Why Octopus Blood Is Blue

Our blood is red because it uses an iron-based molecule called hemoglobin to carry oxygen.

Octopus blood uses a copper-based molecule called hemocyanin instead. When it carries oxygen, it turns blue.

Hemocyanin works well in cold water with little oxygen, which helps octopuses live in some of the harshest parts of the ocean.

The trade-off is that it works less well in warmer water, which may be one reason some octopuses struggle as seas heat up.

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