The Octopus Has Three Hearts — and Blue Blood

Animal Wonders #3

The Takeaway

An octopus has three hearts. And the blood flowing through them is not red, but blue. In both the number of hearts and the color of its blood, its body is built differently from ours.

First, What Kind of Animal Is an Octopus?

The octopus belongs to the cephalopods, the same group as squid and cuttlefish. It is a distant relative of shellfish, with a soft body and no bones.

It lives on the sea floor — sandy beds and rocky reefs — crawling along the bottom to catch and eat crabs and shellfish. And this crawling way of life, it turns out, is directly connected to how its hearts are built.

Three Hearts, Three Different Jobs

The three hearts do not all do the same work.

• Branchial hearts (two): Sitting at the base of each gill, these are dedicated pumps that send blood into the gills, where it picks up oxygen from the seawater.
• Systemic heart (one): The main pump, which sends the oxygen-loaded blood out to the whole body.

Having two separate pumps just for the gills is a system unique to cephalopods. The work of moving blood is split into two stages: breathing first, then the body.

The Blood Is Blue Because of Copper

Human blood is red because it carries oxygen using hemoglobin, which contains iron.

An octopus carries oxygen with a protein called hemocyanin, which contains copper instead. When hemocyanin binds with oxygen, it looks blue — so octopus blood is blue.

Iron or copper: change the metal used to ferry oxygen, and even the color of the blood changes.

Why Did It End Up Built This Way?

Hemocyanin is good at carrying oxygen even in cold, oxygen-poor environments. It is a system well suited to cold seas and deep water.

On the other hand, it carries oxygen less efficiently than hemoglobin. To make up for that weakness, the octopus is thought to have developed a body with reinforced circulation — including those two extra hearts dedicated to the gills.

The blue blood and the three hearts are not separate coincidences. They are a matched set, designed for life in cold seas.

When It Swims, Its Heart Stops

This body has an amusing weakness. When an octopus jets through the water (jet propulsion), the systemic heart — the one that sends blood to the whole body — stops.

The thinking is that when the octopus squeezes its body hard to push the water out, the pressure inside rises so much that blood can no longer return to the heart. Studies that measured blood flow in a swimming common octopus have confirmed this shutdown actually happens.

That is why an octopus tires quickly when it swims. Crawling along the sea floor is far easier. An octopus creeping across the bottom is not being lazy — it is living the energy-saving way its body was designed for.

In Human Terms

• One heart → the octopus has three (two for breathing, one for the body)
• Red blood (iron-based hemoglobin) → blue blood (copper-based hemocyanin)
• Swimming is everyday life → swimming stops its heart, so crawling is everyday life

Even for the same task — circulating blood to carry oxygen — evolution has more than one answer. The octopus quietly teaches us that the human body is not the only correct design.

References

• Smithsonian Magazine, “Ten Curious Facts About Octopuses”
• Smithsonian Ocean Portal, “Octopus”
• Nazology Encyclopedia, “Octopus”