Single oxygenated hemocyanin molecule of an octopus: note the two Cu atoms

The underside of the carapace of a red rock crab (Cancer productus). The purple colour is caused by haemocyanin

Haemocyanin (or hemocyanin) is a protein which transports oxygen in the bodies of some invertebrates. Their active centre has two copper atoms which reversibly bind a single oxygen molecule (O₂). Unlike the hemoglobin in red blood cells found in vertebrates, hemocyanins are not bound to blood cells. The molecules are carried in the haemolymph.