Octopuses, squids, and cuttlefish all have three hearts, making cephalopods the most famous multi-hearted creatures in the ocean. Two smaller “branchial” hearts sit near the gills and push blood through them, while a larger “systemic” heart sends oxygenated blood to the rest of the body.1PubMed Central. Octopus insularis as a new marine model for evolutionary developmental biology This arrangement is not a quirk of nature for its own sake; it is a direct response to the unusual demands of cephalopod blood, which relies on a copper-based protein that is far less efficient at carrying oxygen than the iron-based hemoglobin in your veins.
How the Three-Heart System Works
Picture a circuit with two booster pumps and one main pump. Deoxygenated blood returning from the body enters the two branchial hearts, one on each side of the animal. Each branchial heart pushes blood through its adjacent gill, where the blood picks up oxygen from seawater. Once oxygenated, the blood flows into the systemic heart, which then drives it out through the aorta and into the tissues. The systemic heart is muscular and single-chambered, generating the highest pressures in the system. In the giant Pacific octopus, researchers have recorded aortic systolic pressures ranging from roughly 45 to 70 centimeters of water, with branchial vessel pressures considerably lower on both the incoming and outgoing sides of the gills.2Comparative Biochemistry and Physiology. Circulation in the cephalopod, Octopus Dofleini
The branchial hearts exist because cephalopod blood loses a great deal of pressure as it passes through the fine capillary beds of the gills. Without dedicated pumps to push blood into those beds, the systemic heart alone would not generate enough force to keep blood moving at a useful rate through both the gills and the body. Squids, which are even more active than octopuses and have higher metabolic demands, rely on these branchial hearts to boost gill blood pressure and ensure adequate oxygenation during high-speed swimming and jet propulsion.3The FASEB Journal. Squid branchial hearts: boosting blood pressure in the periphery
The heart rate itself is surprisingly slow compared to a mammal’s. In the giant Pacific octopus at cool water temperatures, the systemic heart beats somewhere between 8 and 18 times per minute.2Comparative Biochemistry and Physiology. Circulation in the cephalopod, Octopus Dofleini Embryonic octopuses show branchial hearts beating roughly once per second, slightly out of sync with one another, which is one of the earliest signs of a heartbeat during development.1PubMed Central. Octopus insularis as a new marine model for evolutionary developmental biology That slight asynchrony hints at something unusual about how these hearts are controlled, which we will get to shortly.
Why Blue Blood Demands Extra Pumping Power
The reason cephalopods need three hearts comes down to their blood. Instead of the red, iron-based hemoglobin that vertebrates use to carry oxygen, cephalopods use hemocyanin, a large copper-based protein that floats freely in their blood rather than being packed into red blood cells. When hemocyanin binds oxygen, the copper atoms turn the blood blue; when it releases oxygen, the blood goes pale or colorless.4PubMed. Crystal structure of a functional unit from Octopus hemocyanin
Hemocyanin works, but it is a much less efficient oxygen carrier than hemoglobin. It delivers less oxygen per unit of blood, which means the circulatory system has to work harder to meet the animal’s metabolic needs. For a sluggish snail or clam, that is manageable. But cephalopods are predators. Squids chase down fish. Octopuses wrestle crabs. Cuttlefish hover and dart with precision. That lifestyle requires a circulatory system that can push a large volume of blood through the gills quickly and deliver it to muscles under high pressure. The branchial hearts evolved as a solution to this mismatch between a relatively inefficient oxygen carrier and a high-energy way of life.3The FASEB Journal. Squid branchial hearts: boosting blood pressure in the periphery
It is worth noting that cephalopods also have a closed circulatory system, meaning their blood stays inside vessels rather than sloshing around in open body cavities the way it does in most other mollusks. A closed system can maintain higher pressures and more precisely direct blood flow, but it also creates more resistance, especially at the capillary level. The three-heart design helps overcome that resistance.
