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 (Mya), horseshoe crabs are considered living fossils.
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 Edit
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 (0.22–0.34 mph). 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 (24 in) long (including tail). The juveniles grow about 33% larger with every molt until reaching adult size.