Electroreception

A   Open your eyes in sea water and it is difficult to see much more than a murky,
bleary green colour. Sounds, too, are garbled and difficult to comprehend. Without
specialised equipment humans would be lost in these deep sea habitats, so how do
fish make it seem so easy? Much of this is due to a biological phenomenon known as
electroreception – the ability to perceive and act upon electrical stimuli as part of the
overall senses. This ability is only found in aquatic or amphibious species because
water is an efficient conductor of electricity.

B   Electroreception comes in two variants. While all animals (including humans)

generate electric signals, because they are emitted by the nervous system, some
animals have the ability – known as passive electroreception – to receive and decode
electric signals generated by other animals in order to sense their location. 

C   Other creatures can go further still, however. Animals with active electroreception
possess bodily organs that generate special electric signals on cue. These can be
used for mating signals and territorial displays as well as locating objects in the
water. Active electroreceptors can differentiate between the various resistances that
their electrical currents encounter. This can help them identify whether another creature
is prey, predator or something that is best left alone. Active electroreception has a
range of about one body length – usually just enough to give its host time to get out of
the way or go in for the kill.

D   One fascinating use of active electroreception – known as the Jamming Avoidance

Response mechanism – has been observed between members of some species known
as the weakly electric fish. When two such electric fish meet in the ocean using the
same frequency, each fish will then shift the frequency of its discharge so that they are
transmitting on different frequencies. Doing so prevents their electroreception
faculties from becoming jammed. Long before citizens’ band radio users first had to
yell “Get off my frequency!” at hapless novices cluttering the air waves, at least one
species had found a way to peacefully and quickly resolve this type of dispute. 

E   Electroreception can also play an important role in animal defences. Rays are one
such example. Young ray embryos develop inside egg cases that are attached to the
sea bed. The embryos keep their tails in constant motion so as to pump water and
allow them to breathe through the egg’s casing. If the embryo’s electroreceptors
detect the presence of a predatory fish in the vicinity, however, the embryo stops
moving (and in so doing ceases transmitting electric currents) until the fish has moved
on. Because marine life of various types is often travelling past, the embryo has evolved
only to react to signals that are characteristic of the respiratory movements of
potential predators such as sharks.
F   Many people fear swimming in the ocean because of sharks. In some respects, this
concern is well grounded – humans are poorly equipped when it comes to
electroreceptive defence mechanisms.  Sharks, meanwhile, hunt with extraordinary
precision. They initially lock onto their prey through a keen sense of smell (two thirds of
a shark’s brain is devoted entirely to its olfactory organs). As the shark reaches
proximity to its prey, it tunes into electric signals that ensure a precise strike on its
target; this sense is so strong that the shark even attacks blind by letting its eyes recede
for protection. 

G   Normally, when humans are attacked it is purely by accident. Since sharks cannot
detect from electroreception whether or not something will satisfy their tastes, they tend
to “try before they buy”, taking one or two bites and then assessing the results (our
sinewy muscle does not compare well with plumper, softer prey such as seals). Repeat
attacks are highly likely once a human is bleeding, however; the force of the electric
field is heightened by salt in the blood which creates the perfect setting for a feeding
frenzy.  In areas where shark attacks on humans are likely to occur, scientists are
exploring ways to create artificial electroreceptors that would disorient the sharks and
repel them from swimming beaches.  

H   There is much that we do not yet know concerning how electroreception functions.
Although researchers have documented how electroreception alters hunting, defence
and communication systems through observation, the exact neurological processes
that encode and decode this information are unclear. Scientists are also exploring the
role electroreception plays in navigation. Some have proposed that salt water and
magnetic fields from the Earth’s core may interact to form electrical currents that sharks
use for migratory purposes. 

Questions 1–6

Reading Passage 1 has eight paragraphs, A–H.

Which paragraph contains the following information?

Write the correct letter, A–H, in boxes 1–6 on your answer sheet.

1. how electroreception can be used to help fish reproduce=c

2. a possible use for electroreception that will benefit humans=G

3. the term for the capacity which enables an animal to pick up but not send out
electrical signals=B
4. why only creatures that live in or near water have electroreceptive abilities+= /H

w wrong ans A
5. how electroreception might help creatures find their way over long distances=B

Wrong ans H
6. a description of how some fish can avoid disrupting each other’s electric
signals=D

Questions 7–9

Label the diagram.

7=

8=

9=

Choose NO MORE THAN TWO WORDS from the passage for each answer.

Write your answers in boxes 7–9 on your answer sheet.

Choose the correct letter, A, B, C or D.

Shark’s 7 ………respiratory movements alert the young ray to its presence

Embryo moves its 8 …………tails……… in order to breathe

Embryo stops sending 9 ……………electric signals…… when predator close by

Wrong electric currents

Questions 10–13

Complete the summary below.

Choose NO MORE THAN THREE words from the passage for each answer.

Write your answers in boxes 10–13 on your answer sheet.

Shark Attack

A shark is a very effective hunter. Firstly, it uses its 10 …… olfactory organs

………….. to smell its target. When the shark gets close, it uses 11 ……electric
signals………….. to guide it toward an accurate attack. Within the final few feet the
shark rolls its eyes back into its head. Humans are not popular food sources for most
sharks due to their 12 …… sinewy muscle …………...  Nevertheless, once a shark has
bitten a human, a repeat attack is highly possible as salt from the blood increases the
intensity of the 13 …… electric field …………... 

Remember, you have 60 minutes to complete the Reading test! You should spend
about 20 minutes on each of the three sections.

You have completed the first section of your Reading test. Now move on to Reading
passage 2.