Animal blood comes in a rainbow of colors – here’s why

Whether it’s a little snail or a huge whalemost living organisms depend on blood flowing through their bodies to stay alive.

This precious liquid fights infections, supplies nutrients and gases to the organs and evacuates waste. But what many of us imagine as blood — red and full of iron, for example — differs by species.

For example, some crustaceans, squid and octopus have blue blood due to hemocyanin, an oxygen-carrying protein, which contains copper, says Stephen Palumbi, a marine biologist at Stanford University. (Think Star Trek’s Green Blood Vulcans, jokes Palumbi.) In marine animals, hemocyanin is colorless, but turns blue when it binds to oxygen.

In humans, the protein hemoglobin carries oxygen. “Hemocyanin is just a different way of moving oxygen,” Palumbi explains over email. “It often happens that evolution invents different things for the same purpose.”

Hemocyanin, which evolved nearly 2.5 billion years ago, originally served to detoxify oxygen for primordial organisms in Earth’s anaerobic or oxygen-poor 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 supply oxygen throughout an organism’s body.

Hemoglobin evolved much later, perhaps around 400 million years ago. Coates says this is likely because vertebrates have more complex respiratory systems than simple organisms. This is because most of the blood of mammals, fish, reptiles, amphibians and birds is red because of hemoglobin, the protein of which is made up of hemes or iron-containing molecules that fuse with oxygen. .

Hemerhythrin is another iron-containing pigment that attaches to oxygen molecules and imparts a purple-pink hue to the blood of certain molluscs, such as lamp shells and sea squirts.

Then there’s the Antarctic icefish, which has no blood pigment thanks to a genetic mutation that removed hemoglobin from its body. In the fish’s frigid southern habitat, oxygen is abundant and the gas seeps directly through the fish’s gills and skin.

Insects have no blood, but instead possess a comparable fluid called haemolymph, which carries hormones and gases through their system, with the exception of oxygen – they absorb it directly through the openings along the way. along their sides or back.

“It’s like they have a line of nostrils on the side of their body,” says Julie Peterson, an entomologist at the University of Nebraska-Lincoln. Hemolymph may have yellowish or blue-green pigments from the insect diet.

Blood as a weapon

Some animals can use their blood in a theatrical defense mechanism called reflex bleeding or autohemorrhagein which they begin to bleed profusely to ward off predators.

Horned lizards in the southwestern United States and Mexico shoot arcs of blood from their eyes when they feel threatened by a predator, such as a coyote. The delinquent gets a yucky surprise, while the lizard lives to squirt another day.

Some insects, like the multicolored Asian ladybug, have “a very noxious, disgusting, unpleasant, smelly fluid” that mixes with their hemolymph, Peterson says. They will squirt this cocktail out of their eyes or legs when disturbed. (See more animals with stinky tusks.)

One of their relatives, the aptly named bloody-nosed beetle, spits red hemolymph, which looks like blood, from its mouth for the same purpose.

To avoid parasites, New Guinea prehensile green skinks 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, turns the blood, bones, mouth, tongue and other body parts of reptiles lime green.

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

If a person had a similar amount of biliverdin, it could be fatal. So “why aren’t these lizards jaundiced and dead?” Austin asks. He theorizes that their bodies evolved ways to deal with biliverdin as a strategy to kill blood parasites, especially 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, have no circulatory system; gas exchange takes place directly through their skin. Oxygen goes directly to their tissues, while nutrients are supplied by diffusion from the gut.

Jellyfish and sponges also receive oxygen by diffusion. For starfish and sea cucumbers, water is an equivalent of blood, moving nutrients and gases through their systems via a water-based vascular system. (Read why animals developed four types of skeletons.)

Blood groups

Humans have eight blood types, but we’re not the only animals with this inherited trait, says Jethro Forbes, a critical care specialist at Cornell University College of Veterinary Medicine, via email.

Wild animals probably also have blood types, although domesticated species are best studied. Cats have three blood types, chickens up to 28, and domestic ferrets “don’t seem to have different blood types,” Forbes says. (Read about a chicken with black bones, organs and meat.)

Why the disparity? It is probable because domestic ferrets are extremely inbred, with little genetic diversity, while there are dozens of chicken breeds, and therefore more varied blood groups.

Useful blood

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

However, the process of collecting and bleeding crabs – up to half a million a year – can kill them, and it has caused species numbers to plummet in the mid-Atlantic region of the United States in recent years. That’s why scientists are scrambling to find synthetic alternatives that would reduce the need to catch wild animals.

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