Читать книгу Extreme Nature - Mark Carwardine - Страница 13

NAME	great hammerhead Sphyrna mokarran
LOCATION	tropical and warm temperate seas
SKILL	hunting with electricity

© James D Watt/imagequest3d.com

All sharks can, to some extent, detect other creatures by sensing the minuscule amount of electricity they create simply by virtue of being alive. In most sharks, this sense is principally an adjunct to more dominant senses (usually hearing, smell and vision) and is particularly important in the final split-second of an attack. But for hammerheads, it’s the main thing, and it could be one reason why their heads are shaped in the curious way they are.

Sharks have special electrical receptors – hundreds of tiny, dark pores called ‘ampullae of Lorenzini’ – which are filled with a conductive gel that transfers electrical impulses to a nerve end in each pore. Ordinary sharks have these all over the snout and lower jaw, forming a curious pattern of dark holes resembling a sparse five-o’clock shadow.

But hammerheads also have a mass of them across the underside of their oblong heads, which scan across the sandy sea-bottom like metal detectors, searching for prey animals that can’t be found in any other way – creatures such as stingrays and flatfish that bury themselves, lie still and usually have no appreciable scent.

The hammerheads are able to detect the slight direct currents caused by interaction between the bodies of their prey and the seawater and the even slighter alternating currents caused by muscle contractions around an animal’s heart. The eight species of hammerhead can sense it better than most other sharks, and the biggest of these, the great hammerhead, which measures up to 6m (20ft) long, may be able to sense it best of all.