From here we journey on down to the deepest place on earth – the Mariana Trench – almost 11 kilometres from the surface, a vast chasm that ruptures the deep sea floor. Yet even eight kilometres down, where the basic chemistry of life was once thought impossible, we find strange species swimming through the darkness. The sheer weight of water above creates almost unendurable pressures. The deeper you go, the more extreme conditions become. There are fish that walk instead of swim, worms that feed exclusively on bones and shrimps that spend almost their entire lives imprisoned with their mate in a cage of crystal sponge. Food is hard to come by and finding a mate is even harder, but life adapts in ingenious ways. On the desert wastes of the abyss, a whale carcass generates a frenzy as slow-moving sharks as big as great whites fight for what may be their first meal in a year. We encounter savage hordes of Humboldt squid hunting lanternfish in the depths and coral gardens flourishing in absolute darkness, with more species of coral to be found in the deep than on shallow tropical reefs. We discover alien worlds, bizarre creatures and extraordinary new behaviours never seen before. This episode takes us on an epic journey into the unknown, a realm that feels almost like science fiction. Scientists already think that there is more life in the deep than anywhere else on earth. We have barely begun to explore it, and yet it is the largest living space on the planet. The deep is perhaps the most hostile environment on earth, at least to us – a world of crushing pressure, brutal cold and utter darkness. As we begin to understand the true complexity of the lives of our ocean creatures, so do we recognise the fragility of their home.
But with rising temperatures, summer sea ice is retreating – their battles to survive are becoming ever harder. Deep in the polar north, we meet walrus mothers and their newborn calves, searching for an ice floe to rest on. But our ocean system, in relative equilibrium for millennia, is changing at a worrying rate. Ocean currents move heat around our planet and maintain a climate favourable for life. Here, in winter, pods of orcas use dramatic tail slaps to stun herring, and humpback whales follow the noises to find the feast. But in the Arctic, a warm current from the south keeps some Norwegian fjords ice-free all year round. Toward our planet’s poles, the ocean’s surface is locked in ice. Females change gender, and a new male challenges an older male to a face-off. But when a female reaches both a critical body size and age, it can undergo an extraordinary metamorphosis. At the start of summer a male mates with the females. In Japan, a kelp-covered shipwreck is home to the Asian sheepshead wrasse, or Kobudai. In temperate seas around the globe, spring brings greening oceans.
But when they find them, the whales team up with the dolphins to form super-pods – a formidable army to take advantage of the bounty of these seasonal seas. Predatory false killer whales off the coast of New Zealand are in search of dolphins. Where the plankton thrive, fish thrive too, and ocean travellers will migrate thousands of miles to take advantage of these productive seas. Phytoplankton produce as much oxygen as all the plants on land and lie at the base of marine food chains everywhere. At night, in a previously unseen event, tiny organisms that light up when disturbed react to their wingbeats, creating an enchanting bioluminescent firework display.
In spring thousands of mobula rays gather in Mexico’s Sea of Cortez. Here, unlike the tropics, the seas change with the seasons. Sun heats the sea, creating rain, winds and huge storms that drive up towards higher latitudes. The tropical oceans drive our planet’s weather. Fledglings must eventually take to the wing, but danger lurks beneath the waves – metre-long giant trevally fish leap clear out of the water to snatch the birds. In the Seychelles, half a million terns nest on an island. On another reef, a tusk fish demonstrates a surprising level of ingenuity – tool use – as it uses corals as an anvil to break open clams. Starting in the tropical coral reefs – the most diverse ocean habitat – a baby dolphin is taught the secrets of a coral reef, as its family rubs against a particular gorgonian which may have medicinal properties. Using cutting-edge technology, One Ocean takes us on a journey from the intense heat of the tropics to our planet’s frozen poles to reveal new worlds and extraordinary never-before-seen animal behaviours. In recent years, our knowledge of life beneath the waves has been transformed.
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