Why Do Octopuses Have Three Hearts?

Why do octopuses have three hearts?

Octopuses have three hearts with different jobs: two branchial hearts send blood through the gills, while one systemic heart pumps oxygenated blood around the body. The arrangement supports oxygen pickup and delivery; it is not three interchangeable backup pumps. Their oxygen-carrying protein, haemocyanin, contains copper and becomes blue when oxygenated. These are anatomy facts, not a reason to approach or handle an octopus.

Three hearts sounds like an excellent start to a very complicated Valentine's card. The biology is more practical: follow where the blood goes, and the number becomes much easier to remember.

The Natural History Museum's octopus guide describes the split between the pair serving the gills and the heart serving the body. The important part of the answer is their different destinations, not just the total.

What does each heart do?

“Branchial” means relating to gills. “Systemic” identifies the circulation serving the wider body.

The Smithsonian Ocean Portal's circulation explanation makes this distinction explicit.

Keep two processes separate. The hearts move blood. The gills are where blood picks up oxygen from the surrounding water. Saying that a heart “makes oxygen” would confuse transport with gas exchange.

Likewise, three hearts does not mean three complete, independent animals inside one body. They are components of one circulation. Calling two of them spares obscures the work they normally perform.

How can you remember the blood's route?

Use this simplified reading sequence:

1. Blood returning from the body's tissues needs more oxygen.
2. The paired branchial hearts send it through the gills.
3. Oxygenated blood reaches the systemic heart.
4. That heart sends it onward through the body.

This sequence is a learning summary of the heart roles, not a complete anatomical diagram showing every vessel, chamber or organ.

Draw the gill route as a pair of branches, not as a queue in which all the blood must pass through one gill heart and then the other. The two-heart count describes the paired arrangement; it does not mean the same blood must visit both in succession.

If you are making a school note, label what each arrow represents. “Blood moving toward a gill” is much clearer than an unexplained blue arrow. Labels also keep the explanation readable without relying on colour.

Why is octopus blood blue?

The oxygen carrier is haemocyanin, also spelled hemocyanin. The Natural History Museum explains that it uses copper, in contrast with the iron-containing haemoglobin associated with our blood.

When oxygenated, the octopus's blood has a blue colour; the museum notes that it loses that colour when deoxygenated. This is chemistry, not a royal family connection.

Do not confuse blood colour with the colour of the animal's skin. A photograph of an orange or brown octopus is not a blood-oxygen measurement. Nor can a viewer diagnose its circulation by watching a change in its outer appearance.

The useful pairing is therefore: hearts move the blood; haemocyanin carries oxygen within it. Knowing only that the blood is blue leaves out the pumping system. Knowing only that there are three hearts leaves out what is being transported.

Is blue blood automatically better in cold water?

That claim needs more care than a short animal-fact caption allows. Moving oxygen involves picking it up and releasing it where it is needed. Strong binding alone does not settle how well the whole system works.

In a 2015 study in Frontiers in Zoology, researchers compared the Antarctic octopus Pareledone charcoti with the South-east Australian Octopus pallidus and Mediterranean Eledone moschata. They examined oxygen transport properties rather than counting hearts.

For P. charcoti at 0°C, the researchers reported poor oxygen unloading by haemocyanin, supported by dissolved oxygen and compensating features including a higher oxygen-carrying capacity. The result is more interesting than “blue blood works perfectly in the cold”: the particular animal's physiology matters.

This is a species-specific research finding, not permission to apply that temperature to another octopus or alter an aquarium. The study does not establish that every octopus responds identically to warming or cooling.

Does heart count explain every way an octopus moves?

No. Anatomy, movement and habitat are related, but one number is not a complete behaviour guide.

MBARI, the Monterey Bay Aquarium Research Institute, describes flapjack octopuses moving with fins, pulses of the webbed arms and water expelled through a funnel. Its observations include animals resting on the seafloor and returning toward it after swimming.

That example matters because “an octopus moving” does not always mean the same movement. A statement about one species using one method should not silently become a claim about every octopus in every habitat.

An ordinary video can show visible motion; it does not directly measure which heart is beating, the blood's oxygen content or the animal's total energy use. If a caption makes those physiological claims, look for the underlying research and its conditions.

Never provoke an animal to make it move for a comparison. Use existing institutional footage and published diagrams.

What can a photograph actually establish?

A photograph and an anatomy source answer different questions.

A labelled museum photograph may help identify the animal shown. Depending on the view, it may show arms, fins or a body outline. It cannot reveal every internal structure merely because its caption mentions them.

Use this original evidence check for a school project or reading note:

Do not copy the strongest claim in a caption into the weakest evidence column. A useful note can simply say, “The photograph illustrates the animal; the anatomy reference supports the heart count.”

The same separation helps when reading other ocean-life explainers: an illustration makes a subject easier to picture, but it does not establish lifespan, physiology or behaviour by itself.

How would you correct a misleading fact card?

Suppose an imaginary card says:

An octopus has three spare hearts, blue skin because of its blood, and the same swimming behaviour as every other octopus.

None of those conclusions follows from the three-heart fact.

A better version would say:

“Two hearts pump blood through the gills, and one pumps oxygenated blood through the body. The blood's blue colour is associated with oxygenated haemocyanin. Identify the species before making specific claims about its movement.”

That is an original editing exercise, not a quotation from a real book or a claim that a particular publisher made the error.

Notice what the correction does not add. It supplies no universal swimming speed, heart rate or survival time. Those figures would need their own evidence. More numbers do not make a fact card more accurate unless the numbers answer a defined question.

How can you learn more without disturbing wildlife?

Start with the linked museum explanations, research and institutional animal records. If you observe marine wildlife, follow current local rules, keep your distance and never approach, feed, touch or handle it.

NOAA's ocean-etiquette guidance also says not to attract, chase or harass wildlife and to contact local authorities about an animal that appears in trouble. Do not attempt a rescue or move an octopus yourself; seek qualified local wildlife advice.

This article is educational, not animal-care, wildlife-handling or diving instruction. There is no home experiment involving an animal's blood, temperature or breathing.

For more questions about the difference between visible action and what it means, explore animal behaviour. The memorable answer remains three hearts. The better answer names their jobs.

Sources

• Natural History Museum: Octopus hearts and blue blood
• Smithsonian Ocean Portal: Cephalopod circulation
• Oellermann and colleagues: Blue blood on ice, 2015
• MBARI: Flapjack octopus
• NOAA: Ocean etiquette