Octopuses have three hearts and blue blood

Introduction: an anatomy that defies our intuition

Three hearts for a single animal

Unlike the vast majority of animals, the octopus doesn't have just one heart but three, an anatomical quirk that reflects the astonishing evolutionary originality of this cephalopod. Two of these organs, called branchial hearts, are dedicated exclusively to pumping blood through the gills to oxygenate it, while the third, called the systemic heart, then takes over to distribute this freshly oxygenated blood throughout the rest of the animal's body. This triple arrangement has no direct equivalent among the vertebrates most of us are familiar with.

This dual-circuit organization allows the octopus to optimize its oxygen intake in an aquatic environment where this gas is naturally less available than in air, a major physiological constraint for any marine organism of substantial size with an active metabolism. Marine biologists often consider this system to be one of the most original circulatory adaptations observed among invertebrates, differing so sharply from the single-heart model found in nearly all vertebrates, including ourselves.

Blood that has nothing in common with ours

Another striking peculiarity: octopus blood isn't red but blue, a color explained by the use of a protein called hemocyanin, built around copper atoms, rather than the iron-based hemoglobin that turns our own blood red in humans and most other vertebrates.

This fundamental biochemical difference shows just how far evolution can go in producing radically different solutions to the same physiological problem, in this case transporting oxygen through the blood of a living organism. Researchers in comparative biochemistry consider this kind of evolutionary convergence and divergence a valuable source of insight into the physical constraints that have shaped marine life across geological eras, long before complex nervous systems even existed.

Why hemocyanin instead of hemoglobin

A molecule suited to cold, oxygen-poor waters

Hemocyanin, unlike hemoglobin, transports oxygen by binding it directly to copper atoms rather than iron, a chemical difference that gives this molecule better efficiency in cold, oxygen-poor waters, conditions common in many marine habitats occupied by cephalopods.

This biochemical adaptation represents a significant evolutionary advantage for animals like the octopus, which often have to survive in environments where dissolved oxygen concentration varies widely depending on depth, temperature and the salinity of the surrounding water, factors that directly influence the biological availability of oxygen. Cold water in particular tends to hold more dissolved gas, which partly favors this molecular strategy in deeper, chillier habitats.

A blue color immediately visible in drawn blood

When researchers draw a blood sample from an octopus for scientific analysis, the characteristic bluish color of the liquid becomes immediately visible to the naked eye, a phenomenon that long puzzled naturalists before modern chemistry revealed the precise explanation tied to the presence of copper in hemocyanin.

This trait isn't unique to octopuses, either: many other invertebrates, including certain crustaceans and mollusks, also share this characteristic of blue blood, proof that this biochemical solution has proven effective on multiple separate occasions throughout the animal kingdom's evolutionary history. Horseshoe crabs, for instance, are famous for a similarly blue-tinted blood prized for its use in medical testing.

The specific role of each of the three hearts

The branchial hearts, engines of oxygenation

The octopus's two branchial hearts sit on either side of the animal's body, each directly connected to one of its two gills. Their sole function is to pump oxygen-depleted blood toward these respiratory organs, where it can be recharged with oxygen before continuing its journey through the body.

This functional specialization allows for efficient oxygenation of the blood before its general redistribution, a mechanism especially useful given the relatively high metabolism of octopuses compared to many other marine invertebrates of similar size. Without this dedicated pumping stage, the animal's overall oxygen supply would quickly fall short of its needs.

The systemic heart, conductor of general circulation

The third heart, called the systemic heart, then takes over, pumping freshly oxygenated blood toward all the organs and tissues of the octopus's body, somewhat similar to how the single heart of vertebrates functions, but here split across several distinct organs.

A remarkable fact reported by several studies in marine biology: this systemic heart literally stops beating when the octopus starts swimming actively, a phenomenon with direct and surprising consequences on the animal's movement behavior. The researchers who first documented this phenomenon had to repeat their observations several times before becoming fully convinced it wasn't an isolated anomaly limited to just a few specimens, given how counterintuitive the finding initially seemed even to experienced physiologists.

Why swimming exhausts octopuses so much

An energy cost directly tied to this cardiac quirk

When the systemic heart stops beating during active swimming, the octopus's general blood circulation slows down considerably, which limits oxygen supply to the muscles being used during this effort and causes noticeably faster exhaustion than during more passive movement.

This physiological constraint explains why octopuses generally avoid swimming over long distances, preferring to save this activity for brief movements, for example to quickly escape a predator or catch nearby prey. Sustained swimming, in other words, is treated almost like an emergency reserve rather than a routine mode of travel.

