Red king crab blood contains leukocytes and the second-most common respiratory pigment called hemocyanin. Arthropod hemocyanin is a distinct variation specific to arthropods and is a metalloprotein that uses copper atoms that are bound to its structure. Two copper atoms are needed to bind one O2 molecule. Because it is a large protein molecule, it is found in the blood plasma, but not in body tissues or muscles. Hemocyanins are named appropriately because when oxygenated, their color changes from colorless to blue.