Octopuses have three hearts.

Two of an octopus’s hearts pump blood to its gills, where it picks up oxygen, while the third pumps oxygenated blood to the rest of the body. Octopus blood is also blue rather than red, because it relies on a copper-based protein called hemocyanin instead of the iron-based hemoglobin found in humans.

The system has a quirk: the main, body-pumping heart actually stops beating when the octopus swims, which is part of why octopuses tend to prefer crawling along the seafloor over sustained swimming — it’s far more exhausting for them than it looks.