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How does hemocyanin make cephalopod blood blue?
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cephalopod
Blue-ringed octopus (genus Hapalochlaena).
Cephalopods, which belong to the class of mollusks, possess blood that appears blue due to the presence of a respiratory pigment called hemocyanin.12
Unlike the iron-based hemoglobin found in vertebrates, hemocyanin is a copper-containing protein. This pigment is colorless when deoxygenated but turns blue when it binds with oxygen. The chemical reaction occurs as oxygen combines reversibly with the copper atoms in the protein, typically in a ratio of one oxygen molecule to two copper atoms. This oxygenated form, sometimes referred to as oxyhemocyanin, is responsible for the characteristic blue coloration of the blood as it circulates through the cephalopod body.1345