When it comes to strange biology, the octopus is already an overachiever—three hearts, blue blood, shape-shifting abilities, and a brain in each arm’s nerve cluster. But one of its quirkiest traits? Two of those hearts stop beating when it swims.
The Three-Heart Setup
Octopuses don’t just have one heart like us—they have three:
- The systemic heart – This is the main pump, circulating oxygen-rich blood around the body.
- Two branchial hearts – One sits at the base of each gill. Their job is to push blood through the gills so it can absorb oxygen from seawater.
The systemic heart keeps the whole animal alive, while the branchial hearts work like specialized turbochargers for oxygen intake.
Why Two Hearts Pause Mid-Swim
Here’s the twist: when an octopus actively swims, both branchial hearts stop beating. This sounds like a terrible idea—cutting off blood flow to the gills during movement—but it’s actually an energy-saving adaptation.
Swimming, for an octopus, is energetically expensive. By shutting down the branchial hearts, the animal reduces the resistance (back-pressure) the systemic heart faces while pushing blood through the body. Instead, oxygen demand during swimming is partly met by reserves in its blood and tissues, and by a temporary reliance on less efficient gas exchange.
Blue Blood and Oxygen
Octopuses have copper-based blood (hemocyanin) rather than our iron-based hemoglobin. This turns their blood a blue color and allows it to carry oxygen efficiently in cold, low-oxygen environments—like the deep sea. But hemocyanin also has a quirk: it’s less efficient in warmer water and when the animal is active, which is another reason constant swimming is unsustainable for octopuses.
Built for Crawling, Not Endurance
Most octopus species prefer to crawl along the sea floor using their arms, which keeps all three hearts happily beating. This is far less taxing than jet-propelled swimming, which they reserve for quick escapes or bursts of speed when fleeing predators.
Think of it like a sports car that burns fuel at an alarming rate—it’s powerful, but you wouldn’t use it for a Sunday drive unless you had to.
A Perfectly Imperfect Design
From a human perspective, a heart that stops beating during exercise sounds like a flaw. But for the octopus, it’s part of a carefully balanced survival strategy—combining explosive bursts of speed with a lifestyle mostly dedicated to stealth, camouflage, and slow, calculated movements.
Three hearts, blue blood, and a stop-start rhythm—nature’s reminder that there’s more than one way to thrive beneath the waves.