Hemocyanins (also spelled haemocyanins) are respiratory proteins in the form of metalloproteins containing two copper atoms that reversibly bind a single oxygen molecule (O2). Oxygenation causes a color change between the colorless Cu(I) deoxygenated form and the blue Cu(II) oxygenated form. Hemocyanins carry oxygen in the blood of most molluscs, and some arthropods, including the horseshoe crab, Limulus polyphemus. They are second only to hemoglobin in biological popularity of use in oxygen tr… (More on Hemocyanin)