Haemocyanin (haemocyanine) is a copper-containing respiratory pigment found in the blood (haemolymph) of most crustaceans (crabs, lobsters, shrimps) and molluscs. It appears blue when oxygenated because the copper atoms bind oxygen and cause the blue coloration, similar to how iron in haemoglobin gives blood its red color. Erythrocruorin is an iron-based pigment in some worms.