Horseshoe crab is the oldest marine animal that lived in the depths of the sea more than 450 million years ago. This arthropod gets its interesting name from its long, spiked tail located at the back of the body.
Modern representatives of the horseshoe crab are no different from the representatives of this species that lived several million years ago. Almost all of its body consists of a dense shell that hides the cephalothorax, the only exception is a long tail in the form of a long spine. In this case, the cephalothorax has two simple middle eyes and two complex - lateral ones.
This "living fossil" has no teeth; the forelimbs, which are grouped around the slit mouth, serve as their replacements. With these limbs, the horseshoe crab breaks food and swallows it. The rest of the limbs, six pairs in total, are located on the abdomen and serve for movement and breathing (gill legs). The tail serves as a rudder, controlling movement, and a kind of ballast that keeps this arthropod in the optimal body position for it.
An interesting fact is that the hemolymph (blood) of the horseshoe crab is blue. This is due to the presence of a specific pigment - hemocyanin, which ensures the saturation of the horseshoe crab's body with oxygen.
Horseshoe crabs reproduce by laying eggs, reaching the age of 10 years. During spawning, the female crawls out of the water onto the shore (this fact makes scientists assume that in ancient times the horseshoe crabs could be an animal living on land) and lays up to 1000 eggs in the sand, which the male fertilizes. From fertilized eggs, larvae first appear (with underdeveloped internal organs) about 4 cm in size, which after a week become fully adult individuals.
Modern horseshoe crabs live up to 30 years, reaching a length of up to 90 cm, which is much higher than the growth of their ancestors living in the Paleozoic period (their length was up to 3 cm). Four species of this arthropod have survived to this day, common off the coast of Southeast Asia (India, Indonesia, the Philippines, Vietnam, China, Japan), the Gulf of Mexico of North America, in the waters of the Atlantic.
