Do You Bleed Blue? Or Purple?

I grew up associating “Do you bleed blue?” with the Blues hockey team’s slogan since I am originally from St. Louis. However, for some creatures, they do bleed blue…or green, yellow, or even purple! Not every animal gets stuck with run-of-the-mill red blood. Nat Geo produced an interesting article, “In Animal Kingdom, Blood Comes in a Rainbow of Colors,” which outlines some of the major hues. The colors basically depend on how an organism transports oxygen throughout its body. So let’s get into some specific examples!


Blue: Octopuses, squids, crustaceans, spiders, and some mollusks. These animals have a copper-rich protein named hemocyanin. When the blood is oxygenated, it takes on a blue coloration. However, the blood appears clear after hemocyanin is
Clear: Ocellated icefish. Ocellated icefish lack hemoglobin (gives human blood its red tint) and hemocyanin. Consequently, its blood does not possess any color. So how is it oxygenating its body? Because icefish live in frigid temperatures, and cold water is more conducive to storing dissolved oxygen, icefish do not require an active oxygen transporter! Moreover, icefish do not have scales, which scientists suggest plays a role in their unique oxygenation strategy.

Green: Green-blooded skinks, annelids (some marine worms, segmented worms, leeches). Green-blooded skinks utilize hemoglobin, just like humans. Animals that use hemoglobin as an oxygen transporter also use their liver to break down used hemoglobin. This process produces compounds like bilirubin and biliverdin, which are normally excreted through the gastrointestinal tract to prevent build-up. However, green-blooded skinks can thrive with abnormally high levels of biliverdin and tout green blood as a result. For the invertebrate species, they possess chlorocruorin. Similar to how it works for hemocyanin, when the chlorocruorin is oxygenated, it turns the blood green. After deoxygenation, the proteins alter the blood back to a lighter shade of green.

Purple: Some marine worms (e.g. penis worms, brachiopods). When the blood protein that is found in these species, hemerythrin, is oxygenated it forces the animal’s blood to turn purple. If deoxygenated, the blood protein keeps the animal’s liquid life a neutral, colorless substance.

Yellow: Beetles, sea squirts, and sea cucumbers. These animals can thank vanabin for their golden veins. I was a little confused about this color/compound combination because vanabin contains a chemical, vanadium, that apparently turns yellow when oxygenated, thereby giving the blood a yellow coloration. However, contrary to the aforementioned blood proteins, vanabin does not assist in oxygen transport. Sea cucumbers appear to have adequate levels of hemocyanin in their blood to meet all of their oxygen needs, making vanabin superfluous. The role of vanabin is still puzzling researchers but has been suggested to possess self-defense properties.

Bernard Dupont, France, Sea Cucumber (Stichopus herrmanni), CC BY-SA 2.0,

Red: Humans and most vertebrates. Good old red blood. Most non-human vertebrates possess the same lame blood color as humans. Hemoglobin is responsible for our blood coloration. It is a blood protein that contains iron and turns our blood a bright red when it is oxygenated. Deoxygenated blood is a deeper shade of RED. It may seem like our blood is blue sometimes (maybe because you have seen too many artery/vein diagrams or have just been staring at your forearm for too long). Regardless, our blood is, and always will be, red. The reason it appears blue on your forearm through your skin is due to the different ways that light waves penetrate your skin. Cool, right?!

In essence, I hate to break it to you, but no matter how much you love your favorite sports team, you will always bleed red. I would recommend watching this Youtube video: What Color is Your Blood? for more information, plus they include a quick rundown on some of the weird blood colors that I mentioned above. I also pulled information from the following websites/articles:

Animal Blood Comes in 5 Crazy Colors– Includes a nice graph that summarizes the main blood hues.
Animals that have Blue or Yellow Blood– Helped clarify the yellow blood/vanabin confusion.
Chemistry of Blood Colors– I loved the graphic they designed for this webpage and included it earlier in my post. They give further details on the chemistry behind the blood proteins.

One of my favorite red-blooded creatures, Nick. He usually brings trash from the enclosure for extra fish, but sometimes he has to get creative. 😉 Super happy that he played Fetch with me today!





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