Horseshoe crab

Limulidae Temporal range: Early Triassic–Present PreꞒ Ꞓ O S D C P T J K Pg N
Tachypleus gigas, one of the four extant species
Jurassic-aged limulids. Crenatolimulus (A,B), Limulus (C), Mesolimulus (D,E)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Chelicerata
Order:	Xiphosura
Superfamily:	Limuloidea
Family:	Limulidae Leach, 1819[1][2]
Genera
See text

Horseshoe crabs are arthropods of the family Limulidae and are the only surviving xiphosurans. Despite their name, they are not true crabs or crustaceans. Rather, they are chelicerates. This makes them 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 the majority 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 changed little in appearance since they first evolved in the Triassic, earning them the title of "living fossil".

Only four species of horseshoe crab are alive today. Most are marine, though the mangrove horseshoe crab is often found in brackish water. Additionally, certain extinct species transitioned to living in freshwater. Horseshoe crabs are primarily benthic animals but can swim if needed. In the modern day, their distribution is somewhat limited, only being found along the east coasts of North America and southern Asia.

Horseshoe crabs are often caught for their blood, something valuable to the medical industry. The blood's usefulness comes from the 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 continuous existence, many areas have enacted regulations on harvesting and established captive breeding programs.