Horseshoe crab

Horseshoe crabs are arthropods of the family Limulidae and the only surviving xiphosurans. Despite their name, they are not crabs or even crustaceans; they are chelicerates, more closely related to arachnids like spiders, ticks, and scorpions. The body of a horseshoe crab is divided into three main parts: the cephalothorax, abdomen, and telson. The largest of these, the cephalothorax, houses most of the animal's eyes, limbs, and internal organs. It is also where the animal gets its name, as its shape somewhat resembles that of a horseshoe. Horseshoe crabs have been described as "living fossils", having changed little since they first appeared in the Triassic around 250 million years ago, and similar-looking fossil xiphosurans extend back to the Ordovician around 445 million years ago.

Only four species of horseshoe crab are extant today, the Atlantic horseshoe crab (Limulus polyphemus), native to the eastern coast of North and Central America, as well as the mangrove horseshoe crab (Carcinoscorpius rotundicauda), tri-spine horseshoe crab (Tachypleus tridentatus) and Indo-Pacific horseshoe crab (Tachypleus gigas), which are native to South, South East, and East Asia.

Most horseshoe crabs are marine, though the mangrove horseshoe crab is often found in brackish water, and the Atlantic horseshoe crab is resident in brackish estuarine ecosystems such as the Delaware and Chesapeake bays. Additionally, certain extinct species transitioned to living solely in freshwater. Horseshoe crabs primarily live at the water's bottom but they can swim if needed.

Horseshoe crabs are often caught for their blood, which contains Limulus amebocyte lysate, a chemical used to detect bacterial endotoxins. Additionally, the animals are used as fishing bait in the United States and eaten as a delicacy in some parts of Asia. In recent years, horseshoe crabs have experienced a population decline. This is mainly due to coastal habitat destruction and overharvesting. To ensure their continued existence, many areas have enacted regulations on harvesting and established captive breeding programs.