A marked preference for crawling along the seafloor

Rather than swimming, the octopus overwhelmingly prefers crawling along the seafloor, using its eight arms, richly equipped with suckers, to move efficiently across the rocks, sand or coral reefs that make up its natural habitat.

This mode of movement, far less energy-intensive than active swimming, allows the animal to conserve its physiological resources for other vital functions, such as hunting, reproduction or avoiding the many predators found throughout coastal marine ecosystems. It also gives the octopus finer control over its surroundings, since crawling allows constant contact with the seafloor's textures and crevices.

What this physiology reveals about cephalopod evolutionary history

An adaptation inherited from very ancient ancestors

Cephalopods, the group that includes octopuses, squid and cuttlefish, rank among the oldest invertebrates on the planet, with an evolutionary lineage stretching back several hundred million years, long before the first terrestrial vertebrates appeared.

This three-heart, hemocyanin-based circulatory architecture is therefore a deeply rooted evolutionary legacy, shaped by millions of years of adaptation to life in ocean environments with wildly varying conditions across different geological eras. It predates the rise of many animal groups we now consider far more familiar or advanced.

A system that continues to fascinate researchers today

Even today, research institutions such as MBARI, the Monterey Bay Aquarium Research Institute, along with NOAA, the American oceanic and atmospheric agency, continue studying the complex physiology of cephalopods, particularly to better understand how these animals manage to survive in environments as diverse as tropical reefs or freezing abyssal depths.

This research also contributes to a broader understanding of the evolution of circulatory systems within the animal kingdom, a field where cephalopods offer a particularly instructive contrast with vertebrate anatomy. Comparative physiologists frequently cite the octopus as a textbook example of convergent evolution in biology.

Other remarkable octopus traits

Extraordinary intelligence for an invertebrate

Beyond their unique cardiovascular system, octopuses are also renowned for their remarkable intelligence among invertebrates, capable of solving complex problems, opening closed containers and, according to certain behavioral studies, even individually recognizing certain caretakers in an aquarium setting. Some individuals have even been observed using coconut shells or discarded containers as portable shelters, a behavior many biologists consider a rudimentary form of tool use.

This intelligence goes hand in hand with an equally unusual nervous system, since a large portion of the octopus's neurons are distributed directly throughout its eight arms rather than concentrated solely in its central brain, giving it a fairly unique form of functional autonomy within the animal kingdom. Each arm can, to a genuinely surprising degree, process sensory information and react independently to its immediate environment without waiting for central brain input.

An equally impressive camouflage ability

Octopuses also possess a remarkable ability to change the color and texture of their skin almost instantly, thanks to specialized cells called chromatophores, allowing them to blend into their immediate surroundings to escape predators or ambush prey.

This unique combination of physiological traits, ranging from the three-heart circulatory system to this sophisticated camouflage ability, makes octopuses one of the most studied and fascinating groups of marine animals for the international scientific community. Some researchers even suggest that in-depth study of these mechanisms could eventually inspire new approaches in soft robotics or adaptive materials capable of changing texture.

Conclusion: an animal that pushes the limits of our biological imagination

A lesson in evolutionary diversity

The octopus's highly unusual anatomy, with its three hearts and copper-based blue blood, beautifully illustrates the extraordinary diversity of solutions evolution has developed over time to allow life to thrive in environments as varied as our planet's oceans.

This unique physiology also reminds us that our own biological intuitions, largely shaped by our experience as terrestrial vertebrates, aren't always enough to anticipate the many ways life has organized itself elsewhere in the animal kingdom.

A subject far from exhausted

Despite decades of scientific research devoted to cephalopods, many aspects of octopus physiology and behavior remain poorly understood, suggesting this creature will continue surprising researchers who study it for a long time to come.

This enduring fascination with such a singular animal confirms, once again, that our planet's oceans still hold biological mysteries comparable, in their strangeness, to anything we might imagine finding on another planet. For anyone interested in living things, the octopus remains a constant reminder that exploring our own marine world can still hold as many surprises as exploring distant, still-unreachable worlds, and that some of the strangest biology imaginable is swimming, or rather crawling, right here on Earth.

By Maxime Marquette, columnist

Sources

Primary sources

MBARI, Monterey Bay Aquarium Research Institute — 2026

NOAA, American oceanic and atmospheric agency — 2026

Nature, section dedicated to cephalopods — 2026

Secondary sources

National Geographic France, Animals section — 2026

Futura Sciences, Planet section — 2026

Sciences et Avenir — 2026