Horseshoe crabs are marine and brackish water arthropods of the family Limulidae and the only living members of the order Xiphosura.[2] Despite their name, they are not true crabs or crustaceans; they are chelicerates, most closely related to arachnids.
Horseshoe crabs live primarily in and around shallow coastal waters on soft, sandy or muddy bottoms. They tend to spawn in the intertidal zone at spring high tides.[3] They are eaten in some parts of Asia, and used as fishing bait, in fertilizer and in science (especially Limulus amebocyte lysate). In recent years, population declines have occurred as a consequence of coastal habitat destruction and overharvesting.[2] Tetrodotoxin may be present in one horseshoe crab species, Carcinoscorpius rotundicauda.[4]
Fossil records for horseshoe crabs extend back as far as 480 million years ago, with extant forms being living fossils.[5] A 2019 molecular analysis places them as the sister group of Ricinulei within Arachnida.[6]
Horseshoe crabs resemble crustaceans but belong to a separate subphylum of the arthropods, Chelicerata.[7] Horseshoe crabs are closely related to the extinct eurypterids (sea scorpions), which include some of the largest arthropods to have ever existed, and the two may be sister groups.[7][8] Other studies have placed eurypterids closer to the arachnids in a group called Merostomata.[9] The enigmatic Chasmataspidids are also thought to be closely related to the horseshoe crabs.[10] The earliest horseshoe crab fossils are found in strata from the Lower Ordovician period, roughly 480 million years ago.[5]
The Limulidae are the only recent family of the order Xiphosura, and contains all four living species of horseshoe crabs:[1][2]
The entire body of the horseshoe crab is protected by a hard carapace. It has two compound lateral eyes, each composed of about 1,000 ommatidia, plus a pair of median eyes that are able to detect both visible light and ultraviolet light, a single parietal 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. Having relatively poor eyesight, the animals have the largest rods and cones of any known animal, about 100 times the size of humans,[15][16] and their eyes are a million times more sensitive to light at night than during the day.[17] They use their chelicerae—a pair of small appendages—for moving food into the mouth. The next five pairs of appendages, the first of which are the pedipalps, are used for locomotion (ambulatory legs). The mouth is located in the center of the legs, whose bases are referred to as gnathobases, and have the same function as jaws and help grind up food.[18] In extant species their appendages are uniramous, but the fossil genus Dibasterium had four pairs of branched walking legs.[19] The pedipalps on a male change shape on their terminal molt, becoming boxing glove-like claspers that are used for grasping the female during mating. The last pair of legs for both male and female are the main legs used for pushing when walking on the ocean floor. The remaining leg pairs have a weak claw at the tip.[20] Lost legs or the telson (tail) may slowly regenerate, and cracks in the body shell can heal.[21]