Why Do Octopuses Have Three Hearts?

Octopuses have three hearts: two branchial hearts that pump blood through the gills, and one systemic heart that circulates oxygenated blood to the body. They need this extra pumping power because their copper-based hemocyanin blood is far less efficient at carrying oxygen than our iron-based hemoglobin.

We have a single powerful heart pumping blood throughout our entire body. Most other mammals and animals have a single heart that circulates oxygen and nutrients throughout the body. All these single hearts might not look or function the same, but they get the job done.

Now meet the cephalopods—the squid, octopuses, and cuttlefish (also nautiloids, but we’ll come to them later) living in the sea. They weren’t happy with just one heart, so they went and evolved themselves three. Their three hearts together serve to pump blood throughout their flexible and mostly boneless body to supply oxygen.

Why do they need three hearts instead of the single heart that works for so many others in the animal kingdom?

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How Octopuses Circulate Their Blood

Cephalopods have a closed circulatory system, the only class in the phylum Mollusca that do (this is the same phylum to which snails and clams belong). A closed circulatory system means that their blood flows within vessels, unlike an open circulatory system, where all the blood or hemolymph bathes all the tissues, such as is found in insects, prawns and other mollusks.

The three hearts in cephalopod circulation have different functions. There is the main heart, like our one human heart, called the systemic heart that pumps the blood throughout the body. The other two hearts are the branchial hearts or the gill hearts, located near each gill. All these structures are in the mantle, a muscular structure right behind the head of the cephalopod. The mantle also houses other organs, such as the digestive system and reproductive glands.

The actual movement of the blood through the three hearts isn’t that unusual. The deoxygenated blood returns to the systemic heart from the rest of the body. The systemic heart reroutes this blood to the branchial hearts. The branchial hearts then send the blood to capillaries in the gills to get oxygenated. This oxygenated blood is sent back to the systemic heart, at which point it makes its way to the rest of the body to replenish its tissues with oxygen. And so, the process repeats itself.

Other animals have a similar system set up, but instead of two more hearts, blood is sent directly to the lungs or gills to get its oxygen. Humans have the pulmonary artery arising from our heart, which takes deoxygenated blood to the lungs to get oxygenated. The pulmonary vein brings oxygen-rich blood back to the heart for distribution.

The reason cephalopods take the three-heart route, rather than the direct route, might have to do with their blood.

Octopus’s Blood Is The Reason For Its 3 Hearts

We might call royalty ‘blue-blooded’, but the true ‘blue bloods’ are the cephalopods. Unlike our blood, which is red due to our iron-containing oxygen transporter hemoglobin, the cephalopods use a copper-containing protein called hemocyanin. The hemocyanin, when bound to oxygen, gives their blood a blue color.

When the protein isn’t bound to oxygen, it becomes colorless! You won’t see this colorless deoxygenated blood because on contact with oxygen in the water or in the atmosphere, the blood will turn blue again.

Hemocyanin isn’t as efficient at transporting oxygen as hemoglobin. While hemocyanin does exhibit some cooperative oxygen binding, it is far less efficient overall—roughly only about 25% as effective as hemoglobin at carrying oxygen per unit volume of blood.

When hemoglobin is partially oxygenated, due to conformational changes in the protein, its affinity to oxygen increases further, giving it a much steeper oxygen-binding curve and greater transport efficiency.

However, hemoglobin isn’t great at low temperatures, where more early cephalopods resided. In these low temperature, low-oxygen pressure environments, hemocyanin had the upper hand.

Hemocyanin, unlike hemoglobin, which is present within red blood cells, is a free-floating protein complex within cephalopod blood, which makes their blood more viscous. Not being inside a cell might aid in its oxygen transport capabilities at low temperatures.

To compensate for the otherwise lowered oxygen transport efficiency and increased viscosity, cephalopods needed to circulate their blood at higher pressures. The solution was developing three hearts. Over years, octopuses and squids ascended to warmer shores, making their three hearts pumping blood at a higher pressure very important.

All of that being said, not all creatures that have blue blood need three hearts. The cephalopod nautilus, the mesmerizing spiral-shelled marine animal, only has a single heart that pumps blood, unlike its three-hearted relatives. It probably doesn’t need the two extra hearts because of their relatively smaller size and the creature’s very sedentary lifestyle.

Arthropods like the famous horseshoe crab and some scorpions also have blue hemocyanin blood. The arthropod hemocyanin differs from molluscan hemocyanin in structure and protein subunits. The arthropods might not have evolved three hearts because with their open circulatory system, they didn’t need three hearts.

Conclusion

Even with these three hearts, octopuses and squids can’t bolt around carefree. In fact, octopuses prefer a more laid-back lifestyle because their systemic heart cannot sustain intense activity.

When an octopus uses jet propulsion to swim—typically to escape a predator—its systemic heart actually stops beating. The muscular contractions required for jet-propelled swimming physically compress the heart, preventing it from pumping. This means swimming quickly is extremely exhausting and the octopus must stop and recover. This is why you probably won’t see an octopus actively zooming about in the ocean. To preserve energy, they prefer to scurry and crawl on the ocean floor, always moving, always watching.

The late Dr Martin Wells, cephalopod researcher, enthusiast and beloved teacher, once said that the octopus is an alien. Well, with three hearts, blue blood, and those oddly sentient-looking eyes, they might as well be!

Alien or not, we love them in all their zany glory!