Octopus: Three hearts, blue blood, eight clever arms

The octopus is one of the most amazing animals in the sea: three hearts, blue blood, and eight arms as nimble as eight hands. It can change colour in an instant, squirt ink to escape, and squeeze through gaps far smaller than itself. Let's meet this "magician of the deep".

Its body

The octopus is a cephalopod mollusc. It has three hearts: two pump blood through the gills, and one serves the rest of the body. Its blood carries oxygen using a copper-based protein, which makes it blue — quite unlike the red blood we know.

Its eight arms carry hundreds of suckers each, and every sucker can "taste", so an octopus really explores the world with its "hands". Thousands of colour cells dot its skin, letting it change colour and pattern in seconds — for camouflage and for expression.

What it can do

When danger comes, an octopus squirts a cloud of ink and escapes while its opponent cannot see. It can also squeeze into small holes, shells and even bottles — because an octopus has no hard bones at all, so its body can reshape at will. It is highly intelligent: it uses tools, opens jars and solves mazes, making it one of the cleverest invertebrates known to science.

Where it is found

Octopuses live in oceans worldwide, from shallow water to the deep. They are mostly solitary, hiding in rock crevices, dens or shells, and are outstanding predators.

A note for grown-ups

You could play "guess how many hearts I have" with a child, then talk about blue blood. Helping a child see that many animals are built completely differently from humans is one of the loveliest parts of nature.