Why an Octopus Has Three Hearts
An octopus moves oxygen through its body using a system built around its gills. Two branchial hearts—one beside each gill—push blood through those oxygen-extracting organs. A third, the systemic heart, then pumps the oxygen-rich blood onward to the rest of the body.
That division of labor matters because an octopus’s blood carries oxygen differently from ours. Its carrier is hemocyanin, a copper-containing protein dissolved in the blood’s fluid, rather than iron-based hemoglobin inside red blood cells. When hemocyanin binds oxygen, it appears blue. It also functions effectively in cold water, where oxygen delivery can be challenging for animals living in frigid seas.
The Cost of a Fast Getaway
The three-heart arrangement has a drawback: during swimming, the systemic heart pauses. Octopuses commonly swim by jet propulsion, drawing water into the mantle cavity—the space around their organs—and forcefully expelling it through a narrow opening. The resulting thrust is useful for a quick escape, but it is energetically demanding. With the heart that supplies the body paused, sustained swimming is especially costly.
Crawling on their arms avoids that same jet-powered sprint. It is a more economical way to travel when there is no urgent need to flee, even if it is slower. The contrast helps explain why an octopus might choose a seemingly unhurried crawl over swimming: the best commute is not always the fastest one.
