Blood transports oxygen, nutrients, and waste throughout an organism’s body. The familiar red color of human blood is due to a specific respiratory protein that binds oxygen. However, this color is not universal. The ultimate shade of an animal’s blood is determined by the metal ion at the core of the oxygen-carrying molecule it utilizes. Some animals have evolved a different molecular architecture, leading to blood that appears various shades of blue, violet, or purple.
Animals That Possess Blue or Purple Blood
The animals that display blue or purple blood are primarily invertebrates, belonging to two major phyla: Mollusca and Arthropoda. Cephalopods, such as octopuses, squid, and cuttlefish, all possess this unusual coloration in their circulatory fluid. Certain arthropods, most notably the horseshoe crab and some species of spiders, also utilize the same unique oxygen-transport system.
When the blood of these animals is oxygenated, it takes on a distinct blue hue. However, the blood can sometimes appear a pale violet or purple when it is highly concentrated or deoxygenated, leading to the term “purple blood.”
The Copper-Based Respiratory Pigment
The protein responsible for this blue-to-purple coloration is called Hemocyanin. It is a large, multi-subunit protein that binds oxygen using copper atoms, rather than the iron atoms found in human hemoglobin. Specifically, two copper atoms are required to bind a single molecule of oxygen in the protein’s active site.
The structural difference is significant because Hemocyanin is not contained within blood cells; instead, it is dissolved freely within the circulatory fluid, which is often called hemolymph. When the copper atoms bind to oxygen, they change from their colorless, deoxygenated state (cuprous ion, Cu(I)) to their oxidized, oxygenated state (cupric ion, Cu(II)). This chemical change causes the protein to absorb red light and reflect blue light, making the oxygenated hemolymph appear blue.
This copper-based system offers particular functional advantages in the environments where these animals live. Hemocyanin is more effective than hemoglobin at transporting oxygen in conditions of low temperature and low oxygen concentration. This adaptation is beneficial for marine invertebrates, such as deep-sea octopuses and horseshoe crabs, which inhabit cold waters where oxygen solubility is a greater challenge.
However, the Hemocyanin system is generally less efficient at moving oxygen at higher temperatures and normal atmospheric pressures compared to the iron-based system. Its large size and dissolved nature also mean that it transports roughly one-quarter the amount of oxygen per volume compared to human blood. This molecular difference in oxygen-binding ability is a trade-off that allows these animals to thrive in their aquatic habitats.
Blood Diversity Across the Animal Kingdom
While the copper-based Hemocyanin is responsible for blue blood, the complete spectrum of blood colors in nature reveals an astonishing chemical diversity. Other pigments exist that utilize different metal ions or protein configurations.
Some marine invertebrates, specifically peanut worms (sipunculans) and lamp shells (brachiopods), possess a different pigment called Hemerythrin. This protein uses iron to bind oxygen, similar to hemoglobin, but it does not contain the porphyrin ring structure characteristic of hemoglobin. When Hemerythrin is oxygenated, the blood appears a pale violet or pink, providing a true purple-tinged alternative to the blue of Hemocyanin.
Another distinct respiratory pigment is Chlorocruorin, which gives the blood of certain segmented worms, like some leeches and polychaete worms, a green color. Chlorocruorin also uses an iron center to bind oxygen, but a slight difference in its chemical structure causes it to appear light green when oxygenated.