Electric fish - a biological application of physics

Kjell Prytz, Göran Nordström

University of Gavle, Sweden, Girep 2006, Amsterdam

Electrolocation Bat - Echo Location Dolphin – Echo Location

With biological applications we may teach basic physics

Weak electric fishes• In muddy/dark waters

vision is of limited importance

• Certain fishes have therefore not evolved the vision sense but rely instead on electrolocation

• The fish uses modified muscle cells to make itself an approximate electric dipole

• Through electric interactions it registers its environment

A weakly electric black ghost knifefish (Apteronotus albifrons),

length approximately 1 dm.

Strong and weak electric fishes around the world

The Elephant Nose Fish - A sensitive aquarium fish

• Big brain in relation to its body

• Generates pulses, 20-30 pulses/second

• Electric generator organs in the rear (the white lines)

• Electro receptors concentrated around the head

Length around 10-20 cm

Investigating potential from fish

10mV and 1 ms per division

Some tasks• How is the potential on the skin of the fish affected

by foreign objects

• with different electric properties?

• with different shapes?

• at different distances?

• How sensitive are the electro receptors?

• Why has no such sense evolved for land living animals?

Simulation - Problem Formulation

• Approximately electrostatic – use Gauss law for the electric field

• Construct a dipole in water (appr. equivalent to series capacitor)

• Give charge density of dipoles• Place objects of different kind in its

neighbourhood• Apply dielectric constant of the different

media

Simulation with FEMLAB

Potential (surface plot) and electric field lines, object excluded

The potential along a line on the skin at the head

Simulation with FEMLAB

Potential (surface plot) and electric field lines, object included

The potential along a line on the skin at the head

Evaluation from simulation

• We observe ’distorsion’ of the dipole field close to the foreign object as well as the expected continuity conditions at the borders of different media

• The effect of the foreign object is to alter the potential at the head by a few V

• Since sensitivity is about 1 V we conclude that the object can be sensed

Application areas

• Research and education in biophysics, physics and biology

• Suitable for both high school and university level

• Pedagogical attempt to make physics alive which might stimulate interest.

• Aspects of gender problem in physics