Humans have red blood. This is caused by the protein Haemoglobin. Most vertebrates have this type of blood because it is the most effective at transporting oxygen. Haemoglobin contains iron, which allows oxygen to "bind" with it. 

Octopi, spiders, and squids have blue blood because of Haemocyanin in their blood. This has a different atomic structure that contains copper atoms instead of iron atoms.

Some worms and leeches have green blood because of a protein called Chlorocruorin. The protein is similar to Haemoglobin except it has an aldehyde group in place of a vinyl group in the chemical structure. 

Some marine worms have violet blood, and this is because of Haemorythrin. Haemorythrin contains individual units that contain iron atoms. 

Some animals have yellow blood because of the pigment,Haemovanabine. Haemovanabine is a vanadium-based compound. 

I was wondering if an intelligent being, as in sapient and sentient, could have one of the other proteins besides Haemoglobin. The most likely are either Haemocyanin(blue), because it is the second best oxygen transporter, or Chlorocruorin(green), because it is only one compound different from Haemoglobin(maybe those movies were right about alien blood). 

Octopi are pretty intelligent, as in brain function, but they are aquatic, and more blue-blooded creatures are in the water. Green blood is interesting, but only some worms have it. Violet blood would be really cool, but it is a combination of the last two, aquatic worms. Yellow blood is interesting, but apparently we don't know if the compound that makes the blood yellow transports blood.

Marine life is a weird field with many different blood colors. I want to narrow my question down to only terrestrial creatures because the majority of them have red blood.

Information came from these sites.

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    $\begingroup$ The example of the octopus answers your question. If you didn't want to include marine animals or invertebrates, you need to very clearly say so. $\endgroup$ May 22, 2023 at 8:31
  • $\begingroup$ @Trish I did wonder about that one, but no-one seems to have had a question that specifically mirrors the key part of this one. There's lots of stuff that's related, such as Would it be possible for mammals to evolve blue blood? and Alternative elements for oxygen transport in an alien blood and Cyanoglobin plausible?. $\endgroup$ May 22, 2023 at 10:00
  • $\begingroup$ @Trish but maybe the question itself is somewhat lacking. The existence of the octopus and co give a trivial "yes", but on Earth at least land-calamari will find itself at a disadvantage to hemoglobin-havers in various ways. Maybe the OP needs to clarify their setting. $\endgroup$ May 22, 2023 at 10:04
  • $\begingroup$ Basic answer is: if alien beings from another planet, anything goes. Pretty much any color is plausible. If Earth-evolved, then likely only red because the other colors are relegated to marine organisms, and it isn't very plausible that they could invent a technological civilization given the only environment that keeps them alive. You don't invent metallurgy underwater. Counter-argument would be Harrison's Yilanè from West of Eden, I suppose... $\endgroup$
    – John O
    May 22, 2023 at 16:03
  • $\begingroup$ @StarfishPrime I have now specified it to a terrestrial intelligent creature. I am interested in why everyone is so fixated on Haemocyanin(blue) when Chlorocruorin(green) is much closer to Haemoglobin on a molecular level. Chlorocruorin still uses iron, so it is still a one to one connection(one iron for one oxygen). Haemocyanin is a two to one ratio(two copper for one oxygen). $\endgroup$
    – Martamo
    May 22, 2023 at 16:58

3 Answers 3


Absolutely. There is no any reason to believe that hemoglobin is the optimal protein and life can't possibly evolve any other way. We couldn't be sure about it even after we found a few independently evolved alien sentient species with red blood.

Examples from Earth ecology tell us nothing, all these species are related to the some extent, so at some point of evolution they branched off a common ancestor making significantly different "design" unlikely to develop. Alien life may follow the same patterns (convergent evolution) or be radically different.

One of your links says "The pigment that gives this (yellow) colour is haemovanabine, a vanadium-based compound, whose function of this compound is not oxygen transport", so, blood color is not necessarily defined by oxygen transporting protein even on Earth.


I don't know if this is what you are looking for but there was a case of a family that had blue skin due to a recessive gene that caused them to be this way!

Specifically, the iron in their hemoglobin is converted from the ferrous $Fe^{2+}$ to the ferric $Fe^{3+}$ form. Instead of being red in color, the arterial blood of these people is brown, their skin is blue, and many other problems arise such as headaches, dizziness, shortness of breath, nausea, and poor muscle coordination as mentioned in the Wikipedia article.

So in principle, you already have an example of an intelligent species that can have non-red blood, and it's within our own species.

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    $\begingroup$ No no, their blood color is indeed different. You can read about it here: en.wikipedia.org/wiki/Methemoglobinemia. It's actually brown and its caused by them having the $Fe^{3+}$ instead of $Fe^{2+}$. The skin color becoming blue is a site effect of this condition. Nothing to do with race etc @Pelinore $\endgroup$
    – cconsta1
    May 22, 2023 at 8:58
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    $\begingroup$ @Pelinore maybe you should read the very first link in the answer, which states that the skin color of the people involved was due to their blood, and includes references? $\endgroup$ May 22, 2023 at 9:42
  • $\begingroup$ @StarfishPrime, I edited my post to include some more relevant information from the link you mentioned. $\endgroup$
    – cconsta1
    May 22, 2023 at 9:48


The only obstruction I can think of is that big brain needs a constant supply of big blood and big oxygen. Maybe non-red oxygen transporters don't transport big enough oxygen to support big brain.

It seems that Hemocyanin is about 1/4 th as efficient as a carrier of oxygen as haemoglobin: https://en.wikipedia.org/wiki/Hemocyanin

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And an octopus has about as much Hemocyanin as you have Haemoglobin:


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So your hemocyanin creature gets 1/4 as much oxygen as the red blooded equivalent. The octopus has an octopus brain which is different from a human brain. I'd imagine a human brain would die with so little oxygen.

However all is not lost. Some people are Anaemic, which lowers their oxygen transport efficiency by up to 1/2. And these people are still intelligent.

So give your octopus man a whopping 25 grams of hemocyanin per 100 ml of blood. His brain will get as much oxygen as an anaemic human, and he is smart as an anaemic human, and still alive but slow moving and chronically fatigued. And his blood is bluegreen.

This is without making creative changes to the brain to allow it to use less oxygen more efficiently. Such creative changes are beyond the scope of modern science. We don't understand the biological nature of intelligence.

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    $\begingroup$ -1 for formating that excludes screen readers by putting text in pictures. $\endgroup$
    – Trish
    May 22, 2023 at 9:46
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    $\begingroup$ I hadn't actually noticed that presentation until I read Trish' comment. Seriously, doing block-quoting with screenshots of text is a downright weird and inconvenient thing to do. Even for non-screen-reader users, the ability to follow links and copypaste text is very valuable. It can't even have been particularly convenient for you to have done in the first place. $\endgroup$ May 22, 2023 at 9:52
  • $\begingroup$ @StarfishPrime You might be surprised. $\endgroup$
    – Daron
    May 22, 2023 at 10:28
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    $\begingroup$ @Daron I am surprised by someone taking screenshots of text, for use in a medium composed primarily of text. $\endgroup$ May 22, 2023 at 10:42

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