Octopus Hearts

Why Does an Octopus Have Three Hearts?

An octopus (order Octopoda) runs on three hearts. One of them, the systemic heart, drives oxygenated blood out to the body. The other two are branchial hearts, one sitting at each gill, and their job is to send deoxygenated blood back through the gills to pick up oxygen again. So you end up with hearts = 1 systemic + 2 branchial = 3. Here's the odd part: the systemic heart stops beating whenever the animal swims, which helps explain why octopuses would rather crawl than swim any real distance.

Their blood comes out blue. That's down to hemocyanin, a copper-based respiratory pigment they use in place of the iron-based hemoglobin that vertebrates carry. Hemocyanin does a poor job in warm water but holds up nicely in cold seawater where oxygen is scarce, and moving it around takes the extra muscle the branchial hearts supply. There's also the matter of their nervous system, often summed up as nine brains: a central brain, then a smaller ganglion tucked into each of the eight arms, which leaves every arm able to act somewhat on its own.

Applications

The three-heart count turns up again and again in marine biology, comparative physiology and science writing for general audiences. Researchers study species like Octopus vulgaris (the common Atlantic octopus) to probe cephalopod intelligence, and philosopher Peter Godfrey-Smith's book Other Minds did a lot to bring this strange anatomy to a wider readership. You'll also see the fact pop up in classroom lessons on convergent evolution and oxygen transport, and in care guides for marine aquariums.

FAQ

Do all cephalopods have three hearts? Pretty much. Octopuses, squids and cuttlefish all carry this same setup, which they got from a shared ancestor. The nautilus is the odd one out, being more primitive: it has four pairs of gills and a circulatory plan that's more involved still.

Why does the systemic heart stop during swimming? Jetting along raises the pressure inside the body and throws off the systemic heart's rhythm. For a short while the muscles keep going on oxygen already banked in the tissues. That's part of why an octopus tires out fast when it's forced to swim.

Do octopuses really have nine brains? There's one central brain and a nerve ganglion in every arm. Roughly two-thirds of their 500 million or so neurons sit out in the arms, so each arm can work through a touch-based problem on its own to some extent.