How many hearts does an octopus have?

An octopus has three hearts: two smaller branchial hearts each pump blood specifically to one of its two gills to pick up oxygen, while a third systemic heart pushes that oxygenated blood out to the rest of the body. This setup helps compensate for the octopus's unusual blood chemistry, which uses a copper-based protein called hemocyanin instead of iron-based hemoglobin, making the blood blue and somewhat less efficient at carrying oxygen than ours.

An octopus does not have only one heart, nor does it have eight, its eight arms are simply limbs, not extra hearts, a natural but incorrect guess given how often the number eight comes up with octopuses.

The systemic heart actually stops beating when the octopus swims forcefully, since the jet-propulsion motion of swimming compresses it, which is thought to be one reason octopuses tire quickly when swimming and generally prefer to crawl along the seafloor instead.