A number of invertebrates have blue blood. The blue colour originates from hemocyanin, a copper containing protein that carries molecular oxygen to the tissues instead of the red-coloured protein haemoglobin in the blood of vertebrates.

Hemocyanins are giant multimeric molecules: their most distinctive structural features are a tertiary fold and an oligomeric state – features that are recognisable in proteins from various animal groups including scorpions, octopi, squid, snails, chitons, some bivalves (molluscs), crabs, lobsters, spiders, myriapods, and insects (arthropods).

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