Why Octopuses Have Three Hearts and Blue Blood

Octopuses have three hearts and blue blood from copper-based hemocyanin, while blue-ringed species carry a powerful venom that can cause paralysis and death.

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An octopus has three hearts, but they do not all do the same job. Two branchial hearts push oxygen-poor blood through the gills, where the blood picks up oxygen. A larger systemic heart then sends that oxygen-rich blood around the body. Octopus blood is blue because it uses hemocyanin, a copper-containing protein, to carry oxygen instead of the iron-based hemoglobin used by humans. The blue color is therefore a real property of the blood, not the visual effect that makes human veins appear blue through skin. Blue-ringed octopuses add another remarkable feature to the story. Their saliva can contain tetrodotoxin, a powerful neurotoxin that can cause rapid paralysis and potentially death after a bite. These animals are generally not aggressive toward people, and dangerous encounters are uncommon, but handling them can be extremely risky. Together, the unusual circulation system, blue blood, and potent venom show how different octopus biology is from our own.

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