# Which protein is more efficient at oxygen extraction for an AIR BREATHING aquatic species? Haemoglobin or Haemocyanin?

Another question concerning the Tetrapi, I would like to know if having haemocyanin-based blood (copper-based blue blood) would be an advantage over having haemoglobin-based blood (iron-based red blood) for an air breathing aquatic species. Is the fact that the majority of animals have red blood grounded in evolutionary pressures or is it just an evolutionary quirk? In a nutshell, do I have to follow strict guidelines or am I spoilt for choice?

What I want from you is:

Find the most efficient blood type to extract oxygen from an atmosphere composed of 28% Oxygen, 72% Nitrogen, 0.4% Carbon Dioxide and 0.6% other gases with an atmospheric pressure of approximately 0.85atm. Please justify your choice. The answer does not require hard science, although some citations and illustrations would be most helpful.

• Haemoglobin is better. Nothing says that different protein based on copper can't be better than Haemoglobin, but out of those two known, haemoglobin wins. – M i ech Jun 26 '17 at 12:10
• Percentages are not enough. What counts is the partial pressure of each gas; so, please, what's the atmospheric pressure on that world? Anyway, Wikipedia says that "most hemocyanins bind with oxygen non-cooperatively and are roughly one-fourth as efficient as hemoglobin at transporting oxygen per amount of blood". And 12000 parts per million CO2 is a lot of CO2 -- at 1 atm this is quite close to the boundary where its presence would affect human capacity for effort; the EU exposure limit is 5000 parts per million. – AlexP Jun 26 '17 at 12:20
• I might change the CO2 percentage to a more suitable level, something like 0.45% Thanks for notifying me! – Andsaur Jun 26 '17 at 12:28
• Are you sure about the majority of animals have red blood? Or do you mean vertebrates? – Henning M. Jun 28 '17 at 20:31
• It's probably worth noting that while copper-based and iron-based blood are found on earth there are many other potential ways of transporting oxygen and carbon dioxide. Your aliens could very well have a completely novel system unrelated to anything that developed on earth. – Mike Nichols Jun 28 '17 at 21:50