One of the most astonishing facts in all of marine biology is this: an octopus has three hearts. Not one, not two — three. And unlike most biological quirks that require a PhD to understand, the reason for this actually makes perfect, elegant sense once you know how octopus blood works.
The Three Hearts of an Octopus
An octopus has one systemic heart and two branchial hearts — sometimes called gill hearts. Each of these serves a completely different function:
- The two branchial hearts sit on either side of the body, one next to each set of gills. Their job is to push deoxygenated blood through the gills, where it picks up oxygen from the surrounding water.
- The systemic heart sits in the center of the body and pumps oxygenated blood out to all the organs, muscles, and arms.
Hearts in a single octopus — a biological adaptation unique to cephalopod mollusks.
This three-heart system evolved because octopus blood is significantly less efficient at carrying oxygen than mammalian blood. Octopuses use a copper-based molecule called hemocyanin to transport oxygen — whereas humans use iron-based hemoglobin. Hemocyanin works well in cold, low-oxygen water, but it requires more pressure and circulation to deliver enough oxygen to the body. Having three hearts solves this problem.
Why Does the Main Heart Stop When Octopuses Swim?
Here's where things get even more interesting. When an octopus swims — propelling itself by shooting water through its siphon — the systemic (main) heart actually stops beating. This is why octopuses prefer crawling along the ocean floor to swimming: swimming exhausts them very quickly because their main heart goes offline during the activity.
Scientists believe this happens because swimming demands so much muscular energy that the branchial hearts have to work overtime pushing blood through the gills at increased rates. The systemic heart pauses briefly to avoid overloading the circulatory system during this burst of intense activity.
This cardiovascular response to swimming is one of the reasons octopuses are ambush predators. They conserve energy by staying still, then strike quickly rather than chasing prey across the ocean floor.
How Does This Compare to Human Hearts?
| Feature | Octopus | Human |
|---|---|---|
| Number of hearts | 3 (1 systemic + 2 branchial) | 1 |
| Oxygen carrier | Hemocyanin (copper-based) | Hemoglobin (iron-based) |
| Blood color | Blue | Red |
| Heart beats per minute | ~30–50 bpm (resting) | 60–100 bpm (resting) |
| Works in cold water | Yes — optimized for it | No — would shut down |
Are Octopus Hearts Similar to Fish Hearts?
Not really. Fish have a two-chambered heart with a simple atrium-and-ventricle design. Octopus hearts, especially the branchial ones, evolved completely independently and serve a more specialized purpose. The octopus circulatory system is sometimes called an "open" system because blood (called hemolymph) can flow into open body cavities — though octopuses also have some closed vessels for faster circulation to critical organs.
What Happens if an Octopus Heart Is Damaged?
Octopuses have remarkable regenerative abilities. They can regrow lost arms, repair skin damage, and recover from significant wounds. However, heart damage is not something they can easily recover from. Since the systemic heart is essential for oxygenating the entire body, damage to it is typically fatal. The branchial hearts, being paired, offer some redundancy — if one is damaged, the other can partially compensate.
The Evolutionary Origin of Three Hearts
Octopuses are cephalopod mollusks, a group that diverged from other mollusks over 500 million years ago. The three-heart system likely evolved as cephalopods became more active predators, needing better oxygen delivery to support their complex nervous systems and rapid movement. Squids and cuttlefish — close relatives — also have three hearts for the same reason.
Interestingly, nautiluses (ancient cephalopods) have four hearts — two branchial hearts for each pair of gills. This suggests that the number of hearts in cephalopods is closely linked to gill architecture rather than body size or complexity.
Frequently Asked Questions
Do all octopuses have 3 hearts?
Yes — all octopus species have the same three-heart structure. It's a defining feature of the octopus (and broader cephalopod) body plan, regardless of species size or habitat.
Can you hear an octopus's heart beating?
Octopus hearts are too small and the body wall too thick to hear a heartbeat without specialized equipment. Marine biologists use ultrasound and electrocardiography to study octopus heart function in laboratories.
Is three hearts better than one?
For an octopus — absolutely. For a human — it would be an unnecessary complexity. The three-heart system is perfectly adapted to the octopus lifestyle: cold-water living, burst predation, and the specific oxygen-carrying properties of hemocyanin.
