Horseshoe crabs are marine arthropods of the family Limulidae and order Xiphosura or Xiphosurida,
that live primarily in and around shallow ocean waters on soft sandy or muddy bottoms.
They occasionally come onto shore to mate. They are commonly used as bait and in fertilizer.
In recent years, a decline in the population has occurred as a consequence of coastal habitat
destruction in Japan and overharvesting along the east coast of North America. Tetrodotoxin
may be present in the roe of species inhabiting the waters of Thailand. Because of their origin
450 million years ago, horseshoe crabs are considered living fossils.
Classification Horseshoe crabs resemble crustaceans, but
belong to a separate subphylum, Chelicerata, and are closely related to arachnids. The
earliest horseshoe crab fossils are found in strata from the late Ordovician period,
roughly 450 Mya. The Limulidae are the only recent family of
the order Xiphosura, and contain all four living species of horseshoe crabs:
Carcinoscorpius rotundicauda, the mangrove horseshoe crab, found in Southeast Asia
Limulus polyphemus, the Atlantic horseshoe crab, found along the American Atlantic coast
and in the Gulf of Mexico Tachypleus gigas, found in Southeast and East
Asia Tachypleus tridentatus, found in Southeast
and East Asia Anatomy and behavior
The entire body of the horseshoe crab is protected by a hard carapace. It has two compound lateral
eyes, each composed of about 1000 ommatidia, plus a pair of median eyes that are able to
detect both visible light and ultraviolet light, a single endoparietal eye, and a pair
of rudimentary lateral eyes on the top. The latter become functional just before the embryo
hatches. Also, a pair of ventral eyes is located near the mouth, as well as a cluster of photoreceptors
on the telson. Despite having a relatively poor eyesight, the animals have the largest
rods and cones of any known animal, about 100 times the size of humans'. The mouth is
located in the center of the legs, where their bases have the same function as jaws and help
grinding up food. It has five pairs of legs for walking, swimming, and moving food into
the mouth, each with a claw at the tip except the last pair. The long, straight, rigid tail
can be used to flip itself over if turned upside down, so a horseshoe crab with a broken
tail is more susceptible to desiccation or predation.
Behind their legs, they have book gills, which exchange respiratory gases and are also occasionally
used for swimming. As in other arthropods, a true endoskeleton is absent, but the body
does have an endoskeletal structure made up of cartilaginous plates that support the book
gills. Horseshoe crab normally swim upside down, inclined at about 30° to the horizontal
and moving at about 10–15 cm/s. Despite this, they usually are found on the ocean
floor searching for worms and molluscs, which are their main food. They may also feed on
crustaceans and even small fish. Females are larger than males; C. rotundicauda
is the size of a human hand, while L. polyphemus can be up to 60 cm long. The juveniles grow
about 33% larger with every molt until reaching adult size.
Breeding
During the breeding season, horseshoe crabs migrate to shallow coastal waters. A male
selects a female and clings to her back. The female digs a hole in the sand and lays her
eggs while the male fertilizes them. The female can lay between 60,000 and 120,000 eggs in
batches of a few thousand at a time. Shore birds eat many of the eggs before they hatch.
The eggs take about two weeks to hatch. The larvae molt six times during the first year.
Raising horseshoe crabs in captivity has proven to be difficult. Some evidence indicates mating
only takes place in the presence of the sand or mud in which the horseshoe crab's eggs
were hatched. Neither what is in the sand that the crab can sense nor how they sense
it is known with certainty. Blood
Unlike vertebrates, horseshoe crabs do not have hemoglobin in their blood, but instead
use hemocyanin to carry oxygen. Because of the copper present in hemocyanin, their blood
is blue. Their blood contains amebocytes, which play a role similar to white blood cells
of vertebrates in defending the organism against pathogens. Amebocytes from the blood of L.
polyphemus are used to make Limulus amebocyte lysate, which is used for the detection of
bacterial endotoxins in medical applications. The blood of horseshoe crabs is harvested
for this purpose. Harvesting horseshoe crab blood involves collecting
and bleeding the animals, and then releasing them back into the sea. Most of the animals
survive the process; mortality is correlated with both the amount of blood extracted from
an individual animal, and the stress experienced during handling and transportation. Estimates
of mortality rates following blood harvesting vary from 3-15% to 10-30%.
Fishery Horseshoe crabs are used as bait to fish for
eels and whelk. However, fishing with horseshoe crab is temporarily forbidden in New Jersey
and restricted to only males in Delaware. A permanent moratorium is in effect in South
Carolina. The eggs are eaten in parts of Southeast Asia and China.
A low horseshoe crab population in Delaware Bay is hypothesized to endanger the future
of the red knot. Red knots, long-distance migratory shorebirds, feed on the protein-rich
eggs during their stopovers on the beaches of New Jersey and Delaware. An effort is ongoing
to develop adaptive-management plans to regulate horseshoe crab harvests in the bay in a way
that protects migrating shorebirds. References
Further reading Chisholm, Hugh, ed.. "King-Crab". Encyclopædia
Britannica. Cambridge University Press. External links
Media related to Limulidae at Wikimedia Commons
LAL Update Horseshoe crab history, biology, research
and conservation of the species Science Friday Video: horseshoe crab season
Horseshoe crab at the Smithsonian Ocean Portal The Horseshoe Crab – Medical Uses; The Ecological
Research & Development Group RedKnot.org links to shorebird recovery sites,
movies, events & other info on Red Knot rufa & horseshoe crabs.