Why does an octopus have three hearts?
Three pumps help an octopus move oxygen around a body built for life underwater.
A circulation system with two jobs
An octopus has two branchial hearts that send blood through its gills. A third, systemic heart pushes oxygenated blood to the rest of the body. This division of labour helps keep blood moving through both the respiratory organs and the animal’s many active tissues. It is one of the distinctive features of cephalopod anatomy, alongside their flexible arms and sophisticated nervous systems.
Blue blood, different chemistry
Human blood relies on iron-containing haemoglobin to transport oxygen. Octopuses instead use hemocyanin, a copper-containing protein that gives oxygenated blood a bluish colour. Hemocyanin is useful in marine environments, including cold water, but octopuses still need a robust circulatory system to supply their muscles and organs. Their blood chemistry and heart arrangement make sense together as adaptations to their habitat.
Why swimming is demanding
An octopus usually moves by crawling or by forcing water through a siphon for jet propulsion. Jetting is fast but costly in energy, and the systemic heart slows down or briefly stops during active swimming. That is one reason many octopuses prefer to crawl when possible. Their three hearts do not make them invincible; they are part of an efficient yet carefully balanced biological system.
External references are provided for exploration; Curio Hour’s explanation is written separately.