One Animal, Three Hearts — Here's Why

If you've ever wanted to meet something that feels genuinely alien without leaving Earth, look no further than the octopus. These cephalopods are so biologically unusual that scientists often describe them as the closest thing to an extraterrestrial we'll ever encounter. Among their many strange features, one stands out immediately: octopuses have three hearts.

But why three? And what does that mean for how they live? Let's break it down.

The Three-Heart System Explained

An octopus has one systemic heart that pumps blood throughout the body, and two branchial hearts — one located at each of its two gills. The branchial hearts push blood through the gills, where it picks up oxygen before being sent to the systemic heart and then out to the rest of the body.

This setup is necessary because octopus blood uses a copper-based molecule called hemocyanin to carry oxygen, rather than the iron-based hemoglobin found in human blood. Hemocyanin is less efficient at transporting oxygen than hemoglobin, especially in warm water, so the extra hearts compensate by pushing blood through the system more forcefully.

There's an interesting side effect: octopus blood is blue. Hemocyanin turns blue when oxygenated, giving their blood its striking color.

A Brain in Every Arm

The cardiovascular strangeness doesn't stop there. Octopuses have a highly distributed nervous system. About two-thirds of their neurons are located in their arms, not their central brain. Each arm can act semi-independently, solving problems and reacting to stimuli without waiting for instructions from the brain.

This means an octopus arm can continue to move and respond to touch for up to an hour after being severed — a fact as unsettling as it is fascinating.

More Remarkable Octopus Facts

  • Three hearts, nine brains: In addition to the central brain, each of the eight arms has its own neural cluster, giving octopuses what scientists sometimes call "nine brains."
  • Masters of camouflage: Octopuses can change both the color and texture of their skin in milliseconds using specialized cells called chromatophores and papillae — even though most species are colorblind.
  • Short but brilliant lives: Most octopus species live only one to two years. Despite this, they demonstrate impressive problem-solving skills, tool use, and even play behavior.
  • Ink as a defense: Their ink doesn't just obscure vision — it also contains chemicals that dull a predator's sense of smell, giving the octopus time to escape.
  • Three heart quirk: When an octopus swims (as opposed to crawling), two of its hearts actually stop beating. This is why octopuses prefer crawling — swimming exhausts them quickly.

Why Do They Stop Swimming?

The fact that two of the three hearts stop during swimming is a significant limitation. It means octopuses tire very quickly when swimming and are far more comfortable moving along the seafloor on their arms. Evolution seems to have "decided" that the efficient crawling lifestyle was worth the trade-off of poor swimming endurance.

What Makes Octopuses So Evolutionarily Unusual?

Octopuses are mollusks, related to clams and snails — yet they developed complex intelligence, sophisticated camouflage, and advanced problem-solving entirely independently from vertebrates. Their intelligence evolved along a completely separate path from our own, which is part of what makes studying them so valuable to neuroscientists and evolutionary biologists alike.

They are living proof that there is more than one way for the universe to build a mind.