How many hearts does an octopus have?

Two of the three hearts serve the gills, pushing blood through them to pick up oxygen. The third, the systemic heart, sends it on to the rest of the body. The arrangement exists because octopus blood is bad at carrying oxygen and needs the extra shove.

It is bad at it because it is the wrong colour for the job. Our blood uses haemoglobin, built around iron, which is why it is red. An octopus uses haemocyanin, built around copper, which is why it is blue and why it works better in cold water with little oxygen in it - a fair trade in the deep sea, a poor one anywhere warm.

The systemic heart has an inconvenient habit: it stops beating when the animal swims. That is why an octopus that can jet away at speed would generally rather crawl - swimming leaves it exhausted. And the brain is distributed too. Roughly two-thirds of its neurons are in the arms, which can taste what they touch and continue solving problems when separated from the rest of the animal.