Although corals feed nocturnally on plankton, sunlight is vital because even though they are animals, each contains millions of single-celled algae. As the community flourishes, animals develop relationships with one another and such a place can feature a huge variety of ocean life. Having multiplied, it hardens into a limestone skeleton and grows to form a reef. It starts life as a larva that becomes a polyp. This episode follows coral reefs which are so crowded that they play host to a perpetual battle for space, even among the coral itself. Further south, as the sea ice breaks up, humpback and minke whales appear. 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. Chinstrap penguins overwinter to the north, beyond the ice, but return during the spring to breed. Under the sea ice, krill shrink in size and revert to their juvenile form in order to save energy. At the other end of the planet, in the Antarctic, winter is even more harsh, but emperor penguins and Weddell seals stay throughout. Everything changes with the arrival of summer, when melting ice brings a variety of migratory visitors. Their movements are limited to a single hole in the ice, therefore putting them at risk of attack from polar bears. 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. The winter in these regions brings temperatures of minus 50☌ and frozen seas that create the biggest challenge. Part four compares oceanic life in the Arctic and Antarctica. A year and a half later the carcass is stripped to the bone. Hagfish, a sleeper shark, and other scavengers arrive to feast on the carcass. Meanwhile another gray whale carcass has sunk to the bottom of the deep sea. Along the coast of California, a migrating gray whale and her calf are targeted by a pod of orcas, who hunt down and kill the calf. In addition, their eggs are nutrition for many, both above and in the sea. Herring initiate the most productive food chain, providing sustenance for humpback whales, and Steller’s and California sea lions. Lunar phases can also have a bearing on events and the mass arrival of Ridley sea turtles on a Costa Rican beach is shown. 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. These feeding grounds have led to the world’s largest albatross breeding colony, on Steeple Jason Island, west of the Falklands. The South Atlantic waters are the roughest, and storms also churn up nutrients to the surface. Off South Africa, a similar situation occurs every June when sardines migrate and are pursued by a caravan of various predators. 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. 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. The first episode looks at how ocean life is regulated around the globe by currents and the varying position of the sun.
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