Looks at the evolution of fish. They have developed a multitude of shapes, sizes and methods of propulsion and navigation. The sea squirt, the lancelet and the lamprey are given as examples of the earliest, simplest types. Then, about 400 million years ago, the first back-boned fish appeared. The Kimberley Ranges of Western Australia are, in fact, the remnants of a coral reef and the ancient seabed. There, Attenborough discovers fossils of the earliest fish to have developed jaws. These evolved into two shapes of creature with cartilaginous skeletons: wide ones (like rays and skates) and long ones (like sharks). However, it is the fully boned species that were most successful, and spread from the oceans to rivers and lakes. To adapt to these environments, they had by now acquired gills for breathing, a lateral line to detect movement and a swim bladder to aid buoyancy. Coral reefs contain the greatest variety of species, many of which are conspicuously coloured to ward off predators or attract mates. Their habitat, with its many hiding places within easy reach, allows them to remain so visible. However, the open ocean offers no such refuge, so there is safety in numbers — both hunters and hunted swim in shoals and have streamlined bodies for pursuit or escape. Most species that live below the thermocline, in the freezing depths of the ocean, have never been filmed, and these are largely represented by still photographs.
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