Cave fish is blind and almost deaf

Two subterranean fish species have two senses

The cave fish Typhlichthys subterraneus is not only blind, it is also deaf for high notes © Niemiller et al.
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In the depths of most caves there is constant darkness, many cave animals have therefore reduced their eyes. So that they still find loot and find their way, their other senses should be even sharper. But now researchers have for the first time discovered a cave fish, which is not only blind, but also almost deaf. This counterproductive adaptation at first glance has a biological meaning: It blocks out the noise that arises in the cave waters due to water turbulence, as the researchers report in the journal "Biology Letters".

"Animals living in perpetual darkness face unique challenges because they still need to find ways to find their food, avoid enemies, and recognize their partners, " said Matthew Niemiller of Yale University, New Haven, and his colleagues, From fish in lakes and rivers, we know that hearing takes on many of these tasks. Because even quiet sounds are transported in the water over long distances. It would therefore be quite obvious if fish living in the permanent dark, had developed a particularly fine ear. Whether this is actually the case and what role these senses play for cave fish has not yet been investigated. Therefore, the researchers have now made up for it.

Hearing test in the aquarium

Niemiller and his colleagues analyzed three closely related species from the family of North American blindfishes (Amblyopsidae) for their study. One of them, Forbesichthys agassizii, lives in aboveground waters, the other two, Typhlichthys subterraneus and Amblyopsis spelaea are cave dwellers. To test the hearing of the three species, the researchers each set each fish individually in a pool and played them through underwater speakers a sequence of short tones in frequencies between 0.1 and 2 kilohertz ago. Each pitch was initially quietly recorded, then in 5 decibel increments louder and louder.

Using electrodes on the head of the fish, they were able to determine whether these sounds were registered by the brain of the fish and thus perceived. In addition, they examined in each fish species how close to their inner ear, the hair cells were. These fine protuberances are moved by the sound and are the main sensors of the hearing.

Dust for high notes

The result: All three fish were the most sensitive in the low frequency range of around 100 hertz, but in the higher altitudes clear differences were manifested. While the above-ground Forbesichthys still quietly, high tones up to 2, 000 Hertz perceived, the two cave types for sounds above 800 hertz were pigeonhole, as the researchers report. In addition, both types of caves had significantly lower hair cell densities than their aboveground relative. "To the best of our knowledge, this is the first time that such a resurgence of the hearing in a cave animal has been proven, " say Niemiller and his colleagues. display

The question arises as to why these cave fish have reduced a meaning that is so valuable in the permanent dark as the Geh r. Obviously, it must be an advantage for them to be unable to hear high notes. To clarify the biological meaning of this adaptation, the researchers next determined the soundscape in the habitat of the two cave fish species. Using microphones, they recorded the sound frequencies in the subterranean tunnels and waterways, and in what volume.

As it turned out, turbulence in the water and the drops falling from the ceiling create a loud background noise - especially in pitches above 800 hertz. Deeper teeth, on the other hand, are swallowed and barely spread. "This raises the exciting possibility that the cave fish have lost their hearing for high frequencies in order to adapt to their particularly loud environment in this area, " the researchers say. How this has happened must now be examined in more detail. (Royal Society: Biology Letters, 2013; doi: 10.1098 / rsbl.2013.0104)

(Biology Letters, 27.03.2013 - NPO)