I grew up in the prairies and one of my favourite ways to relieve the boredom of long summer road trips was to examine the different bugs splattered on the windshield. I would try to figure out which bug each splatter was and I was fascinated by the ways some left green smears or bright, thick, yellow splats. It made me wonder why animals have different colours of blood and how many different colours there are.
Humans blood is made of an amber coloured liquid called plasma and numerous red blood cells (about 25,000,000,000,000), which lend their colour to our blood. Each red blood cell uses haemoglobin (or hemoglobin) to transport oxygen, which gives our oxygenated/deoxygenated blood a bright/dark red colour respectively. With the exception of a few fish, all vertebrates use haemoglobin to carry oxygen.
I was surprised to learn how many other compounds are used by animals to carry oxygen. You may recall last week that I wrote about the vampire squid, which lives in the oxygen minimum zone (OMZ) of the deep ocean. In this hostile environment the vampire squid and many other animals use haemocyanin (hemocyanin) instead of haemoglobin to transport oxygen as it is much more efficient. Since haemocyanin is copper based, it gives the blood a bluish appearance.
Polychaetes, wormy sea creature, use chlorocruorin to move oxygen which makes their blood green when dilute, and vivid red at higher concentrations. Some bottom-dwelling worms and brachiopods use haemerythrin (or hemerythrin) an iron-based compound which is bright pink or violet when oxygenated. And vanadium chromagen, a pigment found in sea squirts, ascidians, and tunicates turns their blood green , blue or orange.
Back to the prairies: insects are quite different because they do not use pigmented oxygen carrying compounds which means bug blood is colorless, or sometimes yellow or green because of pigments in their food.
So maybe Mr. Spock could have green blood. Many movie monsters and aliens are inspired by the weird and wonderful side of Nature. You know the old proverb: Fact is stranger then fiction.