The horseshoe crab is an animal that lives in the deep sea. It does not belong to fish, but to the phylum Arthropoda, class Limostome. Horseshoe crabs are the largest species of arthropods. Because it is shaped like a horseshoe and moves like a spider, some people call it a horseshoe crab or a horseshoe crab. The horseshoe crab appeared at the end of the Devonian period 400 million years ago, and it can be called an ancient remnant of the ocean. It is a type of animal as old as fossil trilobites. Because it has gone through many vicissitudes but has not evolved much, it still maintains the old appearance of primitive organisms, so it is called a "living fossil".

Most horseshoe crabs in the water come in pairs; this is because the female horseshoe crab has four pincers on its front four legs, while the male horseshoe crab has four hooks. It turns out that the male horseshoe crab always puts a hook on the back of the female shark and lets his "wife" carry it around to make a living.

The ugly and lazy horseshoe crab is very dedicated to "love". Once a male and female become a couple, they become inseparable. The plump female horseshoe crab carries a smaller "husband" on her back as she crawls, earning the nickname "undersea mandarin duck". Fishermen in the Beibu Gulf area call them "two parents-in-law". Every spring when the water warms up, groups of horseshoe crabs take advantage of the tide to swim from the bottom of the sea to the beach to give birth. Experienced fishermen are familiar with the horseshoe crab's movement route and have laid out long fishing nets halfway along the way. Once the horseshoe crab is plotted against, it will have no choice but to wait for death in the net, unable to escape even if it has no wings. These married horseshoe crabs, whether they travel from the deep sea to the shallows, or are imprisoned in "jail", they always stay and travel together, never separated. The most surprising thing is that when he grabbed the male horseshoe crab's tail, the male horseshoe crab hugged the female horseshoe crab tightly and the female horseshoe crab did not abandon her husband and ran away. As a result, they were lifted out of the water together.