A copper-containing oxygen-carrying protein found in the blood of certain invertebrates like squid and spiders, functioning similarly to hemoglobin.
From Greek 'haima' (blood) + 'kyanos' (blue) + '-in' (substance). Named in the late 1800s when scientists discovered this blue-colored respiratory pigment in mollusks and arthropods.
Squid blood is actually blue because it uses copper-based hemocyanin instead of iron-based hemoglobin—it's more efficient in cold, low-oxygen ocean environments, which is pure evolutionary genius!
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