The color of an animal’s blood is a striking biological feature determined by the specific oxygen-carrying protein, known as the respiratory pigment, contained within the circulatory fluid. While human blood is famously red, this particular hue is only one of several colors found across the animal kingdom. The existence of purple blood in certain organisms is a fascinating example of how evolutionary biology has employed different chemical structures to achieve the fundamental task of oxygen transport.
Why Blood Color Varies
The visible color of blood depends entirely on the metal ion at the core of the respiratory pigment and whether that pigment is currently bound to oxygen. In the most familiar example, hemoglobin uses an iron atom to bind oxygen, which gives the blood its characteristic red color. Another common variant is blue blood, found in mollusks, arachnids, and crustaceans, which utilize a protein called hemocyanin. Hemocyanin contains copper atoms at its oxygen-binding sites. When oxygenated, this copper center strongly absorbs red light, reflecting a vivid blue color, but when deoxygenated, the blood appears colorless.
Hemerythrin: The Pigment of Purple Blood
Purple blood is a result of the respiratory pigment hemerythrin, a protein found exclusively in certain marine invertebrates. Hemerythrin is distinct because, while it uses iron to transport oxygen, it does not contain the porphyrin ring structure found in hemoglobin. This structural difference means that the iron is bound directly to the protein’s amino acid chains, specifically involving a binuclear iron center, or a pair of iron atoms. When hemerythrin is deoxygenated, the protein is essentially colorless or a faint pink, making the blood appear transparent. When oxygen binds to the two iron atoms, it causes a chemical reaction where the iron centers are oxidized, altering how the molecule absorbs light and causing the blood to transition to a distinct violet-pink or purple hue.
Which Organisms Have Purple Blood
The organisms that possess hemerythrin and therefore exhibit purple blood are all marine invertebrates. These include Sipunculids, commonly known as peanut worms, which are burrowing worms found in marine sediments. Other examples are Priapulids, or penis worms, and Brachiopods, often called lamp shells.
The presence of hemerythrin in these organisms is linked to their habitat in low-oxygen marine environments. Hemerythrin has a high affinity for oxygen, meaning it can effectively pull oxygen from water even when the concentration is relatively low. For certain species of brachiopods, the hemerythrin can bind oxygen five to ten times more strongly than human hemoglobin. This efficiency in oxygen uptake offers a substantial adaptive advantage for survival in deep-sea or sediment-rich waters where oxygen availability is limited.