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Horseshoe Crab
Limulus polyphemus
Key Distinguishing Markings:
- These animals get their name from their general horseshoe shape.
- They have a hard exoskeleton like a crab.
- They are brown with a slender pointed tail.
- A horseshoe crab's tail, while menacing, is not a weapon. Instead, the tail is used to plow the crab through the
sand and muck, to act as a rudder, and to right the crab when it accidentally tips over.
- Unique looking, with nothing else that looks much like them in existence today.
Size:
- Horseshoe crabs are measured across the back (prosomal width)
and adult animals average 7” to 12” ‘prosomal’ width.
- Adult males average 2 pounds, and adult females average 5
pounds.
Distribution:
- Horseshoe crabs are found world wide, but our endemic species is
found from Nova Scotia, Canada to Cancun, Mexico.
- They inhabit coastal bays and near shore waters from spring to
fall, and move offshore to depths of 10 to 100 fathoms in the
winter.
Habitat:
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They inhabit all areas of Maryland’s coastal bays, and areas of
salinity greater than 6 parts per thousand in the Chesapeake Bay.
- Horseshoe crabs prefer shoals and slews in offshore waters.
- Inshore they can be found on sandy beaches and mud flats.
Food Preference:
- Horseshoe crabs are opportunistic feeders meaning they are not picky about
what they eat.
- They like to eat clams, and worms.
Spawning:
- Horseshoe crabs spawn each spring during the high tides of the new and
full moons, and begin spawning once the water temperature reaches 60 degrees
Fahrenheit.
- Males, two-thirds the size of their mates, cluster along the water's
edge as the females arrive. With glove-like claws on its first pair of legs,
the male hangs on to the female's shell and is pulled up the beach to the
high tide line.
- The female pauses every few feet to dig a hole and deposit close to
4,000 pearly green, birdshot-sized eggs. The male then fertilizes the eggs
as he is pulled over the nest. After the spawning is complete, the crabs
leave and the waves wash sand over the nest.
Fun Facts:
The species is estimated to be at least 300 million years old.
They are more closely related to spiders and scorpions than crabs.
Horseshoe crabs can swim upside
down in the open ocean using their dozen legs and five book gills (which
are hiding nearly 200 flattened gills)
to propel themselves.
Horseshoe crabs grow by molting and emerge 25 percent larger with
each molt. After 16 molts (usually between 9 and 12 years) they will
be fully grown adults.
They have 2 compound eyes on the top of their head and also 10
‘eyes’ that let them see UV light.
Horseshoe crab eggs are important food for migratory shore birds
that pass through during the spring mating season.
In the 1900s, horseshoe crabs were dried for use as fertilizer and
poultry food supplements before the advent of artificial
fertilizers.
Horseshoe crabs have blue blood.
The medical profession uses an extract from the horseshoe crab's
blood to test the purity of medicines. Certain properties of the
shell have also been used to speed blood clotting and to make
absorbable sutures.
Raising Horseshoe Crabs in the Classroom
Green Eggs & Sand
(A Tri-State Horseshoe Crab/Shorebird Education
Program)
| Family:
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Limulidae |
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Order: |
Xiphosurida |
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Class: |
Merostomata |
For more information on horseshoe crabs and their management
please contact Steve Doctor.
Photograph courtesy of John White.
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