The horseshoe crab is a marine creature, also known as the representative of "blue blood". The reason why their blood appears blue is because their blood contains a special molecule - copper hemoglobin. The concentration of this molecule in horseshoe crab blood is so high that it gives the blood a blue color.

Copper hemoglobin combines with oxygen more easily than other hemoglobins, allowing horseshoe crabs to survive in deep-sea environments. In the deep sea, the concentration of oxygen is very low, but copper hemoglobin can effectively transport oxygen to various parts of the horseshoe crab's body in this environment.