The blue planet - Tidal seas
This episode deals with marine life that is structured around the rising and falling tides. These are caused by the gravitational pull of the orbiting moon, but in some locations, this can also combine with the power of the sun to create a tidal bore. The world's largest tides are to be found in the Bay of Fundy in Nova Scotia and therefore it is a rich feeding ground. A school of finback whales is closely shadowed by a flock of Cory's shearwaters. However, they only have a limited time to feed before low tide, when they must retreat and other creatures appear. Elsewhere, some of the latter include sand bubbler crabs, bears (which feed on shellfish) and a snail species that can 'surf'. The extreme spring tides allow opportunists to forage further, and racoons are shown tackling a red rock crab. Some larger fish that hunt on the seabed, such as nurse sharks and stingrays, are forced to sit and wait until there is sufficient water in which to swim. A giant horse conch is shown devouring a tulip snail, and hermit crabs battle over its vacant shell. The varying water levels are no obstacle to tarpon: they can breathe air. This enables them to inhabit stagnant areas and hunt in them. The autumn equinox combines with a hurricane off the Bahamas to create a much higher tide than is usual, flooding large parts of the coast. When the sea recedes, it leaves behind salt: food for brine shrimps and the perfect habitat in which flamingos can breed.
The blue planet - The deep
Explores the unknown depths of the ocean. Over 60% of the sea is more than a mile deep and it forms the planet's most mysterious habitat. A sperm whale descends 1,000 metres to look for food and is followed. On the way down, a number of unusual creatures are witnessed, such as transparent squid and jellies, whose photophores give pulsating displays of colour. In such dark places, both being able to see (or sense movement) and the means of quick concealment are equally desirable. To that end, some use bioluminescence as a means of detecting food or evading predators. A descent to the very bottom of the ocean — some 4,000 metres — reveals life even at such cold temperatures, much of it new to science. It is dominated by echinoderms that sweep the sea bed; however, there are occasional large hunters, such as chimaera. In addition, sixgill sharks can grow up to eight metres in length and have remained unchanged for 150 million years. They are described as "living fossils" and relatively little is known about them. As the continental slope flattens out it joins the abyssal plain, which can form huge trenches. At seven miles, the deepest is the Mariana trench, and fish have been found there right down to the very bottom. Attenborough remarks that more is known about the surface of the moon. Species captured on film for the first time include the Dumbo octopus and the hairy anglerfish.
The Blue Planet - The Blue Planet
Looks at how ocean life is regulated around the globe by currents and the varying position of the sun. Near a Pacific seamount, there is a large concentration of marine animals because when the current makes contact with the submerged rock, it forces upwards plankton and other organisms. This in turn attracts other fish to the area that are higher up the food chain, like tuna, and those that are higher still, such as silky sharks. Off of South Africa, a similar situation occurs every June when sardines migrate and are pursued by a caravan of various predators. The South Atlantic waters are the roughest, and storms also churn up nutrients to the surface. These feeding grounds have led to the world's largest albatross breeding colony, on Steeple Jason Island, west of the Falklands. Phytoplankton forms the basis of all sea life, and every night some 1,000 million tonnes of creatures ascend from the deep to search for food. Lunar phases can also have a bearing on events and the mass arrival of Ridley sea turtles on a Costa Rican beach is shown. Herring initiate the most productive food chain, providing sustenance for humpback whales, and Steller's and California sea lions. In addition, their eggs are nutrition for many, both above and in the sea. grey whales make one of the longest migrations of any marine mammal (some 19,000 kilometres) and are ambushed by killer whales, which have learned their journey routes.
The Blue Planet - Frozen Seas
Compares oceanic life in the Arctic and Antarctica. The winter in these regions brings temperatures of minus 50°C and frozen seas that create the biggest challenge. However, there are polynyas in the Arctic, which are free of ice owing to the pressure of currents on either side, and such places do provide refuge for some species, like the walrus and the bowhead whale. A pod of belugas is shown: their movements are limited to a single hole in the ice — therefore putting them at risk of attack from polar bears. Everything changes with the arrival of summer, when melting ice brings a variety of migratory visitors. At the other end of the planet, in the Antarctic, winter is even more harsh, but emperor penguins and Weddell seals stay throughout. Under the sea ice, krill shrink in size and revert to their juvenile form in order to save energy. Chinstrap penguins overwinter to the north, beyond the ice, but return during the spring to breed. Having managed to get ashore, they have to walk a great distance to find a nest site, and the most favoured is Zavodovski Island, an active volcano whose warmth keeps ice from forming. Further south, as the icebergs break up, humpback and minke whales appear, their target the abundant krill. The leopard seal is the Antarctic's top predator. It is most effective underwater, and emperor penguins propel themselves at speed through its territory. Nonetheless, it almost invariably makes a kill.
