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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. |
