Blind Cave-dwelling Fish Are Also Mostly Deaf

A recent report from the publication, Biology Letters has concluded that blind cave fish who spend their lives in the dark have lost much of their ability to hear. Out of the species studied, it was found that two species could not hear high-pitched sounds at all. Study co-author Daphne Soares of the University of Maryland College Park was surprised. She expected them to hear far better than fish that dwell on the surface.

Over many generations, cave-dwelling fish can lose their vision, even their eyes. Without light, eyesight loses its importance in terms of the fishes’ survival. Before this study was released, only two had been conducted to find out what happens to hearing after fish lose their vision, but no difference in hearing was determined between cave fish and those who live in daylight.

For the study, Soares and her colleagues collected two species of blind cave-dwelling fish from lakes in Kentucky: Typhlichthys subterraneus and Amblyopsis spelaea. They also collected specimens of a surface-dwelling species closely related to the cave-dwelling fish called Forbesichthys agassizii from a lake in south-central Tennessee.

The researchers brought their specimens back to the lab and tested their hearing in two ways. First, a test to determine whether sounds across a range of pitches could stimulate nerve activity in the brains of the fish was conducted. Then, a test that measured the density of sound-detecting hair cells in the fishes’ ears.

It was discovered that the two cave-dwelling species could hear just as well as their surface counterparts, up to 800 hertz (which is almost the highest pitch of a trumpet). For pitches higher than 800 hertz, the findings were drastically different. The surface fish could hear frequencies up to 2,000 hertz, which is like the highest pitch of a flute. However, the cave-dwellers were practically deaf to that pitch, as it stimulated little to no nerve activity. This was odd, as cave-dwelling fish also had two-thirds as many hair cells as the surface-dwelling fish.

There were still many factors to rule out as to why cave-dwelling fish were so hard of hearing, so the researchers went back to the caves to find out if background noise may have had a negative impact. Remarkably, the noise, at high-frequency ranges, was far louder. This was the same level of frequency at which the cave-dwelling fish had difficulty hearing. High pitched sounds were suspected to originate from ripples or dripping water from the caves’ ceilings, according to study author Soares.

Over time, cave fish have evolved other improved senses which compensate for their blindness and hearing loss. Most fish have a sixth sense. It’s called the “lateral line” and it allows them to detect water flow. Fish can sense vibrations in water using tiny organs known as neuromats, which are groups of sensitive hair cells on and close under the fishes’ skin. Cave-dwelling fish are known to have far more of these hair cells than their surface-swimming counterparts.

This was a ground-breaking study, as scientists observed a creature that has lost hearing from being in a cave for the very first time. However, the process that causes this hearing loss is unclear. Over time, the fish may have adapted to the noisy cave environment, or maybe they just possess highly flexible hearing systems to begin with.More creatures will be studied by Soares. She hopes to determine whether other creatures that live in the darkness, like cave salamanders, have also become deaf.

It’s interesting to think about the effect that cumulative exposure has on human beings, as well as animals. People often realize they have developed hearing loss when they discover they cannot hear at high or low frequencies. If you suspect that you may be experiencing this type of hearing loss, don’t hesitate to schedule a consultation with our licensed, experienced Audiologists at Clifton Springs Hearing Center.