An octopus has three hearts. Two branchial hearts pump blood through the gills, and one systemic heart pumps it to the rest of the body.
Why Three Hearts?
Octopuses are cephalopods, a group that also includes squid and cuttlefish. Their circulatory system is closed, meaning blood flows through vessels rather than bathing organs directly. The three-heart arrangement is an adaptation to their active, predatory lifestyle and their oxygen-rich blood.
Octopus blood is blue because it uses hemocyanin, a copper-based protein, to carry oxygen. Hemocyanin is less efficient than the iron-based hemoglobin found in vertebrates, especially in cold or low-oxygen water. To compensate, the octopus heart system maintains high blood pressure and flow, ensuring enough oxygen reaches tissues during bursts of activity like jet propulsion or crawling over rocks.
How the Three Hearts Work Together
The two branchial hearts are located at the base of each gill. They receive deoxygenated blood from the body and pump it through the gill capillaries, where oxygen is absorbed and carbon dioxide is released. The oxygenated blood then travels to the systemic heart, which pumps it out to the rest of the body—brain, arms, and other organs.
The systemic heart is the largest of the three and does the main work of circulation. When an octopus swims, the systemic heart often stops beating temporarily. This is because jet propulsion requires strong muscle contractions that create high pressure in the mantle cavity, which can interfere with the heart’s function. As a result, octopuses prefer to crawl rather than swim whenever possible; swimming is tiring and reduces circulation.
Blood and Hemocyanin
Hemocyanin is dissolved directly in the blood plasma rather than packed inside cells like hemoglobin. This makes octopus blood more viscous than vertebrate blood, which adds resistance to flow. The three hearts generate enough pressure to push this thick blood through the body, but the system is still less efficient than that of a mammal.
In cold water, hemocyanin’s oxygen-carrying capacity increases, which helps octopuses in deep or polar environments. However, if the water becomes too warm, oxygen levels drop and hemocyanin becomes less effective, making it harder for the octopus to get enough oxygen. This is one reason octopuses are sensitive to ocean warming and can die in warm, low-oxygen conditions.
Do Other Cephalopods Have Three Hearts?
Yes. Squid and cuttlefish also have three hearts with the same basic layout. Nautiluses, the most primitive living cephalopods, have a simpler system with two hearts, and their blood is less efficient at carrying oxygen. The three-heart system is a shared trait among coleoid cephalopods (octopuses, squid, and cuttlefish), which are more active and have higher metabolic demands than nautiluses.
Common Misconceptions
One common myth is that octopuses have nine brains and three hearts, but that’s only partly true. They do have three hearts, but they have one central brain plus a cluster of neurons in each of their eight arms—often called “mini-brains” because they can process information locally. However, these are not separate brains in the anatomical sense; they are ganglia, and the central brain still coordinates overall behavior.
Another misconception is that an octopus can survive if its systemic heart stops. In reality, the systemic heart is essential for pumping blood to the organs; without it, the octopus would die quickly. The branchial hearts alone cannot sustain circulation to the body.
Frequently Asked Questions
Why is octopus blood blue?
Octopus blood contains hemocyanin, a copper-based protein that turns blue when oxygenated. Hemocyanin is less efficient than hemoglobin, so the three hearts are needed to pump enough blood to meet the octopus’s oxygen demands.
Do octopuses have hearts in their arms?
No, the hearts are all located in the main body (the mantle). The arms contain nerves and muscles, but no hearts. The branchial hearts sit near the gills, and the systemic heart is in the center of the mantle.
How many hearts do squid and cuttlefish have?
They also have three hearts, just like octopuses. This is a common feature among the coleoid cephalopods, which are the most active and intelligent members of the group.
Can an octopus live with only two hearts?
No. All three hearts are necessary for normal circulation. The systemic heart is particularly critical because it pumps blood to the body; without it, the octopus would suffer organ failure and die.
Why does an octopus’s heart stop when it swims?
When an octopus uses jet propulsion, the strong contraction of the mantle muscles increases pressure around the systemic heart, causing it to stop beating temporarily. This is why octopuses prefer crawling and only swim when necessary—it’s energetically costly and reduces blood flow.
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