Octopuses Have Three Hearts and Blue Blood

Quick Answer

Octopuses have three hearts: two branchial hearts that pump blood through the gills, and one systemic heart that circulates oxygenated blood to the rest of the body. Their blood is blue because it uses copper-based hemocyanin to carry oxygen instead of the iron-based hemoglobin that makes human blood red. This system evolved because hemocyanin works more efficiently than hemoglobin in the cold, low-oxygen environments of the deep ocean.

Three Hearts. Not a Metaphor.

If you cracked open an octopus -- which marine biologists do regularly, for science -- you would find three distinct hearts beating in a coordinated rhythm. This is not some vestigial oddity or evolutionary leftover. All three hearts are fully functional and necessary for the animal's survival.

The two branchial hearts sit at the base of the gills. Their job is straightforward: pump deoxygenated blood through the gill tissue, where it picks up oxygen from the surrounding water. Think of them as dedicated booster pumps for respiration.

The single systemic heart takes the freshly oxygenated blood from the gills and pumps it to the rest of the body -- the brain, the arms, the muscles, the organs. This is the heart that does what your heart does.

Here is a strange detail: the systemic heart actually stops beating when the octopus swims. It only operates when the animal is crawling or resting. This is why octopuses prefer crawling to swimming -- sustained swimming exhausts them quickly because their organs are receiving unpumped, passively flowing blood during the effort. The branchial hearts keep working, but without the systemic heart actively pushing oxygenated blood to the muscles, the octopus fatigues rapidly.

This is like trying to run a marathon while your heart takes a break. Octopuses manage it, but they do not enjoy it.

Why Blue Blood?

Human blood is red because it contains hemoglobin, a protein that uses iron atoms at its core to bind oxygen molecules. When iron binds oxygen, it reflects red light. Simple enough.

Octopus blood is blue because it uses an entirely different oxygen-transport protein called hemocyanin. Instead of iron, hemocyanin uses copper atoms to bind oxygen. Oxygenated copper compounds reflect blue light, so oxygenated octopus blood is a vivid blue. Deoxygenated octopus blood is nearly colorless.

This is not a minor chemical substitution. Hemocyanin and hemoglobin are fundamentally different molecules with different evolutionary origins. Hemoglobin is found inside red blood cells -- specialized cellular packages that concentrate the protein. Hemocyanin floats freely in the blood plasma. There are no red blood cells in octopus blood because there is no hemoglobin to package.

The result is a blood that looks, behaves, and functions very differently from ours. And there is a good reason evolution went this route for cephalopods.

The Cold Water Advantage

Hemocyanin is less efficient than hemoglobin at carrying oxygen in warm, oxygen-rich conditions. If you were designing a circulatory system for a mammal living at sea level in a temperate climate, you would pick hemoglobin every time.

But octopuses do not live in warm, oxygen-rich conditions. Many species inhabit deep, cold ocean waters where oxygen concentrations are low and temperatures hover just above freezing. And this is where hemocyanin has a critical advantage.

At low temperatures, hemocyanin maintains its oxygen-binding efficiency far better than hemoglobin does. The copper-based system continues to pick up and release oxygen reliably in conditions where an iron-based system would struggle. Hemocyanin also works better in low-pH environments, which occur in deep ocean waters with high CO2 concentrations.

The three-heart system compensates for hemocyanin's lower overall oxygen-carrying capacity by maintaining higher blood pressure and flow rates. The dedicated branchial hearts ensure that blood passes through the gills at sufficient pressure to maximize oxygen uptake, while the systemic heart pushes oxygenated blood to tissues at pressures high enough to ensure adequate delivery despite the less efficient carrier molecule.

It is an elegant engineering solution: a less efficient oxygen carrier paired with a more powerful pumping system, optimized for conditions where the more common solution would fail.

The Nine-Brain Question

While we are cataloging octopus biological oddities, the heart situation is only part of the story. Octopuses also have what is sometimes described as nine brains.

This is a slight oversimplification, but not by much. The central brain -- located in the head between the eyes -- contains about 180 million neurons and handles higher-level processing, decision-making, and learning. But each of the eight arms contains a cluster of roughly 40 million neurons that can operate semi-independently.

These arm ganglia can taste, touch, and make basic movement decisions without consulting the central brain. If you sever an octopus arm (they can regenerate them), the severed arm will continue to crawl, grip objects, and even bring food toward where the mouth used to be for up to an hour.

The decentralized nervous system means that roughly two-thirds of an octopus's neurons are in its arms, not its head. Each arm is, in a meaningful sense, partially thinking for itself. The central brain sends high-level commands -- "investigate that crevice" -- and the arm figures out the motor details independently.

Combined with three hearts, blue blood, the ability to change color and texture in milliseconds, squeeze through any opening larger than its beak (the only hard part of its body), and intelligence that rivals some vertebrates, the octopus is arguably the most alien creature on Earth.

Tip

Octopuses have been observed using tools, solving complex puzzles, recognizing individual human faces, and escaping from sealed containers in laboratory settings. Despite having a lifespan of only one to five years depending on species, they demonstrate learning abilities and problem-solving skills that put many longer-lived animals to shame.

What Happens When the Hearts Stop

Octopuses have remarkably short lifespans for animals of their intelligence. Most species live only one to two years. The giant Pacific octopus, the largest species, lives three to five years at most.

The end comes after reproduction. Female octopuses guard their eggs obsessively, often for months, and during this period they stop eating entirely. They ventilate the eggs, clean them, protect them from predators, and slowly starve. By the time the eggs hatch, the mother is near death. All three hearts stop shortly after.

Males do not fare much better. After mating, they enter a period called senescence -- rapid physiological decline. They stop eating, their behavior becomes erratic, and they die within weeks to months.

This programmed death is controlled by the optic gland, a structure near the brain that functions somewhat like the pituitary gland in mammals. If the optic gland is removed, octopuses can live significantly longer and continue eating after reproduction. But in the wild, the gland ensures that every octopus is on a one-way trip.

Three hearts, blue blood, nine semi-independent brains, the ability to reshape their bodies at will -- and a life measured in months. Evolution built something extraordinary and gave it an expiration date.