Why Do Octopuses Have Three Hearts?

An octopus has three hearts because moving oxygen through its body requires two distinct pumping jobs.

Two of the hearts sit beside the gills. These are called branchial hearts. Each one receives blood that has already circulated through the body and pumps it through one of the octopus’s two gills. There, the blood releases carbon dioxide and collects oxygen from the surrounding water.

The newly oxygenated blood then flows to the third and largest heart, known as the systemic heart. This heart pumps it to the brain, digestive organs, muscles, and eight arms. The three hearts therefore work as a connected system. Two push blood through the gills, while one supplies the rest of the animal.

This arrangement is especially useful because an octopus has a closed circulatory system and must force blood through the narrow vessels of its gills. Its blood also uses hemocyanin, a copper containing molecule, to carry oxygen. Hemocyanin gives octopus blood its blue color and functions effectively in cold water and environments where oxygen may be limited.

Octopuses are not alone in possessing this arrangement. Squid and cuttlefish also have two branchial hearts and one systemic heart. Three hearts are therefore not an accidental excess. They are a division of labor that helps active cephalopods extract oxygen from water and deliver it throughout their bodies.