Why Do Octopuses Have Three Hearts?

Most animals get by with a single heart. The octopus uses three, pumping blood that isn’t even red. The setup looks bizarre at first glance, but it’s a precise solution to a problem every octopus faces every day: how to move enough oxygen through an active, boneless body using blood that isn’t especially good at carrying it.

Meet the Three Hearts

An octopus has one main heart, called the systemic heart, and two smaller supporting hearts called branchial hearts, according to TONMO. The systemic heart is the larger of the three and does the job most similar to a human heart, pushing oxygen-rich blood out to the arms, organs, and the rest of the body. The two branchial hearts sit just in front of each of the octopus’s two gills, and their only job is to push oxygen-depleted blood through those gills so it can pick up fresh oxygen from the surrounding seawater.

The Real Reason: Blue Blood

The three-heart system exists largely because of what’s flowing through it. Instead of the iron-based hemoglobin that makes human blood red, octopuses use a copper-based protein called hemocyanin to carry oxygen, according to BBC Science Focus. Hemocyanin works well in cold, low-oxygen water, which suits an ocean-dwelling animal, but it’s simply less efficient at carrying oxygen than hemoglobin is.

To make up for that shortfall, octopuses need to circulate their blood more forcefully and more often than a vertebrate would. Splitting the workload across three separate pumps — two dedicated purely to loading blood with oxygen at the gills, and one dedicated purely to distributing that oxygen-rich blood around the body — lets the whole system run at a higher pressure than a single heart could easily manage on its own.

Why Swimming Wears an Octopus Out

Here’s the part that surprises most people: the systemic heart actually stops beating whenever an octopus swims using jet propulsion. When the mantle contracts hard to fire out a jet of water, it pinches shut the blood vessels feeding that heart, briefly cutting off its supply, according to OctoNation. That’s a major reason octopuses generally prefer to crawl along the seafloor using their arms rather than swim for extended periods — sustained swimming is genuinely exhausting for their circulatory system, and jetting is best reserved for short bursts of escape rather than everyday travel.

Not Just an Octopus Thing

This triple-heart, blue-blood arrangement isn’t unique to octopuses. Squid and cuttlefish, their closest relatives, share the same basic three-heart layout, according to ScienceABC. Interestingly, the nautilus — a more distant, slow-moving cephalopod relative with a coiled shell — gets by with just a single heart, likely because its far more sedentary lifestyle simply doesn’t demand the extra pumping power.

Frequently Asked Questions

Do all three octopus hearts beat at the same time? Yes, but they serve different roles in the same continuous loop: the two branchial hearts push blood through the gills to pick up oxygen, and the systemic heart then sends that oxygenated blood out to the rest of the body.

Is an octopus in danger every time it swims? Not exactly, but swimming is metabolically costly for them since the systemic heart temporarily stops during jet propulsion. This is thought to be a key reason octopuses favor crawling for most movement and save jetting for quick escapes.

An octopus’s three hearts and blue blood are a reminder that evolution doesn’t need to follow the same blueprint twice — sometimes the most effective design looks nothing like our own.