Do horseshoe crabs really have blue blood?
Hi Julia, horseshoe crabs do indeed have blue blood, this is because the protein that carries oxygen in their blood is haemocyanin. This protein is a pigment like the haemoglobin in our blood, but instead of using iron to carry the oxygen it uses copper. When iron is oxidised it goes red (hence our red blood) and when copper is oxidised it goes greeny blue (hence the blue blood).
