What are the different colors of blood in the animal kingdom?

Biological fluid tirelessly traversing our arteries and veins, we are used to the symbolic color of blood: red. It is the color of the blood in humans and in vertebrates, so much so that we would be tempted to believe that it would be the only color that it can take. However, this is not the case; on Earth, the blood actually comes in five different colors.

About 6 liters for a man and 5 liters for a woman: this is the amount of blood that runs through our body permanently in a network of blood vessels with a length of 100'000 km. He perfuses all the organic tissues in order to bring them oxygen. It consists of red blood cells (or erythrocytes, Greek erythros for red) which carry a very particular protein: hemoglobin.

Hemoglobin is more precisely a metalloprotein because it contains iron. The iron atoms are capable of fixing oxygen, and this bond between iron and oxygen gives oxyhemoglobin. Oxyhemoglobin thus appears red under the oxidation of the iron contained in hemoglobin. This is why the blood of most vertebrates is red in color. However, not all animals have blood based on hemoglobin.

Hemoglobin is a metalloprotein containing four iron atoms to fix oxygen. The oxidation of iron gives it its red color. Credits: aboutkidshealth

Some species of beetles, ascidians and holothurians, have vanadocytes: cells very rich in vanadium (concentration 100 times higher than in seawater) acting as blood cells. Inside vanadocytes are vanabine, a vanadium-fixing metalloprotein; it is also called hemovanadine. Vanabine does not fix oxygen, so its role is still unknown. But it is she who gives a pale green and yellow color to the haemolymph of these animals.

Vanabine is a metalloprotein that binds vanadium. The oxygenated form, hemovanadine, confers a green or yellow color to certain species of ascidians ( Didemnum soft on the picture). Credits: Bernard Dupont

In annelids (marine worms, leeches, etc.), a large amount of chlorocruorine is found in the blood plasma (and not in the blood cells themselves). It is a metalloprotein whose affinity with oxygen is very low. When oxidized, it appears green in normal plasma concentrations; and pale red in high plasma concentrations.

Spiders, crustaceans, octopus and squid, as well as some species of molluscs, have a haemolymph (circulatory liquid of arthropods) containing hemocyanin. It is a metalloprotein containing copper (two Cu + cuprous cations ) to bind oxygen. The oxygenated form of hemocyanin is blue. This is why the hemolymph of these invertebrates appears blue.

Hemocyanin is a metalloprotein that binds oxygen through cuprous ions. Its oxygenated form takes on a blue color. This is particularly noticeable in the horseshoe crab. Credits: Mark Thiessen

The hemolymph of brachiopods and certain marine worms is made up of hemerythrin, an oligomeric metalloprotein made of iron that transports oxygen. Unlike other ferric hemoproteins, hemerythrin binds oxygen by forming a hydroperoxide complex ROOH. When the hemerythrin fixes the oxygen, it takes on a purplish purple / pink color. The hemolymph of these invertebrates appears purple.

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