I was working on an urban fantasy story years ago that featured elves, and I went pretty far down the rabbit hole on dangerous iron could be to them — including the iron in most terrestrial blood. I discovered that some animals have copper-based blood instead, and it has some neat tradeoffs that eventually inspired the winged, lactating egg-layers I talk about so much.
- Proteins that actively use copper in their chemical makeup are rare: they only became useful after the atmosphere was oxygenated. The relevant one for our purposes is hemocyanin.
- Unlike hemoglobin, which is bright red in the presence of oxygen, hemocyanin turns blue. Here's a nice article of what hemoglobin looks like when it's not oxygenated.
- Hemocyanin is important for transporting oxygen, but it also has secondary uses, like transporting molting hormones and melanin formation.
- Different animals have different blood types. Dogs are unique in that not all dog blood types have natural antibodies.
- Gorillas are probably the weirdest because they always test as "type O" but almost definitely aren't all "type O."
Hemocyanin is one of the strongest known antigens. Antigens are able to stimulate strong immune responses, so hemocyanin is often harvested from horseshoe crabs to help vaccine development Compared to hemoglobin, most hemocyanins (they aren't all the same!) are inefficient at transporting oxygen — except in cold, low-oxygen environments like the bottom of the ocean. From what I can tell, this has to do with the salt concentration of the blood, at least in blue crabs like the ones in the Chesapeake.
Allergic to Blue Bloods
People who are allergic to crustaceans like crab and shrimp may actually be allergic to hemocyanin proteins. Hemocyanin is a cross-reactive allergen of crustacean, cockroach, and dust mites. Incidentally, there's still a lot we don't know about dust mite allergies — which is frustrating to me in particular because a dust mite allergy may have been responsible for me developing dermatographia, a skin condition that functions like a "pressure allergy" — scratching or rubbing my skin causes me to break out in hives, possibly due to my histamines being surrounded by unusually thin cell walls.
The ocellated icefish lives in such an ice-cold environment that neither hemoglobin nor hemocyanin is a great solution for carrying oxygen around its body. Whereas the Antarctic octopus compensates by having 40% more hemocyanin than its warm-water cousins, the ocellated icefish doesn't bother with oxygen-carriers at all. Cold water holds more oxygen than warm water, it turns out. So the icefish's blood runs clear. Its lack of scales and oversized heart help oxygen get through the body instead. They don't have the enzymes to carry carbon dioxide away, either.
High levels of biliverdin in the hemoglobin of Papua New Guinea's skinks turn the lizards' blood green. In humans, biliverdin gets converted into bile — too much causes jaundice. 50 micromoles per litre of blood is deadly in humans, but these lizards can handle 20x that much. This sort green blood evolved at least four separate times on the island, but scientists are still trying to figure out why it might have been useful from an evolutionary perspective.
📗 If you found this interesting, you may also this newsletter about thermoregulation methods, and how different creatures stay comfortable in extreme environments via sweaty feet, poopy legs, & deep dirt.
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