What is the name of the copper-rich protein that creates the blue blood in the Antarctic octopus?

The story behind the answer

Hemocyanin is the copper-containing protein that carries oxygen in many mollusks, including Antarctic octopuses. Each molecule uses copper ions to bind oxygen; when oxygenated, the copper shifts oxidation state, turning the blood blue—think “cyan” in hemocyanin. Unlike hemoglobin (iron-based, inside red blood cells), hemocyanin dissolves in the hemolymph and works efficiently in cold, low-oxygen seawater.

Context tip: Cephalopods often have multiple hemocyanin variants, and Antarctic species are adapted to keep oxygen delivery effective in frigid waters. Remember it this way: hemo (blood) + cyan (blue) = hemocyanin. Cytochromes are electron carriers, iron is the metal in hemoglobin, and methionine is an amino acid—none of these make octopus blood blue.