Way down in the ocean depths hangs out a creature so weird you might mistake it for an alien. We already know octopuses are brilliant, can shapeshift to blend into their surroundings, and have arms that basically think for themselves, they're an absolute marvel of evolution. But their wildest secret is tucked away inside their squishy, boneless bodies. While we, and basically all other mammals, depend on just one four-chambered heart, the octopus runs on a totally different kind of biological motor.

The Three-Heart Engine

These animals rely on a surprisingly complex circulatory system driven by exactly three distinct hearts. To get why they need so many, you have to look at how they split up the workload:

The Systemic Heart: Sitting right in the middle of the octopus, this is the biggest and strongest heart of the bunch. Its main gig is blasting oxygenated blood out to the brain, muscles, and other organs.
The Branchial Hearts (Gill Hearts): The remaining two hearts are noticeably smaller and sit right next to the octopus's two gills. Their only job? Pushing depleted, oxygen-poor blood through the gills so the ocean water can recharge it.

The Problem with Blue Blood

Image Credit: Canva

The whole reason behind this bizarre, multi-heart plumbing system comes down to the chemistry of their famous blue blood. Unlike our blood, which relies on iron-packed hemoglobin to haul oxygen around, an octopus uses a copper-based protein called hemocyanin.

Now, hemocyanin is an incredibly clever evolutionary trick that lets the octopus survive the freezing, oxygen-starved waters of the deep sea. But it has a pretty massive catch, compared to our hemoglobin, it’s only about 25 percent as efficient at moving oxygen.

Because their blood is so inefficient, these creatures desperately need extra pumping power. By the time the two branchial hearts push blood through the gills, it’s packed with oxygen again, but the pressure has dropped to dangerously low levels. If that weak trickle of blood went straight to the rest of the body, the animal simply wouldn't survive. That’s exactly where the massive systemic heart jumps in, repressurizing the freshly oxygenated blood and forcefully slamming it out to wherever the octopus needs it.

Here’s a crazy quirk, though, when an octopus actually swims, this main systemic heart completely stops beating. Moving fast puts a ton of physical strain on their soft bodies, so rather than fighting the pressure, the primary heart just shuts off for a bit. That’s why you’ll almost always spot an octopus dragging itself along the seafloor instead of swimming through the water.

Are They Alone?

Even though the octopus feels like a total one-off, it actually shares this weird anatomy with a couple of other animals on the evolutionary family tree:

Squids and Cuttlefish: Because they're in the same cephalopod family, these guys rock the exact same circulatory layout. They also depend on that trio of hearts (two branchial, one systemic) and copper-tinged blue blood to survive harsh marine environments.
This three-heart setup is a perfect example of how evolution can look at the exact same problem and come up with wildly different answers depending on the animal.