Fish that generate electric fields to navigate, fight and attract mates are equipped with a dimmer switch of sorts that can turn down their signals to save energy, a new study finds.

Electric fish, such as some sharks and eels, emit weak electrical signals from a battery-like organ in their tails. The fish studied, called Sternopygus macrurus, are active at night and must avoid predators, such as catfish, that can sense their electric field.

Generating such impulses can be energetically costly.

Now, researchers have located a dimmer switch in the membranes of cells called electrocytes within this electric organ. The switch takes the form of sodium channels that the fish can insert and remove from the electrocyte membranes. More sodium channels mean a stronger electric impulse.

The fish can turn their electric bursts up or down at a moment's notice. That's likely because a reservoir of sodium channels is stored in the electric cells. When serotonin is released in the fish brain, it initiates the release of adrenocorticotropic hormone from the pituitary gland. This triggers the mechanism that puts more sodium channels in the membrane.

"This is happening within a matter of two to three minutes," said study researcher Michael Markham of the University of Texas at Austin. "The machinery is there to make this dramatic remodeling of the cell, and it does so within minutes from the time that some sort of stimulus is introduced in the environment."

When the fish are inactive, they remove the sodium channels from the cell membranes to reduce the intensity of the electric impulse.

"By adding new ion channels to the electrocyte membrane only during periods of activity or social encounters and removing these channels during inactive periods, these animals can save energy and reduce predation risks associated with communication," the researchers write in the Sept. 29 issue of the journal PLoS Biology.