How Cold Water Changes the Equation
Hemocyanin’s performance degrades in cold water. At low temperatures, the protein becomes less willing to release oxygen to the tissues, which creates a potential crisis for cephalopods living in polar oceans. Researchers comparing Antarctic octopuses to relatives in warmer waters found that the Antarctic species Pareledone charcoti had evolved a partial workaround: its hemocyanin binds oxygen less tightly than that of warm-water species, and its blood carries a higher total concentration of hemocyanin to compensate for the poor unloading efficiency at near-freezing temperatures.5PubMed Central. Blue blood on ice: modulated blood oxygen transport facilitates cold compensation and eurythermy in an Antarctic octopod
At 0°C, hemocyanin still performs poorly in these animals, but there is a saving grace: cold water holds more dissolved oxygen than warm water does, so the blood can carry some oxygen in physical solution rather than relying entirely on hemocyanin. At warmer temperatures around 10°C, the same hemocyanin releases most of its bound oxygen effectively, meaning the animal can function across a wider temperature range than you might expect.5PubMed Central. Blue blood on ice: modulated blood oxygen transport facilitates cold compensation and eurythermy in an Antarctic octopod
This matters for understanding the three-heart system because it shows that the circulatory design of cephalopods is not a solved problem. It is a system under constant evolutionary pressure. The branchial hearts are not simply a luxury; in cold-water species, the ability to keep blood flowing briskly through the gills may be the difference between getting enough oxygen and slowly suffocating. Antarctic octopuses are working at the edge of what their blood chemistry can support, and the circulatory machinery has to compensate.
Hearts That Beat Without a Brain
One of the strangest features of the cephalopod three-heart system is that the hearts do not depend on the brain to keep beating. In experiments on octopuses, cutting the visceral nerves that connect the hearts to the central nervous system did not stop the hearts. They continued to beat in a powerful, well-coordinated manner even in complete isolation from the brain. Going further, severing all nervous connections between the two branchial hearts and removing the nerve supply to the systemic heart still did not shut them down. All three hearts kept beating as strongly as before.6The Company of Biologists. Nervous control of the heartbeat in octopus
This means the cephalopod heart system is myogenic: the heartbeat originates in the muscle tissue itself, not from a nerve signal. The brain can modulate heart rate up or down depending on what the animal is doing, but it does not initiate the beat. This is actually similar to how the human heart works. Your heart has its own internal pacemaker cells and would keep beating if you somehow disconnected every nerve running to it. The brain adjusts heart rate through the autonomic nervous system, but the rhythm itself is locally generated. The interesting wrinkle in cephalopods is that you have three independent myogenic oscillators that have to stay roughly coordinated without a single central pacemaker tying them together.
The slight asynchrony observed in embryonic octopus branchial hearts, where one beats at a slightly different rate than the other, makes more sense in this light. Each branchial heart is running on its own local clock. In a healthy adult, the nervous system likely fine-tunes the timing so all three hearts work in concert, but the underlying beats are independent.
What Happens When an Octopus Swims
There is a well-known factoid that the systemic heart of an octopus stops beating when the animal swims. This is broadly accurate for octopuses, though the details vary by species. During jet propulsion, the muscular contraction of the mantle that forces water out through the siphon also compresses internal organs, including the systemic heart. The result is that the systemic heart essentially stalls during active swimming, and the animal relies on the branchial hearts and the pressure generated by mantle contractions to keep blood moving. This is one reason octopuses tend to prefer crawling over swimming: sustained jet propulsion is exhausting because the tissues are being starved of fully circulated oxygenated blood the entire time. You can often see an octopus sit still and “rest” after a burst of swimming, and that pause is partly about letting the systemic heart catch up.
Squids handle this differently. Because squids swim by jet propulsion almost constantly, their circulatory systems are better optimized for it. Their branchial hearts generate higher pressures, their systemic hearts are more robust, and the overall system tolerates the rhythmic compression of jet propulsion more gracefully. This is part of why squids can sustain the kind of high-speed pursuit predation that octopuses simply cannot.
