Sea otters can break the shell on a shellfish by hitting it against a stone resting on their belly. This can look like clapping. Some even have a favourite stone they carry around in their armpits.
Nightvision, ejectable stomachs and regrowable arms mean starfish are more than meets the eye.
Besides wondrous creatures, new discoveries and spectacular filming, Sir David Attenborough's follow up to The Blue Planet comes with a stark warning about the future