Fish that use electric fields to sense their environments dim their signals to save energy during the day when they are resting.
Sternopygus macrurus, a South American river fish, is a natural practitioner of energy efficiency. It can reshape the charged-molecule channels in its electricity-producing cells to tone down its electrical signature within a matter of minutes.
This is a really expensive signal to produce. The fish is using up a lot of its energy budget,” said neurobiologist Michael Markham at the University of Texas at Austin, lead author of a paper in PLoS Biology on the fish. “These animals are saving energy by reducing the strength of the signal when they are not active.”
Thousands of fish and other oceanic creatures use electrical fields to help them perceive their environments. The most famous is the electric eel, which a colleague of Markham’s termed “a frog with a cattle prod attached,” but most animals use the electrical signals in more subtle ways.
The fish’s standard electrical signal runs at 100 hertz; if you turn the electrical signal into sound, it sounds like a hum high whine.. In laboratory experiments, the fish can detect tiny bugs half a centimeter wide and easily navigate obstacles by detecting the changes the objects cause in the electrical field.
Other fish put out different types of electrical fields, some of which vary a lot more. Markham’s team chose S. macrurus specifically because its discharge is fairly regular.
All fish generate electricity with a specialized type of cell called an electrocyte. These cells can generate current by manipulating the amount of charged sodium and potassium ions that they allow to flow into and out of themselves. An electrical current propagates on the membrane of the cell as a result.
Thousands of cells combine to generate the 5 millivolts per centimeter electrical field the fish uses. By using fewer sodium channels, the signal gets dimmed and energy is conserved.
“The wave form of the electric signal changes and at the level of the individual cell, it is changing its discharge,” Markham said. “This is the first time in a vertebrate animal that you can show such a clear connection between an animal’s behavior and the changes at the molecular level.”
For Markham, the system is interesting because the ways cells reshape their membranes — scientists call the process ion channel trafficking — are very similar to the ones that our hearts and nervous systems use.
The same molecular machinery that drives our nervous system, muscles and heart has evolved into an organ just to produce electricity, he said. The specialized organ, then, acts as a kind of biological laboratory for evolutionary experiments with electricity production.
“If there is a slight mutation in the ion channels in your heart, that’s very likely to be a fatal mutation,” Markham said. “The electric organ from an evolutionary standpoint is a much more forgiving place for experimentation.”
In the future, they hope to use their research on ion channels to better understand the kinds of electrical malfunctions that cause disorders like epilepsy.
“There’s a kind of gee whiz interest factor to working with these fish, but obviously, we’re pursuing a bigger agenda,” Markham said.
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