Shielded cells help fish ignore noise

Fish don’t have to filter the chatter of cocktail parties to hear the latest about Al vs. W, but they do have to distinguish important ripples in water from noise. Now, researchers may have figured out how fish do it.

Fish pick up water movements—like the kicks of an insect just right for lunch or the whoosh of an incoming predator—through sensory cells called neuromasts.

In rushing water, the neuromasts on a fish’s skin get uselessly excited, say Horst Bleckmann and his colleagues at Rheinische Friedrich-Wilhelms-Universität Bonn in Germany. However, a second type of neuromast, protected in canals below the skin surface, can ignore the chatter of rushing water and pick up more relevant vibrations, the researchers report in the Nov. 2 Nature.

“We are the first to study this in flowing water,” Bleckmann says. Previous experiments exposed fish to a vibrating object in still water, he explains. Those studies, he adds, didn’t demonstrate the different real-world functions of the two kinds of neuromasts.

Neuromasts dot a fish’s skin surface as well as a subsurface canal, called a lateral line, that runs along each fish’s flank and branches across its head. The receptors pick up their information via tufts of hair covered by a gel-filled cap. Water sluicing by a fish presses against the cap and bends the hairs, triggering impulses in a nerve below.

Using goldfish, the researchers eavesdropped on the nerves connected to both canal and skin-surface neuromasts. When the researchers jiggled a tiny sphere near the fish in otherwise quiet water, skin-surface neuromasts picked up vibrations.

When the researchers added a current of 10 centimeters per second in the fish’s tank, however, these receptors got so stimulated they didn’t sort out the sphere’s vibrations. The canal neuromasts, on the other hand, didn’t respond to the current but picked up the sphere.

“They’re exquisitely sensitive,” notes John C. Montgomery of the University of Auckland in New Zealand. He led research that in 1997 found that skin-surface neuromasts help fish figure out how to orient themselves in currents.

Fish species that live in relatively tranquil water have neuromast-rich skin surfaces, Montgomery notes. In contrast, dashing swimmers in rushing water carry most of their neuromasts in their lateral-line systems.

“The environmental correlation has been known for years,” he says. “What [Bleckmann’s group] has done is add the physiology behind it.”