The Shark S Sense of Electroreception
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Student’s Guide
The Shark’s Sense of Electroreception Electric Field of a Trout
Olivier Tardif-Paradis Mathieu Riopel
Cégep Garneau Centre de démonstration en sciences physiques (support for the development of demonstrations)
Great white shark near a school of fish – Credit:Terry Gross
APP/Student Guide 1 Electroreception: An Evolutionary Advantage
Background
Stefano Lorenzini, born in Florence in 1652, was a physician and ichthyologist. In 1678, he described the presence of pores leading to transparent tubes located around the mouths of sharks but for which he was unable to determine the purpose. This Italian scientist would never have imagined that these tubes, now called the ampullae of Lorenzini, allow sharks to perceive a world of electrical images that are imperceptible to human senses.
It was not until three hundred years later, in 1960, that this organ was finally identified as a specialized sense that perceives Figure 1: Pores on the snout of a shark. These pores lead to transparent tubes called the ampullae of Lorenzini. Credit: Alber Kok electric fields. It was then understood that these tubes offer a clear evolutionary advantage for the sea predators that have them. All living creatures produce an electric field when they contract their muscles. A shark can detect the weak electric stimuli from a prey and deduce its presence. How does this sixth sense function? What exactly is it capable of detecting?
The golden trout is a potential prey for sharks in the Pacific Ocean around the coasts of California in the USA. Although most trout live only in fresh water, some species, such as the golden trout (subspecies of rainbow trout) spend their adult life in the ocean and swim back up certain rivers in California to spawn, like salmon do. The golden trout can reach up to 71 cm long and weigh 5 kg in adulthood, so they make a very interesting meal for many sharks.
In this problem, you will have to describe the electric field produced by the beating of a trout’s heart and understand its role in relation to the shark’s electroreception sense. To help you understand the phenomenon, a trout, an aquarium, an electrocardiograph and lots of questions will be presented.
Figure 2: Adult golden trout Credit: Citron / CC-BY-SA-3.0
PBL/Student’s Guide 2 Three-step Cycle
List all the relevant information you gathered when you read the problem. Based on this information, state what you need to know to solve the problem. As you discover new information, you should summarize and update the relevant information you have gathered and ask new questions.
List the Following:
What We Know To Determine Summary
PBL/Student’s Guide 3 Preparing for the Experiment
1) The vectors in the figure below represent the electric field of a point charge. Between which of the points represented here by the letters A, B, C, D, E and F is the modulus of the potential difference DV the highest? The lowest?
E
D
B A Figure 3: Electric field of a point charge.
F C
(highest): Between points ______and ______. DVmax
DVmin (lowest): Between points ______and ______.
PBL/Student’s Guide 4 2) Figure 4 shows the field lines representing the electric field of a dipole made up of point charges. Using these field lines, draw the electric equipotential lines.
Figure 4: Electric field lines of an electric dipole
X
3) In Figure 4, at point X
a) Show a vector that corresponds to the electric field produced by the positive charge
b) Show a vector that corresponds to the electric field produced by the negative charge
c) Show a vector that corresponds to the sum of these two electric fields (resultant electric field).
Note: The charges do not produce fields of the same size at point X.
PBL/Student’s Guide 5 The Trout and the Shark
Some sharks are more sensitive to electric fields than any other animal, because they have a sensitivity threshold of up to 0.5 μV/m.
Using an electrocardiograph, it is possible to measure the size of the electric field generated by a trout. For this experiment, a trout is placed in an aquarium. Then two sensors connected to an electrocardiograph are submerged in the aquarium near the trout as shown in Figure 5. The electrocardiograph records the signal in millivolts (mV) and it measures the electrical potential difference between the two sensors, as shown in Figure 6.
To simplify the model, we use an electric dipole to represent the muscular contraction of the trout’s heart. When the potential difference generated by the trout is the highest, the distance between the point charges of the dipole representing the system is 1 cm, the size of the trout’s heart, as shown in Figure 7 Figure 5: Assembly to measure the beating of a 16-cm brook trout’s heart ) V m (
e c n e r e f f i d
l a i t n e t o p
l a c i r t c e l E
Time (s) Figure 6: Electrical potential difference measured at Points A and B by the sensors shown in Figure 7. This is the beating of a brook trout’s heart recorded when the trout is in the position shown in Figure 7.
PBL/Student’s Guide 6 View from above the fish in the aquarium with sensors A and B at the moment when the electric potential difference is measured by the electrocardiograph.
y
x
Figure 7: Scale drawing of trout seen from above in the aquarium when the measurement is taken by the electrocardiograph. The positions of sensors A and B and the electric dipole representing the heart are all shown.
PBL/Student’s Guide 7 Did You Know?
An electric field does not behave exactly the same way in vacuum as in water. The dielectric permittivity of a substance, , is the factor by which the electric field is decreased or increased relative to vacuum. At low frequencies, at room temperature and at a certain level of salinity, the dielectric permittivity of water has a value of 80.
Electric field of a dielectric:
E E = 0 D k
where is the modulus of the electric field in the dielectric and is the modulus of the electric field ED E0 in vacuum.
Therefore we can assume that Coulomb’s constant in vacuum, , will differ based on the value of the dielectric permittivity.
k k = 0 D k
PBL/Student’s Guide 8 Questions
1) What is the value of the maximum electric charge accumulated in the dipole representing the trout’s heart?
2) What is the maximum distance at which a shark can detect the electric signal emitted by the trout’s heart if the predator is at a point on an axis passing through the centre of the heart? Assume the axis is parallel to the x-axis in Figure 7.
PBL/Student’s Guide 9 3) During the experiment to measure the beating of the trout’s heart, would it be useful to put the aquarium in a wire mesh cage? Explain your answer.
PBL/Student’s Guide 10