It's a classic mystery of the deep. Why does the hammerhead shark （双髻鲨)have the bizarre

It's a classic mystery of the deep. Why does the hammerhead shark (双髻鲨)have the bizarrely shaped head from which it gets its name？

There have been a variety of suggested explanations. Some simply say that the sharks use their heads to "hammer" and pin down their favourite food. More plausibly, others have speculated that the wide lobes(圆形突出部分)of the hammerhead allow it to have longer electrorecep-tots, the organs that all sharks use to detect the electric fields produced by nearby prey. This might allow hammerheads to sense subtler electric fields from more distant prey than their narrow-headed cousins.

Now it turns out that the shark's head does indeed help it find and capture prey, but not in the way that zoologists expected. Stephen Kajlura and Kim Holland of the University of Hawaii at Manoa set out to test the conventional theory by tricking young sharks into chasing phantom (虚构的)prey. Using a system of wires on the bottom of a shallow pool, they set up electric fields that mimicked those created by the bottom-dwelling shrimp and fish that form. the sharks' usual diet.

Sure enough, hungry sharks abruptly turned towards an electric field when they detected it. But when the researchers measured the distance at which this happened they found it was the same for 13 young hammerheads as it was for 12 young sandbar sharks(沙堤鲨),which have normal-shaped heads.

The two types of sharks proved equally adept at sensing the electric fields: each was able to detect the source from up to 30 centimetres away. That ruled out any improved sensitivity from the wider head. However, the hammerheads enjoy another more prosaic(平淡无奇的)advantage: their wider heads let them sweep more than twice as wide a swathe of the seafloor as they swim, which must boost their chance of encountering food.

The researchers also found that hammerheads could turn more sharply when they detected the phantom prey. "They're a much more bendy shark , "says Kajlura, who is now at the University of California at Irvine. In part, that's because they have more slender bodies than the sandbar sharks. However, Kajiura has other unpublished data that suggests that the hammerheads' broad heads can act as fins to improve manoeuvrability(机动性).

So far, the researchers have only experimented with young sharks, so adult hammerheads may gain some other advantage from their head shape.

The passage is mainly about ______.

A．the different species of sharks

B．the differences between hammerhead and sandbar sharks

C．how hammerhead sharks detect their prey

D．special functions of the hammerhead sharks