If you thought our seas were cold, grey and lifeless, think again! In the fifth and final episode of Wild Isles, Sir David Attenborough introduces us to the colourful underwater world of Britain and Ireland’s oceans. This episode will take viewers below the waves to uncover the stories of the animals that live along 22,000 miles of our coastline.

In winter, clear evidence of the sea’s abundance can be seen on a beach in Norfolk. Thousands of grey seals congregate on the shoreline to give birth. The British Isles are home to 40% of the world’s population of grey seals, and the number being born on our shores rises every year.

Beyond the beach, the vibrant shallows contain several important habitats. Rich beds of sea grass act as important carbon storage areas, as well as providing home for seahorses. When night falls, tiny algae produce bright flashes of light when disturbed. This bioluminescence allows nearby cuttlefish to see their moving prey in the darkness.

On the shallow seabed, the spider crab undergoes a mass migration of epic scale. For just a few days every year, hordes of crabs march together as one. Others, like the slow-moving royal flush sea slug have a more mysterious way of migrating. They use the water currents to carry them to new areas, using their bodies like wings to swim up into a water column, hanging in the water like dancing fairies.

Where the water is forced through narrow gaps along the coast, powerful currents are created. The most awe-inspiring is the Corryvreckan Whirlpool on the Scottish west coast - the third largest whirlpool on the planet. Here, rising nutrients and penetrating sunbeams fuel a fresh blossoming of life, and our waters become a rich soup of tiny floating algae. The algae provide food for a group of alien-like animals called zooplankton, which in turn feed our largest fish, the basking shark. This gentle giant can grow to more than the length of a double decker bus, and it filters the water, open-mouthed, feeding on the tiny zooplankton.

Herring and mackerel form huge swirling shoals as they feed on plankton, and this attracts the attention of aerial hunters. Our seas support 68% of the world’s population of northern gannets and in summer, huge numbers of another seabird, the Manx shearwater, rely on our shores. In a touching piece to camera, Sir David Attenborough bids a fledgling chick farewell as it starts its journey many thousands of miles across the oceans to South America. There is no better example of how important the abundant seas of Britain and Ireland are to the survival of wildlife worldwide.

Confirmed for BBC One on 9 April at 7pm to 8pm.