How many hearts does an octopus have?

Answer

Two branchial hearts serve the gills and one systemic heart serves the body.

Context

Octopus circulation runs in a closed loop with two stages. The systemic heart pushes oxygen-rich blood out to the organs and muscles, and the blood comes back low in oxygen. The two branchial hearts, one at the base of each gill, then drive that returning blood through the gill tissue, where it picks up oxygen and gives off carbon dioxide before it reaches the systemic heart again.

Smithsonian reporting adds a notable detail: the systemic heart stops beating while the octopus swims. With less blood reaching the body during jet propulsion, swimming is tiring, so octopuses tend to crawl across the seafloor and generally save swimming for short bursts.

A commonly offered reason for the extra pumps is that gill capillaries add resistance, and a dedicated heart at each gill can keep blood moving through them at adequate pressure. Octopus blood also carries oxygen with copper-based hemocyanin, which holds less oxygen than a vertebrate's hemoglobin-filled blood cells, so efficient circulation matters. This functional explanation is widely accepted but is an interpretation of the anatomy rather than a single proven cause.

The three-heart layout is not unique to octopuses. Squid and cuttlefish, the other coleoid cephalopods, share the same arrangement of two gill hearts and one systemic heart.