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Horseshoe
crabs
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The horseshoe crab is a strange, ancient creature that is even older
than the dinosaurs. It is not a crab or even a crustacean. They are
more closely related to spiders and scorpions of the Class Arachnida.
How did it get its name? Possibly
because its shell resembles a horse's hoof. This shape is ideal for
bulldozing through the mud and sand, and clinging to the bottom in
rough water. A horseshoe has an exoskeleton, but unlike a crab's,
this does not incorporate calcium and is made of chitin and protein
instead. The shell is hard in adults, but more flexible in juveniles.
Like other arthropods, a horseshoe crab must moult to grow bigger.
During their first year, they may moult 5-6 times, growing 20-25%
with each moult. It takes about 7 years to reach maximum size. Sometimes,
you might come across what appears to be dead horseshoe crabs on Chek
Jawa. These might just be moults. Moults are lightweight, have transparent
eyes and no bad smell. More about moulting.
Horseshoe Food: A harmless creature,
the horseshoe crab bulldozes quietly along on the sea bottom feeding
on worms, clams and anything edible including dead animals. They may
also scrape off algae.
Eating with its legs! The horseshoe
crab has no jaws. It has to grind down its food with the rough spiny
areas (called gnathobases) near the base of the walking legs. The
first pair of legs are tiny with small pincers which pick up and pass
titbits into its four pairs of 'food processing' legs. Walking movements
grinds up the food and the bits flow into the mouth, which is between
the second pair of legs and conveniently faces backwards. So a horseshoe
crab can only eat while it walks! In fact, the Class it belongs to
is called Merostomata, which means 'thigh mouth'.
Galloping Horses? Horseshoes generally
creep slowly over the sea bottom. However, they can move more speedily
if they have to. They can use their last pair of legs, called pushers,
to lurch forwards. These legs are longer, have spines which flare
out when pushed against the sand, and are toothed. These legs are
also thought to direct water flow over the gills and to clean the
gills. Horseshoe crabs can also swim for short distances, using their
swimmerettes and gill flaps. They can also 'hop' over the sand slowly
by bending their hinged body then pushing forwards against the tail,
which is anchored in the sand.
Super gills: Horseshoes breathe
well in oxygen-poor water. They have five pairs of flap-like appendages
which contain book gills. Horseshoe crab blood (like that of some
other crabs and arthropods) contains copper compounds which carry
oxygen, the way iron does in our blood. So horseshoe crab blood is
blue when exposed to air!
Eyes Everywhere: A horseshoe crab
has a lot of eyes! It has a pair of compound eyes at the top of the
shell. Unlike an insect, these can't form an image and are used mainly
to find mates. It also has five simple eyes on the top of the shell,
a series of light sensors on the top and side of their tails, and
a pair on the underside near the mouth! The eyes of a horseshoe crab
are more sensitive at night, when they are active and seek out mates,
and less so during the day.
In a Tailspin: The sharp tail
of the horseshoe crab is is connected to the body in a ball-and-socket
joint so it is very mobile. The tail is not venomous and is not used
as a weapon. It is merely used as a lever to right itself if it is
overturned. If you see an upside down horseshoe crab struggling with
its tail waving around, do give it a helping hand. It will not hurt
you. The tail (called a telson or caudal spike) is also used as a
rudder when moving underwater. If a horseshoe loses its tail, it is
doomed. So please be gentle with its tail and don't dangle a horseshoe
crab by its tail.
Horsing around: Horseshoes mate
during high spring tides when they can reach the highest part of the
beach. The males are smaller and usually hitch a ride on the females
using their specially adapted hooked first legs. Sometimes several
males latch onto each other forming a chain on a female. The female
digs a pit near the high water mark and lays about 200-300 eggs. The
males release sperm over the eggs and the nest is covered. They may
come back again at the next high tide and a female may lay a total
of 2,000-30,000 eggs. In the US, migrating birds time their arrival
to feed on this bonanza of horseshoe crab eggs. Eggs hatch at the
next full moon when the tide is at its highest again. The hatchlings
(called trilobite larvae) look like miniature adults but without tails,
and are bright green! The larvae burrow into the sand and after a
few moults begins to develop tails.
Role in the habitat: Like other
scavengers, horseshoe crabs help keep the place clear of dead animals.
Although the adults have few natural predators (apparently, only sharks
and turtles will eat adults), their eggs and hatchlings are eaten
by many creatures.
Human uses: Horseshoe crab blood
has a substance that is so sensitive to bacteria that purified extracts
of the blood are used to test for the presence of bacteria in human
medication (e.g., intravenous fluids) and in medical tests. For more
on how this test was discovered and exactly how it works, see the
Horseshoe Crab
website. About 200,000 crabs are bled every year for this substance.
About 20% of a horseshoe's blood is extracted and in the US, laws
require that the animal be returned to the sea. But about 10% die
in the process. A team from the National University of Singapore's
Department of Zoology has cloned a substance to replace wild-extracted
horseshoe blood. For more about this breakthrough see Campus
News Online. Horseshoe crabs have also contributed in other ways
to human health. Much of the basic principles of vision is based on
studies of the horseshoe crab's eyes.
Status and threats: Humans are
the main threat to horseshoes. Habitat loss, pollution and overharvesting
have seriously depleted horseshoe populations. In the 1950's, they
were harvested in the US and ground up as fertiliser and livestock
feed. This only stopped when their numbers plunged drastically. Harvesting
began again in the 1980's, this time they were used as fish bait in
commercial eel traps; only the eggs (ripped out of females) are used.
Nothing, not even eels, like to eat horseshoe flesh. Overharvesting
of horseshoes also seriously affect birds migrating along the US Atlantic
coastline, as they depend on the egg bonanza to fuel them on their
long trip.
Horseshoe crab populations are vulnerable to overharvesting because
they reproduce slowly. Few hatchlings make it through the natural
predator net, they reach sexual maturity only at 9-12 years and are
rarely found far from where they were born. There are so many of them
only because they live for a long time, some up to 20-30 years.
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Righting itself
Underside
Buried at low tide
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quick
facts
15-25cm, sometimes seen in the mangroves, sand bar and seagrass
lagoon
Classification:
OrderXiphosura
Class Merostomata
Subphylum Chelicerata
Phylum Arthropoda |
