Читать книгу Extreme Nature - Mark Carwardine - Страница 31

NAME	sawfish Pristis species
LOCATION	shallow, warm coastal waters
ABILITY	using its saw for slashing and sifting

© Marty Snyderman/imagequest3d.com

A sawfish has external teeth, set around a sensitive, flat snout – the saw, or rostrum (here shown from the underside). Swung from side to side, the saw can be used as a powerful weapon to slash shoaling fish such as mullet and herring, which it then eats off the sea-bottom. Generally speaking, though, the sawfish is a slow and peaceable animal, spending its time in shallow, muddy water, raking the mud with its saw for crustaceans and other prey. The saw-teeth get worn by all this grubbing, but they grow continuously from their bases and so don’t wear out.

Like its close relatives, the rays, it’s perfectly camouflaged against the bottom of the sea, and like its more distant relatives, the sharks, it swims in an undulating way. And like both groups, its hard bits are cartilage, not bone, and its teeth are adapted scales. It has another similarity. Using special cells, the ‘ampullae of Lorenzini’, on its saw and head, it can detect electrical fields generated by prey.

One problem for females is that they give birth to live saw-babies. But a youngster’s saw is covered with a sheath to make birth relatively painless. A much greater problem for all sawfish (possibly seven species) is the fact that their coastal waters are being polluted and developed and that they have been overfished to the point where all are endangered, some critically. A sawfish’s saw is also its downfall. Not only has it been sought after as a trophy, but it also fatally entangles the fish in nets.