Octopuses have three hearts and blue blood — two facts most people have encountered at some point. What tends to get left out is the part where one of those hearts stops working every time the octopus tries to swim. That detail changes the picture considerably, and it explains something you'd notice immediately if you ever watched an octopus move.
The three hearts divide labor in a specific way. Two of them — called branchial hearts — sit at the base of the octopus's gills and are dedicated entirely to pushing blood through gill tissue, where it picks up oxygen from the surrounding water. The third — the systemic heart — takes that freshly oxygenated blood and pumps it out to the rest of the body: the arms, the organs, the brain. It's the systemic heart that keeps the whole animal alive and functioning. And it's the systemic heart that stops beating when the octopus swims.
The mechanism is mechanical rather than neurological. When an octopus needs to move quickly — escaping a predator, typically — it uses jet propulsion, drawing water into its mantle cavity and blasting it out through a muscular siphon. The powerful contractions required to generate that jet physically compress the area around the systemic heart, preventing it from pumping. A 1987 study in the Journal of Experimental Biology confirmed it directly: the systemic heart arrests completely during jetting. The two branchial hearts keep running, but the main circulatory pump stops.
The consequence is immediate and significant. During sustained swimming, the octopus's organs and muscles are temporarily cut off from freshly oxygenated blood. The animal can sustain this for a short burst — long enough to escape — but it exhausts quickly, and it needs time to recover once it stops. This is why, given any choice in the matter, an octopus crawls. Crawling doesn't trigger the same cardiac arrest. Moving along the seafloor on eight arms is far more sustainable than jetting, even if it's considerably slower.
The blue blood is connected to the same underlying problem. Where human blood uses iron-based hemoglobin to carry oxygen, octopus blood uses hemocyanin — a copper-based protein that turns blue when oxygenated. Hemocyanin is less efficient than hemoglobin at carrying oxygen under normal conditions, which is part of why octopuses needed a more elaborate pumping system in the first place. The two dedicated gill hearts exist partly to compensate for the lower oxygen-carrying capacity of their blood, ensuring it moves through the gills slowly enough and under enough pressure to pick up adequate oxygen despite the inefficiency.
Hemocyanin does have one advantage: it performs better than hemoglobin in cold, low-oxygen environments. In the deep, cold water where many octopus species live, blue blood is actually better suited to the conditions than red blood would be. The three-heart system, strange as it looks from the outside, is a functional solution to a specific set of environmental and physiological constraints — not a design flaw, but an adaptation to a particular way of living.
One detail that researchers have noted in passing: a male octopus introduced to a female in a tank showed his systemic heart skipping beats as she approached. The same cardiac arrest mechanism that gets triggered by swimming, it turns out, can also apparently be triggered by something that looks a lot like excitement. Which, depending on how you feel about octopuses, is either endearing or deeply unsettling.
Three hearts, blue blood, and a circulatory system that breaks down under the physical stress of escape. The octopus is, as one cephalopod researcher famously put it, effectively an alien — and its cardiovascular system is one of the better pieces of evidence for that argument.
