What color is octopus blood, and why?

Answer

Octopuses carry oxygen with hemocyanin, a copper-based protein. Human blood uses iron-based hemoglobin, which is why ours is red.

Context

Hemocyanin binds oxygen with two copper atoms, and the oxygen-carrying form looks blue. In octopuses and other molluscs it floats freely in the blood fluid, not inside cells, and blood with little oxygen is nearly colourless. Red blood cells with iron-based hemoglobin are a different solution to the same problem, moving oxygen from respiratory surfaces to tissues.

Cold water shows why the protein matters. National Geographic reported that in very cold water oxygen binds so tightly to hemocyanin that tissues have trouble getting it, and the Antarctic octopus Pareledone charcoti compensates with about 40 percent more hemocyanin than two warmer-water octopus species. A related paper on octopus hemocyanin found evidence of natural selection linked to temperature adaptation.

Octopus blood is also sensitive to acidity. Smithsonian writers note that if seawater pH falls far enough, octopuses struggle to circulate enough oxygen, which is why researchers have raised concern about ocean acidification.

Blood colour varies widely beyond red and blue. The ocellated icefish has colourless blood with neither pigment, relying on oxygen-rich cold water, and the green-blooded skink of Papua New Guinea has high levels of biliverdin, a hemoglobin breakdown product.