| Class Merostomata | Order Xiphosura > Family Limulidae | about
learn only 3 things about them ...
They are NOT crabs. They are more closely related to spiders
They don't use their tail to sting people. Don't dangle
them by their tail. They will be helpless if the tail
provided a substance used to test human medications. Singapore
scientists made breakthroughs in cloning this substance
seen? Horseshoe crabs are sometimes encountered on our
undisturbed shores, more often in our northern shores especially near
What is a horseshoe crab? The
horseshoe crab is a strange, ancient creature that has been around
since before the dinosaurs. It is not a crab or even a crustacean.
It is more closely related to spiders and scorpions of the Class Arachnida.
There are only four species of living horseshoes crabs in the world.
Limulus polyphemus is found on the Atlantic coast. In Southeast
Asia there are three: the Mangrove horseshoe
crab (Carcinoscorpius rotundicauda) which is also the smallest
horseshoe, and the Coastal horseshoe crab
(Tachypleus gigas) and in Japan, China and southern Sabah are
found the Chinese horseshoe (Tachypleus tridentatus).
Features: Adult body 15-25cm in diameter. Its shape is
ideal for bulldozing through the mud and sand, and clinging to the
bottom in rough water. It probably got its common name because its
shell resembles a horse's hoof.
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 the shore.
These might just be moults. Moults are lightweight, have transparent
eyes and no bad smell. More
Sometimes confused with stingrays.
In murky waters, these two different animals do have a similar profile,
both being round and flat with a long tail. Stingrays are fishes that
are related to sharks.
What does it eat? 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. These legs are also toothed, and
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 contains copper compounds which carry oxygen,
the way iron does in our blood. So horseshoe crab blood is blue when
exposed to air! But the horseshoe crab is NOT the only blue-blooded
arthropod. Some true crabs and other arthropods also have blue blood.
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. Various creatures may settle on a horseshoe crab
keelworms and other
encrusting plants and animals.
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. Links to more info below.
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: The Coastal
horseshoe crab (Tachypleus gigas) is listed as 'Endangered'
and the Mangrove horseshoe crab
(Carcinoscorpius rotundicauda) as 'Vulnerable' on the Red List
of threatened animals of Singapore. Populations of these ancient creatures
in Singapore have been severely reduced over the last two decades
due to habitat loss.
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.
Coastal horseshoe crab
Changi, May 05
Mangrove horseshoe crab
Pulau Sekudu, Dec03
Different kind of legs
Changi, May 05
Changi, May 05
Mouth in the middle of the legs.
Changi, May 05
Using its tail to flip over to the right side
Changi, Jun 05
Often seen in a pair, the smaller male
on top of the larger female.
Pulau Sarimbun, May 05
Juvenile horseshoe crab.
Kranji, Jun 08
With encrusting animals.
Changi, Jul 04
Often entangled in abandoned drift nets.
Changi, Oct 07
Horseshoe crab eggs seen in sand.
Mandai, Apr 11
Males have modified
to hold on to the female.
Chek Jawa, Mar 11
have modified front legs.
Chek Jawa, Mar 11
crabs recorded for Singapore
in red are those listed among the threatened
animals of Singapore from Ng, P. K. L. & Y. C. Wee, 1994.
The Singapore Red Data Book: Threatened Plants and Animals of Singapore.
crabs in general
Singapore breakthrough in cloning Factor C
Singapore's horseshoe crabs
horseshoe crab species not found in Singapore