A blue copper-containing protein in the blood of many mollusks and arthropods that carries oxygen, similar to hemoglobin in vertebrates.
From 'haemo-' (blood) + 'cyan' (blue, from Greek 'kyanos'). Scientists named it for its characteristic blue color caused by the copper it contains.
Octopuses and horseshoe crabs use haemocyanin instead of hemoglobin—the copper-based version is actually better at carrying oxygen in cold, low-oxygen ocean environments.
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