Husband and wife fish, scientific name "horseshoe crab" (HOU), also known as "mandarin duck fish", is an animal as old as trilobites (only fossils are left). This fish only finds one mate in its life. If its partner dies, it will never marry another horseshoe crab again. If one horseshoe crab is caught, the other one will follow. So people call it husband and wife fish. If people want to catch them, they first throw a big net into the sea, and then sprinkle some sticky fish food in the net to attract horseshoe crabs to eat. Horseshoe crabs will swim when they smell the fragrance, often two at a time, and their blood is blue.