Horseshoe crab is an arthropod of the suborder Lycaenidae, and there are four species of horseshoe crabs in the world: American horseshoe crab, scorpion horseshoe crab, giant horseshoe crab, and Chinese horseshoe crab (Chinese national second-class protected animal). The body surface is covered with several butyric exoskeletons and is tan in color. The cephalothorax has a well-developed horseshoe-shaped dorsal carapace and is often referred to as the horseshoe crab (but is not actually related to crabs). Adults and males often live together in sandy and shallow waters, often crawling or diving in sediment. Horseshoe crabs first appeared in the Americas and then spread to Asia with the movement of the Earth's plates.
The body is approximately scoop-shaped and divided into three parts: head, thorax, abdomen and tail. Without antennae, the cephalothorax is broad and half-moon-shaped, with 6 pairs of appendages on the ventral surface, the first pair of appendages are claws, which are used for feeding. The ventral carapace is small, slightly hexagonal, with several sharp spines on both sides, 6 pairs of flaky swimming limbs underneath, and a pair of gills above each of the last 5 pairs for breathing. The tail is sword-shaped, slender, three-fusiform, and connected to the relevant nodes of the abdomen to support the body, especially when the dorsal abdomen is flipped. Three longitudinal crests are faintly visible on the back of the nail, with a pair of single eyes on either side of the front of the middle crest, and a compound eye on each side of the outer side of the crest, of which the single eye can be used to sense light, and the compound eye is used to find the opposite sex.
They have a large body size, up to 60 cm long (including tail length), and weigh 3-5 kg, making them less likely to be eaten by other animals. The ancestors of the horseshoe crab appeared in the Devonian period of the Paleozoic period of geological history, when dinosaurs had not yet risen and primitive fish had just emerged, and they were one of the oldest animals on Earth. Fossil horseshoe crabs dating back 500 million years have been found, and they are close relatives of the long-extinct trilobites. Most of the animals of their contemporaries evolved or became extinct, and the horseshoe crab has retained its original and ancient appearance since its appearance more than 400 million years ago, so it is called "living fossil".
It is a carnivorous animal, a food-eating animal and a mollusk, and sometimes also eats seabed algae. Like other arthropods, the horseshoe crab grows up by shehulling, and is about 1.3-1.4 times longer than the previous body size by a single shelling, but the mortality rate is also high, which can be said to be a kind of life-threatening growth. Due to its slow growth rate, it is a small animal that does not grow up and takes nearly 8-13 years to complete reproduction. It is still uncertain how many years it will take to grow into adults, but it is thought that males shed their skin 15 times and become adults in the 13th year, while female horseshoes 16 times and become adults in the 14th year.
Horseshoe crabs have high medicinal value, and their flesh, shell and tail can be used in medicine. The blood of the horseshoe crab is blue due to the presence of copper ions and is a blue-blooded animal. The "horseshoe reagent" extracted from this blue blood can accurately and quickly detect whether the internal tissues of the human body are infected by bacteria. They are also widely used in the pharmaceutical and food industries to monitor toxin contamination, making it a medically popular animal. In addition to its medicinal value, horseshoe crabs are of high food value (although the smallest individual scorpion horseshoe crab is poisonous, other species of adult horseshoe crabs are edible). ）