An octopus has three hearts and blue blood.
Two hearts pump blood through the gills and the third pumps it around the body. Their blood is blue because it carries oxygen with a copper-based protein called hemocyanin instead of iron-based hemoglobin.
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What each heart does
Two small "branchial" hearts sit at the base of the octopus's two gills and pump blood through them to pick up oxygen. The third, the systemic heart, then pumps that oxygen-rich blood around the rest of the body.
Why the blood is blue
Our blood carries oxygen with iron-based hemoglobin inside red blood cells. Octopus blood uses hemocyanin, a copper-rich protein dissolved straight into the blood, which gives it a bluish colour. In cold water with little oxygen, hemocyanin carries oxygen more efficiently than hemoglobin, which suits life in the sea.
The catch
The systemic heart stops beating while an octopus swims. That makes swimming tiring, and it's one reason octopuses usually prefer to crawl.
Common myth
Octopuses don't have tentacles. Their eight limbs are arms. In biology, "tentacles" are the extra feeding limbs that squid have and octopuses lack.
Test yourself
How many hearts does an octopus have?
Two pump blood through the gills, and one pumps it around the body.
Sources
Fact-checked 14 October 2026 · Report a mistake
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