Why Do Some Animals Have Blue Blood

Why Some Animals Have Blue, Green, or Purple Blood

By Joseph Stromberg

October 31, 2014 8:30am EDT

Most animals on Earth have red blood due to the iron-based protein hemoglobin that binds oxygen. However, some animals have evolved with different blood pigments that give their blood a different color.

For example, some crustaceans, squid, and octopuses have blue blood due to the oxygen-transporting protein hemocyanin, which contains copper. Hemocyanin is not as efficient as hemoglobin at binding oxygen, so these animals need to have a higher volume of blood to circulate the same amount of oxygen.

Other animals, such as spiders and snails, have green blood due to the presence of the protein chlorocruorin, which contains iron but is not as efficient as hemoglobin at binding oxygen. Still other animals, such as sea cucumbers and earthworms, have purple blood due to the presence of the protein hemerytherin, which contains iron and is similar to hemoglobin in terms of its efficiency at binding oxygen.