ocean floor

Description

Horseshoe crabs are arthropods that live primarily in shallow ocean waters on soft sandy or muddy bottoms. They will, however, occasionally come on shore, e.g. for mating. They are commonly used as bait and in fertilizer, and in recent years there has been a decline in number of individuals, as a consequence of coastal habitat destruction in Japan and overharvesting along the east coast of North America. Horseshoe crabs look similar to crustaceans, but actually belong to Chelicerata, and are therefore more closely related to spiders and scorpions. Limulidae is the only recent family of the order Xiphosurida and contains all the 4 living species of horseshoe crabs: * Genus Carcinoscorpius o Carcinoscorpius rotundicauda, mangrove horseshoe crab, found in Southeast Asia. * Genus Limulus o Limulus polyphemus, Atlantic horseshoe crab, found along the northwest Atlantic coast and in the Gulf of Mexico. * Genus Tachypleus o Tachypleus gigas, coastal horseshoe crab, found in South and Southeast Asia. o Tachypleus tridentatus, Japanese horseshoe crab, found along East Asian coasts. The entire body of horseshoe crab is protected by a hard shell. They have two large compound eyes and multiple smaller simple ones atop the carapace. Beneath the carapace they look quite similar to a large spider. They have five pairs of legs for walking, swimming and moving food into the mouth. The long, straight, rigid tails can be used to flip themselves over if they are turned upside down, so a crab with a broken tail may die. Behind their legs, they have book gills, which exchange respiratory gases and are also occasionally used for swimming. While they can swim upside down, they usually are found on the ocean floor searching for worms and mollusks, 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 centimetres (24 in) long (including tail). The juveniles grow about 33% larger with every molt until reaching adult size. During the breeding season, horseshoe crabs migrate to shallow coastal waters. Males select a female and cling onto 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--120,000 eggs in batches of a few thousand at a time. Many shore birds eat the eggs before they hatch. The eggs take about 2 weeks to hatch. The larvae molt six times during the first year. Unlike humans, 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 for 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.