The blood of horseshoe crabs is not red, but shows a rare blue color. This is because its blood contains a special molecule-copper hemoglobin. The concentration of copper hemoglobin in the blood of horseshoe crabs is very high, which makes the blood of horseshoe crabs appear blue.

Copper hemoglobin is different from hemoglobin in humans and other animals because its molecule contains copper ions. The main function of copper hemoglobin is to transport oxygen and bind to oxygen more easily than other hemoglobins, which allows horseshoe crabs to survive in deep sea environments where oxygen concentrations are very low.