大象传媒

A hammerhead shark; a predator in search of prey. In these Hawaiian waters it's not easy. Visibility is poor and prey are clever. Their lives depend on it. But something in this shark's head is smarter. Out of sight is not enough. Even hidden in its burrow the goby gives off tell-tale clues and there is nothing it can do about it: it blows its cover just by breathing. As water passes over its gills, oxygen is extracted and absorbed. Every breath produces a minute electrical field, which pulses around the fish's head. This bubble of electricity reaches 20cm upwards - an invisible sign-post marking buried treasure. Of course, not everyone can read the sign. It needs a special kind of extra-sensory perception - one that's found inside this unique head. The underside of the head is lined with thousands of electrical detectors. These are connected to long ducts that allow the brain to register even minute electrical fields. It can pick up on the goby's tiny charges from a distance of 20 centimetres - the equivalent of finding a household battery from half a mile away. It scans the ocean bed using its head like a metal detector.. waiting for something to spark it off. Because it can't actually see the electric sign, the shark needs to pass directly over the generator. As it systematically works every sandy area, the head doubles as a hydrofoil, keeping the detectors inside the critical 20 centimetres. The hammerhead is the only shark with a flexible neck - it can turn on a sixpence. Now it's only seconds from the jackpot. The sensors fire and the hammerhead turns. To get a better fix on its target, it swims in a figure of eight through the bubble of charge. It's now directly above. The goby's stone fortress will buy a few seconds but it's not enough.