shape info. To recognize a mate.
right directly behind there legs. They They are also able to regenerate lost
let them breath underwater or on limbs. Also horseshoe crabs can use
land as long as the gills remain wet. their tails to ﬂip themselves over
book gills get there name because when on their backs.
they are alternating air pockets this
causes them to look like a folded
Top: Is a seagull eating a
Middle: Shows the book
gills.You can see how it
resembles an open book.
Bottom: Is a baby
Right: Is an horseshoe crab
on its back.
T y p e o f s y m m e t r y Another group of extinct arthropods
Th e h o r s e s h o e h a s b i l a t e r a l is the Trilobite. They are marine
symmetry. Bilateral symmetry means arthropods that lived in the early
if you would cut it in half it would
be roughly the same on each side. Cambrian period. They went extinct
E x t i n c t o r E n d a n g e r e d ? at the end of the Permian (250
million years ago).
There are several types of horseshoe
What’s the importance?
crabs one that is considered
e n d a n g e r e d i s t h e j a p a n e s e Horseshoe crabs are used in
h o r s e s h o e c r a b ( Ta c h y p l e u s medical testing. Their blood has a
Tridentatus). It is found in the Seto certain clotting reaction used in the
Inland sea and is considered Limulus Amebocyte Lysate test to
endangered because of loss of ﬁ n d b a c t e r i a l e n d o t o x i n s i n
habitat. Also there some extinct medicines. They are also used in
species. Other extinct organisms in ﬁnding cures for diseases that can’t
t h e M e r o s t o m a t a c l a s s a r e be cured by penicillin. Once the
Eurypterids (sea scorpions). They are horseshoe crabs are used for there
an extinct group of arthropods. They blood they are returned to the
predate the earliest ﬁshes. The ocean. In a week blood levels are
largest type was Jaekelopterus witch able to return to normal.
could measure up to 6 ft. 7 in.
Top: Is a dipiction of a
Middle: Is a Japanese
Bottom: Is a picture of
Limulus Amebocyte Lysate
Left: Is a picture of a
Some Interesting Facts:
Horseshoe crabs have blue blood. They have ten eyes. Two of them are
There blood contains hemocyanin witch has compound eyes located on the top of the
copper in it. They don’t have hemoglobin. shell. The other eyes are scattered on
Hemocyanin is technically colorless when the body. They are light sensors that
not exposed to oxygen then when help navigate.
exposed to oxygen it turns dark blue.
Horseshoe crabs grow by molting. They
grow twenty-ﬁve percent larger each time
they molt. About after sixteen molts they
are fully grown.
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