An octopus seems to be built according to different rules from most animals we know: it has eight arms, no bones, can rapidly change its skin colour and texture, and has bluish blood. Yet one of its most striking features is its circulatory system: an octopus has three hearts.
This is neither an exaggeration nor a metaphor. Two of the hearts pump blood to the gills, where it takes up oxygen from the water, while the third sends it on to the organs and muscles. Its blood is blue because it uses a copper-containing protein called haemocyanin to transport oxygen.
Three hearts, three different jobs
In humans, one heart pumps blood to the lungs and the rest of the body. In an octopus, this route is divided among three specialised “pumps”:
- two branchial hearts, one for each set of gills;
- one systemic heart, which sends oxygenated blood to the tissues and organs.
Oxygen-poor blood returns from the body to the branchial hearts. They pump it through the gills, where gas exchange takes place: carbon dioxide is released and oxygen is taken up from the water. The systemic heart then distributes the oxygenated blood throughout the body.
It is a closed circulatory system with well-developed blood vessels. For a mollusc—a group that also includes snails and clams—this is an unusually efficient solution. Octopuses, squid and cuttlefish are active cephalopods: they hunt, hide, accelerate suddenly, and use considerable energy to control their arms and bodies.
Why octopus blood is blue
In vertebrates, including humans, oxygen is carried by haemoglobin, an iron-containing protein. When haemoglobin binds oxygen, blood takes on its characteristic red colour.
Octopuses use haemocyanin instead. This protein contains copper, and its oxygenated form can give blood a blue appearance. Copper does not circulate through the body as a free metal; it is part of the chemical structure of the oxygen-carrying protein.
Haemocyanin is also found in other animals, including horseshoe crabs, some spiders and many crustaceans. However, not all animals with bluish blood are closely related: they have arrived at a similar solution to the same biological problem, oxygen transport.
An adaptation to the marine environment
In water, oxygen is harder to obtain than in air, and the amount available can vary according to temperature, depth and water circulation. For cephalopods, an efficient circulatory system is especially important because they are animals with high energy demands.
Haemocyanin works differently from haemoglobin and is sensitive to changes in water temperature and acidity. For this reason, researchers are closely monitoring the effects of ocean warming and acidification on cephalopods. This does not mean that all octopus species face identical vulnerability, as they live in very different habitats, from coastal areas to cold depths.
Why the octopus needs such a complex system
An octopus is not a slow animal simply because it has a soft body. It can crawl among rocks, explore narrow crevices, capture crabs and clams and, when necessary, rapidly propel itself by expelling a jet of water through its siphon.
All of this requires oxygen. Its eight arms contain muscles, suckers and a very rich neural network. In addition, the octopus has a large brain relative to its body size for an invertebrate, and it can solve simple problems, learn from experience and manipulate objects.
This does not mean that each arm “thinks” separately like an independent animal. More precisely, a large share of neural processing takes place in the arms, enabling rapid responses to touch, taste and movement. The brain coordinates overall behaviour, while the arms handle an important part of local information.
In this context, the three hearts are not an extra curious detail added by nature, but a component of an organism specialised for life as a marine predator.
Octopuses have arms, not tentacles
In everyday speech, an octopus’s arms are often called “tentacles”. From a zoological perspective, arms is the more accurate term. An octopus has eight arms, each equipped with suckers along almost its entire length.
In addition to eight arms, squid and cuttlefish have two longer tentacles, used mainly to catch prey. The distinction may seem small, but it shows how varied adaptations are within the cephalopod group.
An octopus’s suckers are not merely attachment “hooks”. They also detect chemical information, allowing the animal to explore objects through touch and taste almost simultaneously. This combination explains why the arms are so useful for hunting and exploration.
Camouflage complements the work of the hearts
Efficient blood pumping is only part of the octopus’s arsenal. The animal also defends itself and hunts through camouflage. Its skin contains specialised cells called chromatophores, which can rapidly alter the distribution of pigments. Other skin structures contribute to reflecting light and changing its appearance.
The result can be remarkable: some octopuses change their colour, pattern and even the apparent texture of their skin within seconds. They can therefore resemble sand, coral, a rock or algae. Underwater mechanics can be just as spectacular in other creatures: for example, the pistol shrimp produces an extremely rapid jet by closing its claw, using an entirely different physical principle.
A recent discovery: deep-sea octopus “gardens”
Octopus biology continues to offer surprises. Researchers at the Monterey Bay Aquarium Research Institute studied an area of the Pacific seafloor near Davidson Seamount, nicknamed Octopus Garden. There, they observed thousands of pearl octopus females (Muusoctopus robustus) guarding their eggs in rock crevices.
The site is important because warmer water from underwater thermal springs reaches the area near the nests. In cold abyssal water, egg development could take a very long time. At this “garden”, warmer conditions reduced the incubation period to less than two years, according to data published by the institute.
The observation shows that, although octopuses are best known for their intelligence, camouflage and arms, their lives depend deeply on seemingly invisible physical details: water temperature, available oxygen, blood circulation and the site chosen for laying eggs.
In short: why three hearts?
| Element | Role |
|---|---|
| Two branchial hearts | Send blood to the gills for oxygenation. |
| One systemic heart | Pumps oxygenated blood to the body. |
| Haemocyanin | Transports oxygen and gives blood a bluish tint because of copper. |
| Closed circulatory system | Helps distribute oxygen efficiently in an active organism. |
An octopus has three hearts and blue blood because its body is adapted to a demanding way of life in the ocean. The two branchial hearts and the systemic heart work together so that oxygen quickly reaches where it is needed. This is one explanation for why this soft-bodied mollusc has become such an agile, discreet and fascinating predator.
For more animal curiosities, see Did You Know…? 10 Curiosities That Will Spark Your Imagination, which offers complementary details on this subject.
Frequently asked questions
Does an octopus really have three hearts?
Yes. It has two branchial hearts, which pump blood to the gills, and one systemic heart, which sends oxygenated blood to the rest of the body.
Why is octopus blood blue?
An octopus uses haemocyanin to transport oxygen. This protein contains copper and, when oxygenated, can give the blood a bluish colour.
Do all cephalopods have three hearts?
Octopuses, squid and cuttlefish typically have one systemic heart and two branchial hearts. This type of circulation is an important characteristic of coleoid cephalopods.
Do octopuses have arms or tentacles?
From a zoological perspective, an octopus has eight arms. Tentacles are found chiefly in squid and cuttlefish, which also have two longer appendages for capturing prey.
Where is the largest known octopus aggregation?
At Davidson Seamount, near the coast of California, there is Octopus Garden, a deep-sea area where thousands of pearl octopuses guard their eggs near underwater thermal springs.
