Hemocyanin

Hemocyanin (also spelled haemocyanin) is a protein that transporst oxygen throughout the bodies of some invertebrates. These metalloproteins contain two copper atoms that reversibly bind a single oxygen molecule (O2). They are second only to hemoglobin in frequency of use as an oxygen transport molecule. Unlike the hemoglobin in red blood cells found in vertebrates, hemocyanins are not bound to blood cells but are instead suspended directly in the hemolymph. Oxygenation causes a color change between the colorless Cu(I) deoxygenated form and the blue Cu(II) oxygenated form.

Species using hemocyanin for oxygen transportation tend to live in cold environments with low oxygen pressure. Under these circumstances hemoglobin oxygen transportation is less efficient than hemocyanin oxygen transportation. Hemocyanin in these species tend to produce a blueish-purple color of skin, flesh or shell as seen in the Thaurans.