鯊魚聽得到將近 250 英尺外的聲音
Sharks are famous for their sense of smell, but new research shows that their hearing is impressive too.
Scientists from Florida Atlantic University studied wild blacktip sharks in the clear, shallow waters off south-east Florida. They lowered an underwater speaker into the sea and played a series of low-pitched sounds. A drone hovering high above filmed how the sharks reacted, while special microphones measured exactly how loud the sounds were.
The sharks responded to sounds coming from as far as 74 metres away – around 243 feet. That is a greater distance than had ever been recorded for free-swimming sharks. Often, they made a sharp turn away from the speaker, which suggests they could tell which direction the noise was coming from. They were most sensitive to the lowest sounds. When the researchers played a high-pitched sound that sharks are not thought to hear, the animals ignored it.
Unlike most fish, sharks do not have a swim bladder, so the scientists believe they rely on their inner ears to pick up the movement of water caused by sound.
The team also argues that testing sharks in tanks can give misleading results, because sound bounces off the walls. Studying them in the open ocean, they say, gives a much more natural picture.
鯊魚以嗅覺靈敏聞名，但新研究發現，牠們的聽力也很厲害。
佛羅里達大西洋大學的科學家在佛州東南部清澈的淺海研究野生的黑邊鰭真鯊。他們把水下喇叭放進海裡，播放一連串低沉的聲音。高空的無人機拍下鯊魚的反應，特殊的水下麥克風則精準測量聲音有多大。
結果發現，鯊魚能對遠在 74 公尺（約 243 英尺）外的聲音做出反應，這是目前在自由游動的鯊魚身上記錄到最遠的距離。牠們常常猛然轉身，往遠離喇叭的方向游開，表示牠們分辨得出聲音從哪裡來。鯊魚對最低沉的聲音最敏感；研究人員播放一種一般認為鯊魚聽不到的高音時，牠們完全不理會。
鯊魚和大多數魚類不同，沒有魚鰾。科學家認為，牠們是靠內耳來感受聲音造成的水流動。
研究團隊也指出，在水族箱裡研究鯊魚可能得到誤導的結果，因為聲音會在箱壁之間反彈。他們認為，到開闊的海域研究，才能看到鯊魚比較自然的反應。