Only small and simple animals can live without blood. For the rest of us, blood plays a vital role in carrying nutrients and other essential atoms and molecules, like oxygen, throughout the body.

Oxygen is used in the energy-making process of all multicellular animal life. Oxygen naturally dissolves in fluid but special oxygen-binding proteins can carry it more efficiently. These proteins are called respiratory pigments, since they are the component of blood that gives it color. The diversity of respiratory pigments give rise to an equal diversity of colors blood can be found in and creates a veritable rainbow flowing through animals throughout the world.

Andean geese live at high altitudes and their hemoglobin is incredibly effective at binding oxygen. Image by Greg Schechter.

Red

Hemoglobin is the respiratory pigment used in vertebrates, such as humans. Each molecule of hemoglobin contains four iron atoms. When oxygenated, hemoglobin imparts a bright red color to blood. This color darkens to a deep red when deoxygenated. The veins that carry deoxygenated blood appear blue when viewed through the skin, but this is only an optical illusion. Skin scatters light waves as they pass through and make veins appear blue despite the blood inside still being red.

Animals (including humans!) that live at a high altitude tend to have more hemoglobin than those living at sea level and some have evolved hemoglobin that is more effective at binding oxygen at low pressures. Athletes will sometimes train at higher altitude than they plan on competing at in order to trigger the production of greater levels of hemoglobin in the blood. When those athletes return to sea level to compete their increased hemoglobin levels let them pack more oxygen into their blood than previously.

Some invertebrates use a variant of hemoglobin as their respiratory pigment and therefore also have red blood. An interesting example of this can be seen in the deep sea tubeworm (Riftia pachyptila). They have a symbiotic relationship with bacteria stored in their bodies which use sulfur to produce energy. A tubeworm’s hemoglobin binds and transports sulfur in addition to oxygen and keeps their symbiotic bacteria well-supplied with fuel.

Blue

We call people of aristocratic birth “blue bloods” as a metaphor but many invertebrates like spiders, cephalopods, crustaceans, and some mollusks have literal blue blood. They use the respiratory pigment hemocyanin and each molecule contains 96 pairs of copper atoms. Hemocyanin is not confined to blood cells and floats freely within the invertebrate version of blood, also known as hemolymph.

Horseshoe crab hemolymph contains an additional unique protein: Coagulogen. Coagulogen causes coagulation in the presence of bacterial contamination which helps protect horseshoe crabs from infection. This property of horseshoe crab hemolymph is very important to medical science.

Every year, half a million horseshoe crabs are harvested from the beaches where they gather to lay their eggs. About a third of their hemolymph is drawn and collected before the horseshoe crabs are released alive back into the water. Coagulogen is used in medical research to detect bacterial contamination of medications and vaccines. Scientists are working on developing synthetic versions of coagulogen, but for now, we rely on our horseshoe crab “blood donors” to help keep our medications safe.

Green

If a human had green blood it would mean something had gone very wrong but for leeches it’s entirely normal! Many segmented worms (annelids) and some arthropods have green blood due to the respiratory pigments chlorocruorin and erythrocruorin. These pigments are chemically very similar to hemoglobin and also use iron to bind oxygen. Interestingly, chlorocruorin is only green when deoxygenated and turns red when carrying oxygen.

There is a single vertebrate standout with green blood. The aptly named green-blooded skink (Prasinohaema virens) is only found in New Guinea, a large island sitting to the north of Australia. This oddball lizard uses hemoglobin to carry oxygen like its fellow vertebrates but the red of hemoglobin is overwhelmed by the green of another pigment: biliverdin.

Biliverdin is a byproduct of the breakdown of old red blood cells in the liver. In most animals, biliverdin is converted to bilirubin which is eventually excreted in urine. Green-blooded lizards cut this process short. They do not convert biliverdin to bilirubin and biliverdin accumulates in their blood at high concentrations. The result is a lizard whose green blood has stained almost their entire body green, including bones and tongue. This adaptation may help the lizard prevent malaria infections but more research is needed.

The brown sea cucumber. Image by François Michonneau.

Yellow

Yellow blood is fairly unusual and is only seen in tunicates, sea cucumbers, and a few types of beetles. The color is caused by high concentrations of vanabin proteins in their blood. Vanabin contains the element vanadium. Unlike other respiratory pigments, vanabin doesn’t transport oxygen. It is unclear what the function of vanabins are. They might play a role in predator deterrence or preventing parasitic infection.

a bucket of peanut worms. image by Vmenkov.

Purple

There are some animals with purple blood such as brachiopods, peanut worms, and other marine invertebrates. This color is produced by the respiratory pigment hemerythrin. Hemerythrin appears to be a very effective oxygen binding agent and binds oxygen at a greater level than most hemoglobin variants. However, it is generally very poorly understood and will take much more analysis to uncover its secrets.

Early research implies that hemerythrin does more than just transport oxygen. It may help prevent metal toxicity by performing iron storage. It also appears involved in immune function.

Some peanut worms have translucent skin and appear purple due to the color of their blood showing through.

What Else?

These are the most common respiratory pigments (and blood colors) found in the animal kingdom but not the only ones. In fact, some highly unusual animals have evolved to not use respiratory pigments at all.

The crocodile ice fish family have milky pale blood and live in the frigid waters of the southern Atlantic Ocean. Cold fluid naturally holds more dissolved oxygen than warm and the icy cold blood of these fish is capable of carrying enough dissolved oxygen that they do not require specialized binding proteins.

And what about plants? Plants might not have blood but they do have hemoglobin proteins and complex oxygen needs. However, that is the topic for another article!

Kate Dzikiewicz, Science Curatorial Associate