Other Sea Creatures with Unusual Heart Arrangements
Cephalopods are not the only marine animals with more than one heart, though they are the most famous. Hagfish, the slimy, jawless scavengers of the deep ocean floor, have a main heart plus several accessory hearts distributed along their body to help push blood through their long, flexible form. Some species of earthworm have a series of muscular arches around the esophagus that function as pseudo-hearts, though calling them hearts stretches the definition. Among true marine animals, though, the three-heart cephalopod system is distinctive because it involves dedicated, anatomically separate pumping organs with clearly differentiated roles in the circulatory loop.
Fish, by contrast, have a single two-chambered heart. The blood goes from the heart to the gills, picks up oxygen, and then flows directly to the body without returning to the heart first. There is no second pump to boost pressure after the gills, which is why fish blood pressure tends to be lower than that of mammals. Cephalopods essentially solved the same problem fish face, low post-gill pressure, by adding the branchial hearts as inline booster pumps rather than evolving a multi-chambered single heart the way vertebrates eventually did.
Cephalopod Hearts as Engineering Inspiration
The way cephalopod hearts generate flow has caught the attention of engineers working on soft robotics and medical devices. Because these hearts are soft, muscular structures that produce pulsatile flow without rigid valves or hard parts, they offer a template for pumps that need to work in flexible environments. One research group developed a bioinspired soft pumping element modeled on the squid circulatory system, producing both a soft robotic squid and a prototype artificial heart ventricle based on the same principles.7PubMed. Bio-SHARPE: Bioinspired Soft and High Aspect Ratio Pumping Element for Robotic and Medical Applications
The appeal is practical. Traditional mechanical pumps use rotating impellers or reciprocating pistons, which create shear forces that can damage blood cells or surrounding tissue. A soft pump that squeezes fluid through a flexible chamber, the way a branchial heart squeezes blood through a gill, avoids those problems. The three-heart system also demonstrates how a distributed pumping architecture can outperform a single centralized pump when the fluid has to pass through high-resistance networks like capillary beds. That design principle applies to any engineering problem where fluid needs to be moved through narrow, branching channels without excessive pressure loss at any one point.
Why Not Just Evolve Better Blood?
A reasonable question is why cephalopods did not simply evolve a more efficient oxygen-carrying molecule instead of building extra hearts. The answer involves deep evolutionary constraints. Hemocyanin appeared very early in molluscan evolution, hundreds of millions of years ago, and the entire cephalopod body plan developed around it. Switching to a completely different oxygen transport system would require reengineering not just the blood but the gills, the circulatory architecture, and the tissues that receive oxygen. Evolution does not start from scratch; it modifies what already exists. Adding branchial hearts to boost flow was a much simpler evolutionary step than replacing the fundamental chemistry of the blood.
There is also evidence that hemocyanin is not all downside. It functions well at moderate temperatures, it dissolves directly in the blood plasma without needing red blood cells, and its large molecular structure means it contributes to maintaining blood volume and osmotic pressure. For a soft-bodied animal that uses hydrostatic pressure to move its limbs, the physical properties of the blood matter beyond just oxygen delivery. The three-heart workaround allowed cephalopods to keep the benefits of hemocyanin while compensating for its weaknesses, a trade-off that has clearly worked well enough to sustain one of the most cognitively sophisticated groups of invertebrates on the planet.
The Developmental Origin of Three Hearts
Researchers studying octopus embryos have tracked exactly when the three hearts appear during development. In the species Octopus insularis, the first detectable heartbeat occurs at a specific developmental stage when two lateral regions on the posterior side of the embryo, tucked inside the mantle, begin pulsing. These are the branchial hearts, and they start beating before the systemic heart becomes fully functional. The branchial heart rates were measured at roughly one beat per second, with each branchial heart running at a slightly different tempo. The systemic heart beat at a similar rate, about once every 0.93 seconds.1PubMed Central. Octopus insularis as a new marine model for evolutionary developmental biology
The fact that branchial hearts begin beating before the systemic heart is fully online suggests they may play an early role in establishing circulation during embryonic development, not just in the adult animal. Watching these three independent heartbeats emerge in a tiny, translucent embryo is one of those moments where the strangeness of cephalopod biology becomes vividly real. These are animals that build their own circulatory plumbing from scratch in miniature, complete with three separate pumps, inside an egg the size of a grain of rice.