The surprising truth about the octopus’s three hearts.

An octopus does not run on one heart. It runs on three – and when it swims, the most important of the three stops beating. That single design choice explains a lot about how the animal lives.

Two hearts for the gills, one for the body

Two of the hearts sit at the base of the gills and do one job: they push blood through the gills, where it picks up oxygen. The third heart, larger and downstream of the other two, pumps that oxygen-rich blood out to the rest of the body. It is a division of labor most animals never bother with.

Blue blood, built for cold water

Octopus blood is blue. Ours is red because we carry oxygen on hemoglobin, an iron-based protein. The octopus uses hemocyanin instead, a copper-based protein – and copper turns blue when it carries oxygen. Hemocyanin is less efficient than hemoglobin in warm, oxygen-rich water, but it keeps working in the cold, low-oxygen depths where octopuses live.

The heart that quits during a swim

When an octopus swims, the main body heart stops. Swimming is therefore exhausting for the animal: it is moving at speed while its main pump is paused. That is why octopuses spend so much of their time crawling along the seafloor on their arms instead – crawling keeps all three hearts on the job.

A brain that reaches into every arm

The strangeness does not end with the hearts. Roughly two-thirds of an octopus’s neurons are not in its central brain at all – they are distributed through its arms, which can taste what they touch through chemoreceptors in the suckers. Each arm can explore, grip and react with a degree of independence no vertebrate limb has.

Sources

• Smithsonian Magazine: Ten Curious Facts About Octopuses
• Monterey Bay Aquarium: Giant Pacific Octopus