True or false: Octopuses have three hearts

Octopuses are already among the most alien creatures on the planet, and their cardiovascular system is a perfect example of just how different they are from us. While humans get by with a single heart, octopuses run on three — and each one has a specific job to do.

How the Three-Heart System Works

Two of the hearts — called branchial hearts — sit at the base of each of the octopus's two gills. Their sole purpose is to pump deoxygenated blood through the gills, where it picks up oxygen from the surrounding water.

Once the blood is oxygenated, the third heart — the systemic heart — takes over. This is the main pumping heart, and it circulates the now oxygen-rich blood around the rest of the body, delivering fuel to the muscles, organs, and the octopus's famously large brain.

Why Blue Blood?

Here's where things get even more interesting. Octopus blood is blue, not red. That's because instead of iron-based haemoglobin (which gives our blood its red colour), octopuses use a copper-based molecule called haemocyanin to carry oxygen.

Haemocyanin is actually less efficient than haemoglobin at carrying oxygen under normal conditions — but it has a significant advantage in cold, low-oxygen environments like the deep ocean. It releases oxygen to the tissues more readily at low temperatures, making it a smart evolutionary trade-off for a deep-sea animal.

The Downside of Three Hearts

The three-heart system does have one notable weakness. When octopuses swim by jet propulsion — forcefully expelling water to move quickly — their systemic heart actually stops beating. This makes swimming extremely tiring for them, which is why octopuses prefer crawling along the seafloor to open-water swimming. They essentially have to hold their cardiovascular breath every time they make a dash for it.

A Body Built Differently

The three hearts are just one piece of a much bigger picture of biological strangeness. Octopuses also have nine brains — a central brain and one in each of their eight arms — and each arm can act semi-independently, processing sensory information and controlling movement without checking in with the central brain first.

They also have no skeleton, can fit through any gap larger than their beak (the only hard part of their body), and can change colour and texture in milliseconds despite being completely colourblind.

Why It All Matters

The octopus is a reminder that evolution doesn't follow a single blueprint. Three hearts, blue blood, and distributed intelligence all evolved independently from the systems vertebrates use — and they work remarkably well. Octopuses have been around for roughly 300 million years, so clearly, the design holds up.