Animal blood comes in a rainbow of colours—here’s why

From milky blue to lime green, organisms have evolved different types of blood—and ways to move oxygen around the body.

By Liz Langley
Published 28 Feb 2022, 17:47 GMT
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Atlantic horseshoe crabs' blue blood is extracted at the Charles River Laboratory in Charleston, South Carolina. Every year, pharmaceutical companies round up half a million crabs, bleed them to collect a substance valuable for human health, and return them to the ocean.
Photograph by Timothy Fadek, Redux

Whether it’s a tiny snail or a massive whale, most living organisms rely on blood flowing through their bodies to stay alive.

This precious fluid fights infection, delivers nutrients and gases to organs, and ferries away waste products. But what many of us picture as blood—red and full of iron, for instance—differs depending on the species.

For instance, some crustaceans, squid, and octopuses have blue blood due to the oxygen-transporting protein haemocyanin, which contains copper, says Stephen Palumbi, a marine biologist at Stanford University. (Think Star Trek’s green-blooded Vulcans, Palumbi quips.) In marine animals, haemocyanin is colourless, but turns blue when it binds with oxygen.

Some squid, such as the Atlantic brief squid (pictured above at Gulf Specimen Marine Lab and Aquarium) have blue blood because it contains the metal copper.
Photograph by Joël Sartore, National Geographic Photo Ark

In humans, the protein haemoglobin carries oxygen. “Haemocyanin is just a different way to move oxygen around,” Palumbi says by email. “There are lots of times that evolution invents different things for the same purpose.”

Haemocyanin, which evolved nearly 2.5 billion years ago, originally served to detoxify oxygen for primordial organisms in Earth’s anaerobic, or low-oxygen, environment, says Christopher Coates, a comparative immunologist at Swansea University in Wales. Later, when the atmosphere became more oxygen-rich, the protein evolved again to deliver oxygen throughout an organism’s body.

Haemoglobin evolved much later, possibly about 400 million years ago. Coates says it likely came about because vertebrates have more complex respiratory systems than do simple organisms. Indeed, most mammal, fish, reptile, amphibian, and bird blood is red because of haemoglobin, whose protein is made of haemes, or iron-containing molecules that fuse with oxygen.

Haemerhythrin is another iron-containing pigment that attaches to oxygen molecules and gives a purple-pink hue to the blood of some mollusks, such as lamp shells and sea squirts.

Then there’s the Antarctic icefish, which has no blood pigment at all thanks to a genetic mutation that removed haemoglobin from their bodies. In the fish’s frigid southern habitat, oxygen is plentiful, and the gas seeps in directly through the fish’s gills and skin.

Insects don’t have blood, instead possessing a comparable fluid called haemolymph, which transports hormones and gases through their system, except for oxygen—they absorb that directly through openings along their sides or back.

“It’s like they have a line of nostrils down the side of their body,” says Julie Peterson, an entomologist at the University of Nebraska-Lincoln. Haemolymph can have yellowish or bluish-green pigments that come from the insects’ plant diets.

Blood as weaponry

Some animals can use their blood in a theatrical defence mechanism called reflex bleeding or autohaemorrhaging, in which they start bleeding heavily to ward off predators.

Horned lizards of the southwestern U.S. and Mexico shoot arcs of blood out of their eyes when they feel threatened by a predator, such as a coyote. The offender gets a yucky surprise, while the lizard lives to squirt another day.

Some insects, such as the multicoulored Asian lady beetle, have “very noxious, icky, distasteful, smelly liquid” that mixes with their haemolymph, Peterson says. They’ll gush this cocktail out of their eyes or leg joints when disturbed

A relative of theirs, the aptly named bloody-nosed beetle, spews red haemolymph, which looks like blood, from its mouth for the same purpose.

A bloody-nosed beetle releases its defensive spray of hemolymph in Wiltshire, England.
Photograph by Tony Hamblin, Minden Pictures

To avoid parasites, prehensile green tree skinks of New Guinea may have what amounts to dirty blood. A constant buildup of a bile pigment called biliverdin, the waste product of broken-down red blood cells, causes the reptiles’ blood, bones, mouths, tongues, and other body parts to be lime green.

In other animals, the liver would process the extra biliverdin “like the oil filter in your car,” removing impurities so the engine runs well, says Chris Austin, director of the Louisiana State University Museum of Natural Science, who discovered biliverdin as the cause of the skink’s green blood.

If a person had a similar amount of biliverdin, it could be fatal. So “why are these lizards not jaundiced and dead?” Austin asks. He theorises their bodies evolved ways to cope with the biliverdin as a strategy to kill off blood parasites—specifically those that cause malaria.

Bloodless creatures

Some animals don’t have blood or circulatory systems because they simply don’t need them.

Flatworms, for example, lack a circulatory system; gas exchange occurs directly through their skin. Oxygen goes right to their tissues, while nutrients are supplied by diffusing from the gut.

Jellyfish and sponges also get oxygen via diffusion. For sea stars and sea cucumbers, water is a blood equivalent, moving nutrients and gases through their systems via a water-based vascular system. 

Blood types

The eight human blood types are defined by antigens—substances on the red blood cells that trigger immune responses to foreign bodies, such as viruses. Blood types, which are inherited, probably evolved to fight particular diseases, Jethro Forbes, critical care specialist at Cornell University College of Veterinary Medicine, says by email.

This is likely true with many animals, too, though domestic species are the most well studied. Cats have three blood types, dogs have 12, chickens have up to 28, and ferrets “do not seem to have different blood types,” Forbes says. (Read about a chicken with black bones, organs, and meat.)

Why the disparity? It’s likely because domestic ferrets are extremely inbred, with little genetic diversity, while there are dozens of chicken breeds, and thus more varied blood types.

Helpful blood

In some cases, animal blood has useful applications for human health. The milky-blue, haemocyanin-rich blood of Atlantic horseshoe crabs, for instance, clots when it comes in contact with bacterial toxins. This makes it a valuable tool for ensuring medical drugs or products—particularly vaccines—are safe and free from contaminants.

However, the process of collecting and bleeding the crabs—up to half a million annually—can kill them, and it’s caused the species’ numbers to drop in the U.S. mid-Atlantic region in recent years. That’s why scientists are working to find synthetic alternatives that would reduce the need for catching wild animals.

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