Why an Octopus Has Three Hearts

In the depths of the oceans lives an animal with a circulatory system so unusual that even biologists have to look twice.

Most people are familiar with heart problems, yet a creature that carries three hearts in its body sounds almost like science fiction. That is precisely the case with the octopus. This eight-armed animal has a circulatory system perfectly adapted to marine life, demonstrating just how far evolution can go to balance energy, oxygen and survival.

Why the octopus needs three hearts at all

For humans, one heart is enough to supply the whole body with blood. In an octopus, however, that arrangement simply would not deliver enough power. Its body structure, exceptional mobility and life in often cold seawater place far greater demands on its circulation.

“The octopus has one central main heart and two additional subsidiary hearts that specifically force blood through the gills.”

This division allows oxygen-poor and oxygen-rich blood to remain separated as efficiently as possible. It conserves energy and improves performance – a clear advantage in an environment where oxygen in the water is limited.

The octopus heart system in detail

The systemic heart – the central pump

At the centre is what is known as the systemic heart. Broadly speaking, it works in a similar way to our own heart, although its role within the overall system is distributed differently.

• It pumps oxygen-rich blood from the gill area throughout the body.
• It supplies the muscles, skin, nervous system and, above all, the eight arms.
• Once the oxygen has been delivered, the blood flows back – but not directly to the same heart.

The systemic heart can work so effectively only because two other organs take care of the preliminary stage.

The gill hearts – two specialised powerhouses

The two gill hearts are positioned to the right and left of the gills. They perform a task that, in humans, is handled by the right-hand side of the heart and the lungs, but which is separated out in the octopus.

• They receive the used, oxygen-poor blood returning from the body.
• They force this blood directly through the gills, where oxygen is absorbed from the water.
• Only then does the oxygen-rich blood return to the systemic heart.

This creates a two-stage circulation: first, the blood is “recharged” as it passes through the gills; then the main heart distributes it around the body. Separating these stages increases pressure exactly where it is required – at the gills.

Haemocyanin: why the blood is blue and requires more hearts

In humans, haemoglobin makes blood red because it contains iron. In the octopus, haemocyanin acts as the oxygen carrier, with copper at the centre of the molecule. This gives the blood a bluish appearance.

“Haemocyanin binds oxygen differently and often less efficiently than haemoglobin – the three hearts make up for this shortfall through sheer ‘throughput’.”

Haemocyanin binds oxygen comparatively well in cold water, but its transport capacity per volume is still lower than that of haemoglobin. Evolution’s logical response was greater pumping power: more hearts continuously keeping the blood moving.

A circulatory system for extreme conditions

Octopuses often inhabit areas:

• where oxygen concentrations in the water are low,
• where temperatures fluctuate sharply,
• where pressure is high at greater depths.

At low temperatures, blood becomes more viscous and does not flow as easily. With only one heart, the octopus would soon reach its limits. Three pumps maintain stable pressure even when the water becomes colder and denser.

Eight-armed elite athletes

Octopuses are not leisurely animals of the sea floor. They can change colour in an instant, seize prey, squeeze through crevices and escape predators in a single burst. Every one of these actions requires energy – and therefore oxygen.

To meet this high demand, the three hearts operate as a carefully coordinated team:

Interestingly, during rapid jet-propelled swimming, the systemic heart noticeably reduces its activity. As a result, the octopus reaches a kind of endurance limit more quickly – one reason it prefers to crawl, glide and stalk slowly rather than sprint continuously.

Evolutionary advantages in the harsh sea

Over the course of evolution, the three-pump arrangement gave octopuses clear advantages. Species with a more efficient circulatory system could:

• dive deeper and retreat to places with less competition,
• survive in colder regions,
• react more quickly when a predator appeared.

“Anyone who reacts too slowly in the underwater world becomes prey themselves – three hearts provide valuable seconds.”

There is another factor: compared with many other marine animals, octopuses have a very large brain. Thinking, learning and complex behaviour also consume energy. A powerful circulatory system reliably supplies the nervous system with oxygen as well.

What we can learn from the three-heart system

The octopus has long served as a model organism for research. Its nervous system and circulation provide inspiration for technology and medicine. Engineers study how a system with several pumps can compensate for failures. Biologists examine how haemocyanin functions at different temperatures.

Using several hearts in series or in parallel may initially seem wasteful. In the sea, the opposite is true: redundancy improves the chances of survival. If one area temporarily fails or the oxygen level in the water drops, reserves remain available.

A brief look at other marine animals

The octopus is not the only strange example from the oceans, but it is one of the most striking. Other groups also have unusual circulatory systems or blood compounds, including:

• squid with a similar haemocyanin system,
• fish with enlarged hearts for rapid swimming,
• deep-sea inhabitants with slow, energy-saving circulation.

Nevertheless, the octopus’s three-part circulation stands out because it supports an active, intelligent predator that depends on quick reactions and flexible movement.

How this knowledge changes the way we view our own bodies

Anyone who hears about an animal with three hearts automatically thinks of the organ in their own chest. The comparison shows that there is no single perfect design. Instead, the body adapts to its task and environment – ours to breathing air and walking upright, the octopus’s to cold water, changing depths, camouflage and hunting.

The octopus’s anatomy is a reminder of how inventive biology can be. Three hearts are not a luxury, but a precisely tailored compromise: more complexity in the body structure in exchange for better everyday survival beneath the water. The next time you see a picture of an octopus, you can be certain that behind its eight arms is a circulatory system running, quite literally, at full speed.