An octopus has 3 hearts. Two of them, called branchial hearts, pump blood through the gills to pick up oxygen, while the third, the systemic heart, sends that oxygen-rich blood out to the rest of the body. On top of that strange arrangement, octopus blood is blue, not red, because it carries oxygen using copper instead of iron.

Three hearts, two jobs

The setup is more efficient than it first sounds. The two branchial hearts sit right next to the gills and do exactly one task: push blood through them so it can grab oxygen from the water.

The single systemic heart then takes that freshly oxygenated blood and pumps it out to the muscles and organs. Splitting the work across three pumps helps an octopus keep oxygen moving despite running a relatively low-pressure circulatory system.

That low pressure is the core problem the three hearts solve. Octopus blood does not push as forcefully as ours, so dedicating two whole hearts just to charging the blood with oxygen at the gills keeps the rest of the body adequately supplied.

Three hearts is not overkill for an octopus; it is the workaround that makes the rest of the body possible.

Why the blood is blue

Human blood is red because it uses hemoglobin, an oxygen-carrying molecule built around iron. Octopuses use a different molecule entirely, hemocyanin, which is built around copper.

Copper turns the blood a bluish color when it binds oxygen, instead of the red you see in iron-based blood. Hemocyanin is not just a quirk, either: it works better than hemoglobin in the cold, low-oxygen water that many octopuses live in, which is a genuine survival advantage.

Unlike our hemoglobin, which is packed inside red blood cells, hemocyanin floats freely dissolved in the fluid. That difference in chemistry is a big reason octopuses can survive in deep, chilly waters where iron-based blood would struggle to deliver enough oxygen.

Three hearts, blue blood, and one of them quits the moment the octopus tries to swim.

The reason they prefer crawling

Here is the genuinely strange part. When an octopus swims by jetting water out of its body, its systemic heart actually stops beating.

That makes swimming exhausting work, and it tires the animal out quickly. So most octopuses would much rather crawl across the seafloor on their arms, which lets all three hearts keep pumping normally.

It is a rare case where the slow, plodding option is the smart one. Crawling, for an octopus, is the energy-efficient choice.

Built for a different kind of body

All of this fits an animal that is soft, boneless, and living under pressure for most of its life. A three-heart, copper-based system is the kind of solution evolution lands on when the body plan looks nothing like ours.

It is a useful reminder that the way humans are built, one heart and iron-rich red blood, is just one option among many. The octopus quietly proves there is more than one way to keep a complex animal alive.

Other animals with surprising hearts

Octopuses are not alone in the weird-heart club. Squid and cuttlefish, their close relatives, also run on three hearts and copper-based blue blood, since they share the same basic body plan.

Earthworms go even further in their own way, with a series of five paired vessels that squeeze blood along like simple pumps. Different bodies clearly solved circulation in wildly different ways.

The octopus still stands out, though, because it pairs three hearts with blue blood and a brain spread partly into its arms. It is one of the strongest reminders we have that intelligence and complex bodies can evolve along a path completely unlike our own.

Try It Yourself

Want to mess around with the ideas above? On whatifs.fun, Animal Quiz, Aquarium and Ocean Depth all let you do exactly that — free, in your browser, no download.

Keep reading: how long different animals live and how deep the ocean goes. Both go deeper on the same rabbit hole.

🎮 Try it yourself: Animal Quiz

Test how much you really know about the animal kingdom.

Play free at whatifs.fun