Call to arms

An octopus hides itself within armour of shells (left) and peeks out an eye (middle), wary that an enemy may approach at any time. This protective behaviour, captured for the first time by the BBC programme Blue Planet II, sometimes works, but things can always go wrong (above) when a pyjama shark tries to nab it in a tangle of arms and legs. The octopus must be careful – it can sacrifice a limb but a bite to its head could prove fatal.