Why Octopuses Have Three Hearts

If you set out to design an animal from scratch, you probably wouldn't give it three hearts, blue blood and most of its brain in its arms. Evolution did exactly that with the octopus, and every one of those odd choices turns out to solve a real problem.

Two hearts for the gills, one for everything else#

An octopus has one systemic heart and two branchial hearts.

• The two branchial hearts sit at the base of the gills, one on each side. Their only job is to push blood through the gills, where it picks up oxygen.
• The systemic heart then takes that freshly oxygenated blood and pumps it around the rest of the body: the arms, the mantle, the brain.

Why split the work? Gills are a network of very fine vessels, and pushing blood through them costs a lot of pressure. By the time blood comes out the other side, it has lost much of its push. Rather than ask one heart to do two hard jobs in a row, the octopus gives the gills their own dedicated pumps and lets the main heart start fresh.

Squid and cuttlefish, the octopus's close relatives, use the same three-heart layout.

The heart that stops when it swims#

Here's the catch. When an octopus swims by jet propulsion, squirting water out of its mantle, its systemic heart stops beating. Swimming is therefore exhausting for an octopus, and it can't keep it up for long.

That helps explain why octopuses spend most of their time crawling along the seabed on their arms. Crawling keeps the main heart beating; jetting is saved for a quick escape.

Why the blood is blue#

Our blood is red because it carries oxygen with haemoglobin, an iron-based protein. Octopus blood uses haemocyanin, which is built around copper instead. When haemocyanin picks up oxygen it turns blue, so octopus blood is a pale blue colour.

Haemocyanin is less efficient than haemoglobin at carrying oxygen, which is part of the reason octopuses need the extra hearts to keep the blood moving. But it has advantages too: it holds on to oxygen well in cold water with little oxygen, which suits many of the places octopuses live.

Arms with a mind of their own#

An octopus has around 500 million neurons, roughly as many as a dog. But only about a third of them are in its central brain. The other two thirds are spread through its eight arms.

Each arm can taste, touch and make simple decisions on its own. An arm can explore a crevice, find a crab and start pulling it towards the mouth while the central brain is busy with something else. Researchers have even seen severed arms continue to reach and grasp for a while after being cut off.

The suckers are packed with chemical receptors, so an octopus is effectively tasting everything it touches.

A short, intense life#

For all this sophistication, most octopuses live fast. Many species live only one to two years. Females typically guard a single clutch of eggs, stop eating while they do it, and die shortly after the eggs hatch. The giant Pacific octopus, one of the longest-lived, still only manages around three to five years.

In short#

• Octopuses have three hearts: two pump blood through the gills, one pumps it around the body.
• The main heart stops while the octopus swims, which is why they prefer to crawl.
• Their blood is blue because it uses copper-based haemocyanin instead of iron-based haemoglobin.
• About two thirds of their neurons are in their arms, which can act semi-independently.