” Horseshoe crabs are marine arthropods of the family Limulidae and order Xiphosura or Xiphosurida. They are invertebrates, meaning that they lack a spine. Horseshoe crabs live primarily in and around shallow ocean waters on soft sandy or muddy bottoms. They occasionally come onto shore to mate. They are commonly used as bait and in fertilizer. In recent years, population declines have occurred as a consequence of coastal habitat destruction in Japan and overharvesting along the east coast of North America. Tetrodotoxin may be present in the role of species inhabiting the waters of Thailand.
Because of their origin 450 million years ago, horseshoe crabs are considered living fossils. 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. The horseshoe crab has five additional eyes on top of its shell. Despite 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. 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. The horseshoe crab has five pairs of legs for walking, swimming, and moving food into the mouth, each with a claw at the tip, except for the last pair.” https://en.wikipedia.org/wiki/Horseshoe_crab