An octopus has three hearts and blue blood

Two hearts pump blood through the gills, a third sends it round the body, and the blood carries oxygen on copper instead of iron. Here is how the pieces fit.

An octopus is strange in many ways, and its plumbing is one of the stranger ones. It has three hearts, and its blood is blue.

Like ours, an octopus's circulation is closed: the blood stays inside vessels. The difference is in the pumps. Two branchial hearts, one at each gill, push blood through the gills, where it picks up oxygen. That fresh blood then flows to the third, the systemic heart, which pumps it out to the rest of the body.

The arrangement has a cost. The systemic heart stops working while an octopus swims. That is one reason octopuses tire quickly when they swim, and spend most of their time crawling along the bottom instead.

Why the blood is blue

Our blood is red because of haemoglobin, a protein built around iron and packed inside red blood cells. An octopus uses a different protein, haemocyanin, built around copper. Each of its working units holds two copper atoms, which together grip one molecule of oxygen.

Haemocyanin is colourless when it has no oxygen and turns blue when it carries some. It also floats freely in the blood fluid instead of sitting inside cells. Many other molluscs and arthropods use it too, from squid to crabs and tarantulas.

Copper is not simply better. By volume, most haemocyanins carry only about a quarter as much oxygen as haemoglobin does. In an octopus it also makes the blood thick, so it takes high pressure to move it around the body.

What haemocyanin offers is performance in hard conditions. In cold water that holds little oxygen, it carries oxygen more efficiently than haemoglobin, which suits many of the seas where octopuses live.

Three hearts and copper blood can sound like a design gone wrong. They are better read as a different answer to the problem every animal has, getting oxygen to every cell, worked out along a branch of life that split from ours long ago.