Horseshoe Crab
Though they are called “crabs”, horseshoe crabs are actually not crustaceans at all. They are arthropods more closely related to spiders, scorpions and other arachnids.
Kingdom: Animalia
Phylum: Arthropoda
Order: Xiphosura
Family: Limulidae
Genera: Carcinoscorpius, Limulus, Tachypleus
Species: C. rotundicauda (Mangrove Horseshoe Crab), L. polyphemus (Atlantic Horseshoe Crab), T. gigas (Indo-Pacific Horseshoe Crab), T. tridentatus (Chinese or Japanese Horseshoe Crab)
Common Names: Horseshoe Crab
Horseshoe crabs spend much of their time on the sea floor in search of food, which mainly includes molluscs, worms, and crustaceans.
As they grow, horseshoe crabs undergo “molting”, a process in which they shed their exoskeleton (shell), as a larger one takes its place. They will do this up to 16 or 17 times before adulthood.
Horseshoe crabs often live in deep ocean waters for much of the year, but travel to coastal near-shore areas to breed. Females lay between 60,000 and 120,000 eggs. The reason so many eggs are laid is due to high levels of predation, as many young horseshoe crabs are eaten by shore birds and other predators.
Horseshoe crabs are named for the hard shell (or carapace) that surrounds their body, which is shaped like a horseshoe. Their entire upper body is enclosed in this protective “armor”, while their undersides reveal their ten legs and “book gills”, which help them in both breathing and swimming. A long, straight tail pokes out from behind their shell.
The largest species of horseshoe crab, the Chinese horseshoe crab, can grow up to 31 inches long (including the tail), while the smallest, the mangrove horseshoe crab, only grows to about a foot long.
Though they look somewhat fearsome and have a reputation for being venomous or dangerous, horseshoe crabs are in fact completely harmless and pose no danger to humans.
Horseshoe crabs have been around for over 300 million years. Their overall body design and shape has largely remained unchanged during that huge span of time. They are the only living members of their order, Xiphosura. Despite being commonly called crabs, their closest living relatives in the subphylum Chelicerata are the arachnids, including spiders and scorpions.
While there are currently four living species of horseshoe crab in three genera, there are several more extinct species known from the fossil record.
Some species of horseshoe crabs are currently Endangered or Vulnerable, though the conservation status of all living species of horseshoe crab has not been determined. In Thailand, the female of the Indo-Pacific horseshoe crab is a delicacy, and the harvesting of them has contributed to declining numbers.
Damage to and development of coastal areas also limits horseshoe crab breeding areas, which can have a negative effect on the population. They are also harvested for use as bait to catch other animals, and the eggs are consumed in certain countries.
Horseshoe crabs are important for medical research. Their blood, which is blue, contains a type of cell called an amebocyte, which the crab uses to combat disease. Medical scientists use these cells to detect harmful bacteria in human medicine. Because of this, horseshoe crab blood is in high demand. Once their blood is drawn, the collected animals are released back into the wild. Most animals survive the process, but some do not. A synthetic alternative has been developed, which may over time help to provide an alternative to harvesting horseshoe crabs.