D12 Body Electric Field Measurement

Topic

This technique measures the fish's body electric field for three distinct purposes: characterizing the electric organ discharge of electrogenic fish—whether weakly or strongly electric; mapping the spatial distribution of that electric field around the fish's body; and detecting disturbances caused by objects in the environment—the same phenomenon of electrolocation described in an earlier model in this series regarding the disruption of electroreception. This same technique also allows for the study of passive bio-electric fields generated by any fish—even non-electrogenic ones—resulting from ionic currents flowing across their skin and gills.

This body electric field is measured using arrays of reference electrodes made of silver and silver chloride, submerged in the water at known distances from the fish's body. The electric potential difference between a pair of these electrodes yields the electric field component along the axis connecting them; this field component equals the potential difference between the two electrodes divided by the distance separating them.

To map the complete body electric field in two or three dimensions, researchers either use an array containing many electrodes simultaneously or systematically move a single pair of reference electrodes around the fish's entire body. In either case, the amplifiers recording the potential difference must have a very high input impedance—exceeding one billion ohms—because otherwise, the measuring instrument itself would disturb the electric field being measured.

The electric organ discharge of weakly electric fish—one of the body electric fields characterized by this technique—features frequencies ranging from 50 to 2,000 hertz, depending on the species, and voltages between 1 and 100 millivolts when measured near the fish's body. Applying the same spectral analysis described in the previous model of this series to this discharge reveals its fundamental frequency and harmonics, which are characteristic of both the species and the individual producing them.

When an external object enters the fish's body electric field, the disturbance that object introduces into the electric organ discharge is measured by comparing the field with the object present against the field without it: conductive objects concentrate the field lines, generating a positive disturbance, whereas resistive objects disperse them, generating a negative disturbance; this allows the fish to distinguish the electrical properties of the objects it detects in its surroundings.

This disturbance decays with the cube of the distance to the object—as is characteristic of a field with a dipolar structure—and this steep drop-off limits the range of the fish's electrolocation to just one or two times its own body length. Thus, the very body electric field described at the beginning of this model as the basis for electrolocation ultimately also limits its range to the fish's immediate vicinity.

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