Life at sea


Every day the sun and moon pull the seas this way and that. Warm currents travel from the tropics towards the poles, while cold water wells up from the ocean depths. Winds create waves on the surface and submarine volcanoes stir up the seabed.

Marine plants and animals come in all shapes, colours, and sizes. Just below the surface live phytoplankton - microscopic plants that absorb carbon dioxide (one of the "greenhouse gases" given off when fuels such as wood, oil, and petrol are burned). Phytoplankton form the basis of a massive food-chain, providing food for zooplankton, minuscule animals which in turn feed the smallest fish and crustaceans. These tiny creatures serve as food for bigger ones, and so the chain builds up.

Most marine life is found near land, where sunlight can penetrate the shallow water down to the seabed. Plants flourish, mammals such as polar bears and sea otters spend much of the time at the water' s edge, and deep sea fish, turtles, and migratory birds visit to breed and feed.

Away from the land, the sea is more mysterious. Birds wheel above. Jellyfish float on the surface, whales come up for air, and clusters of weeds and minute fish and crustaceans gather where different currents meet. Down below, swarms of smaller fish twist and turn to avoid predators. Below them, bigger fish swim and rest. Lower still, temperatures rarely rise much above freezing point and the only light to be seen is the glow (or "chemoluminescence") given off by tiny bacteria. Here lurk weird-looking sharks and octopuses, waiting to catch the dead animals and plants that float down to the ocean floor.


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Copyright 1996, The World Wide Fund for Nature