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Read the passage carefully. Then choose the correct answer.
 
Baby sharks are born with all the sensors they will ever need to defend themselves and hunt down food but developing embryos still stuck in their egg cases are vulnerable to predators. A new study finds that these baby sharks can detect a potential predator and play dead to avoid being eaten.

Every living thing gives off a weak electrical field. Sharks can sense this with a series of pores - on their heads and around their eyes, and some species rely on this electro-sensory ability to find food buried in the seafloor.

Two previous studies on the spotted catshark and the clearnose skate, a relative of sharks - found similar freezing behaviour in their young. But new research by shark biologist and doctoral student, Ryan Kempster at the University of Western Australia has given scientists a more thorough understanding of this behaviour.

It all started because Kempster wanted to build a better shark repellent. Since he needed to know how sharks respond to electrical fields, Kempster decided to use embryos. "It's very hard to test this in the field because you need to get repeated responses," he said. "And you can't always get the same shark to cooperate multiple times. But we could use embryos because they're contained within an egg case." So, Kempster got his hands on eleven brown-banded bamboo shark embryos and tested their reactions to the simulated weak electrical field of a predator.

In a study published in the journal PLoS One, Kempster and his colleagues report that all of the embryonic bamboo sharks, once they reached later stages of development, reacted to the electrical field by ceasing gill movements (essentially, holding their breath), curling their tails around their bodies, and freezing.

A bamboo shark embryo normally beats its tail to move fresh seawater in and out of its egg case. But that generates odor cues and small water currents that can give away its position. The beating of its gills as it breathes also generates an electrical field that predators can use to find it. "So it cloaks itself," said neuroecologist JosephSisneros, at the University of Washington in Seattle, who was not involved in the study. "The embryo shuts down any odor cues, water movement, and its own electrical signal." Sisneros, who conducted the previous clearnose skate work, is delighted to see that this shark species also reacts to external electrical fields and said it would be great to see whether this is something all sharks, skates, and ray embryos do.

In addition to the freezing behaviour he recorded in the bamboo shark embryos, Kempster found that the baby shark remembered the electrical field signal when it was presented again within 40 minutes and that they would not respond as strongly to subsequent exposures as they did initially. This is important for developing shark repellents, he said, since some of them use electrical fields to ward off the animals. "So, if you were using a shark repellent, you would need to change the current over a 20 to 30-minute period so the shark doesn't get used to that field."

Kempster envisions using electrical fields not only to keep humans safe but to protect sharks as well. Shark populations have been on the decline for decades, due partly to ending up as by-catch, or accidental catches, in the nets and on the longlines of fishers targeting other animals.

A 2006 study estimated that as much as 70 per cent of landings, by weight, in the Spanish surface longline fleet were sharks, while a 2007 report found that eight million sharks are hooked each year off the coast of southern Africa.

"If we can produce something effective, it could be used in the fishing industry to reduce shark by-catch," Kempster said. "In America at the moment, they're doing quite a lot of work trying to produce electromagnetic fish hooks." The eventual hope is that if these hooks repel the sharks, they will not accidentally end up on longlines.

     
  1. What is the strategy used by baby sharks to protect themselves from predators ?
       
    (A) They freeze.
    (B) They fight back.
    (C) They give off electrical field.
       
  2. The original objective of Kempster's study was to
       
    (A) improve the electro-sensory ability of sharks
    (B) develop effective methods to drive away sharks
    (C) determine the sensitivity of sharks to electrical fields
       
  3. Kempster chose to experiment on embryos because
       
    (A) they can be controlled
    (B) they are easily available
    (C) they have stronger electro-sensors
       
  4. But that generates odor cues ... That refers to
       
    (A) beating of the tail
    (B) generating odor cues
    (C) moving fresh seawater
       
  5. Why is Sisneros pleased with Kempster's findings ?
       
    (A) They are similar to his findings.
    (B) They prove that all sharks behave the same way.
    (C) They confirm that all sharks give off electrical signals.
       
  6. In order for the shark repellent to work effectively, the electrical field signal
       
    (A) must be strong
    (B) must be frequent
    (C) must be changed regularly
       
  7. The results of Kempster's study have application in
       
    (A) controlling shark population
    (B) reducing accidental killing of sharks
    (C) producing electromagnetic fish hooks
       
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