Horseshoe crabs are neither horseshoe-shaped, nor are they crabs. However, they are very interesting creatures!
- They have existed for 450 million years, making them a "living fossil".
- Modern horseshoe crabs are almost indistinguishable from prehistoric species.
- They have 9 eyes. They can see ultraviolet light, but probably only detect movement, not clear images.
- Their blood is extremely valuable as a testing agent for medical application, and its blue!
- Their closest living relatives may be spiders.
- They swim upside down.
- They can grow up to 2 feet long.
- One species is thought to possess a neurotoxin.
- They have 5 pairs of legs, the last pair has structures just for cleaning their gills.
- During mating, the female lays 200-300 eggs, which the male covers in sperm.
Horseshoe crabs are marine and brackish water arthropods of the family Limulidae, suborder Xiphosurida, and order Xiphosura. Their popular name is a misnomer, as they are not true crabs, nor even crustaceans, as crabs are, but a different order of arthropod.
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. They are commonly eaten in 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. Tetrodotoxin may be present in one horseshoe crab species, Carcinoscorpius rotundicauda.
Given their origin 450 million years ago, horseshoe crabs are considered living fossils. A 2019 molecular analysis places them as the sister group of Ricinulei within Arachnida.
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 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. Having relatively poor eyesight, the animals have the largest rods and cones of any known animal, about 100 times the size of humans', and their eyes are a million times more sensitive to light at night than during the day. 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. In extant species their appendages are uniramous, but the fossil species Dibasterium had four pairs of branched walking legs. 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. Lost legs or the telson (tail) may slowly regenerate, and cracks in the body shell can heal.