Blind fish that spend their lives in dark, underwater caves have lost a huge chunk of their ability to hear, scientists report in the March 27 Biology Letters. Two of the fish species studied could not hear high-pitched sounds.
“I was really surprised,” says study coauthor Daphne Soares of the University of Maryland, College Park. “I expected them to hear much better than the surface fishes.”
Cave-dwelling fish can lose their vision and even their eyes over many generations. And without light, eyesight can lose its importance in fish survival. Only two previous studies have explored what happens to hearing after fish lose their vision; both found no differences in hearing between cave fish and those that experience daylight.
Soares and her colleagues collected fish of two blind cave-dwelling species, Typhlichthys subterraneus and Amblyopsis spelaea, from lakes in Kentucky. Specimens of a surface-dwelling species, Forbesichthys agassizii, which is closely related to the cave dwellers, came from a lake in south-central Tennessee.
Back in the lab, the researchers tested fish hearing by seeing whether sounds across a range of pitches could stimulate nerve activity in the fishes’ brains. The researchers also measured the density of sound-detecting hair cells in the creatures’ ears.
At frequencies up to 800 hertz, almost the highest pitch of a trumpet, the two cave-dwelling species could hear just as well as their surface counterparts, the researchers found. For higher pitches, it was a different story: The surface fish could hear frequencies as high as 2,000 hertz, roughly the highest pitch of a flute. But the cave dwellers were virtually deaf to those sounds. Those high pitches stimulated little or no nerve activity. The cave dwellers also had two-thirds as many hair cells as the surface fish.
The researchers returned to the caves to find out whether background noise there may have hurt the animals’ ability to hear. The noise was generally far louder at higher ranges — the same frequencies that the cave fish couldn’t hear, the researchers found. The high-pitched sounds may come from ripples or dripping water from the caves’ ceilings, Soares says.
The cave fish may have evolved other improved senses to compensate for their lost sight and hearing, Soares says. Most fish have a sixth sense called the lateral line, which allows them to detect water flow. The lateral line senses vibrations in water using tiny organs known as neuromasts, groups of sensitive hair cells on and just below the creatures’ skin. Cave fish, Soares says, have far more of these neuromasts than their surface-dwelling relatives.
The study represents the first time scientists have observed a creature that has lost hearing from being in a cave, the researchers say. But the process by which cave fish lost their hearing is not yet clear, says Martina Bradic of New York University. The fish could have adapted over time to the noisy environment, or their hearing systems could be highly flexible within a single lifetime.
Soares next hopes to study whether other creatures living without light, such as cave salamanders, have also become partially deaf.
