From red, blue, lime green, and purple, animal blood comes in a wide range of colors, but why? Most living organisms rely on flowing blood to stay alive, and blood and the transportation of oxygen have evolved in many ways over the years.

Blood is responsible for bringing oxygen and nutrients throughout the body. Blood carries carbon dioxide and wastes to the kidneys, digestive system, and lungs to eventually be removed from the body. Blood is essential to fighting infections, carrying hormones through the body, and forming blood clots to prevent blood loss. Organisms have evolved different types of blood and different ways to carry oxygen throughout their body.

Some crustaceans, squid, and octopuses have blue blood because of a protein called hemocyanin. Hemocyanin contains copper and transports oxygen throughout the body. In marine animals, hemocyanin is colorless and turns blue when exposed to oxygen.

Squid on its night dive hunting
Eitan Ben Zvi/Shutterstock

Hemocyanin is blue and moves oxygen around in a different way. Hemocyanin evolved almost 2.5 billion years ago to detoxify oxygen for some of the first organisms in Earth’s low oxygen environment. A study found that the Earth became oxygen-rich about 2.33 billion years ago when the hemocyanin protein evolved again to deliver oxygen throughout the organism’s bodies.

Humans have the protein hemoglobin inside their red blood cells that transports oxygen throughout the body. Hemoglobin is iron-rich and red. Most mammals, reptiles, amphibians, and birds have red blood because of hemoglobin.

However, hemoglobin evolved only about 400 million years ago.

Christopher Coates, a comparative immunologist at Swansea University in Wales, said this likely was because vertebrates have a much more complex respiratory system than simple organisms.

Hemerhythrin is another protein that gives a purple-pink blood color to some mollusks like lamp shells and sea squirts. The iron-rich pigment attaches to oxygen molecules.

There are even animals that have no blood pigment at all. Due to genetic mutation that removed hemoglobin from their bodies, animals like the antarctic icefish have colorless blood. The fish is found in a freezing environment that has an abundant amount of oxygen, and it seeps into the fish’s gills and skin directly, over time eliminating the need for hemoglobin.

Interestingly, insects don’t have any blood. Instead, they carry a fluid called hemolymph that transports hormones and gases throughout their body. Insects absorb oxygen directly through the openings in their sides or back and have no oxygen in their blood-like fluid, hemolymph. Sometimes, insects can have a yellow, blue, or greenish pigment to their hemolymph due to different plants in their diet.

Many animals use blood as a weapon in their natural habitats. Some use autohemorrhaging, also called reflex bleeding, where they begin to bleed to repel off predators.

Horned lizards use their blood in a very unique way to ward off predators. The southwestern U.S. and Mexico native lizard shoots blood out of their eyes when they feel threatened.

Another exciting insect, the multicolored Asian lady beetle, has a distasteful liquid mixed with their hemolymph, which they shoot out of their eyes or leg joints when disturbed by predators.

Walking multicoloured Asian Ladybird
Ger Bosma Photos/Shutterstock

A relative of the multicolored Asian lady beetle is the bloody-nosed beetle. This beetle shoots red hemolymph out of its mouth for the same purpose as its relative.

The New Guinea prehensile green tree skinks have lime-green blood. They have a buildup of bile pigment called biliverdin, a waste product of red blood cells that causes the reptile to have lime-green blood, bones, mouths, and tongues.

Chris Austin, who discovered biliverdin to be the cause of the lime green blood, said other animals remove the excess bile in the liver, “like the oil filter in your car,” but the skinks don’t, which leaves them always a light green color.

He continued that if a human had the same amount of biliverdin, it would be fatal, so how do these lizards handle it? He speculates that their bodies have evolved to cope with biliverdin as a strategy to kill blood parasites, like those that cause malaria.

Some animals don’t even have blood or circulatory systems because they aren’t essential to life. Flatworms don’t have a circulatory system and trade gases through their skin. Oxygen goes right to the tissues, and nutrients are delivered by diffusing from the gut. Jellyfish and sponges receive oxygen through diffusion as well.

Yellow and orange Jellyfish dansing in the dark blue ocean water
Arzu Kerimli/Shutterstock

Humans have eight blood types. Most wild animals likely also have different blood types, although there have not been as many studies conducted with these creatures as with domestic species. Cats have three blood types; chickens have up to 28, while ferrets don’t have any different blood types, likely because they are highly inbred and have little genetic diversity.

Animal blood has proven to be useful for studies. Atlantic horseshoe crabs, for example, clot, have milky blue hemocyanin-rich blood which clots when it is exposed to bacterial toxins. The blood has been valuable for testing medical drugs like vaccines to ensure they are free from contaminants.

However, the incredibly unique species has seen a massive decrease in population numbers in the last few years due to this discovery. Up to half a million of these crabs are caught every year and killed just to collect their blood. Scientists are working on making synthetic alternatives to eliminate the practice of catching and killing these animals.

Sign this petition to demand horseshoe crabs be handled ethically!

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