Scientists have discovered one of the ways cavefish have adapted to subterranean living is by using sound differently to their surface fish relatives. Gabrielle Ahern reports.

25,000 years ago, the ancestors of Astyanax mexicanus were swept away from their river habitat into the caves of North-Eastern Mexico by flood waters. Trapped in the caves and isolated from the surface, the cavefish gradually lost their eyes and pigment as a consequence, but kept their hearing, a finding reported by Arthur Popper in the 1970’s.

Astyanax mexicanus is the only vertebrate where extant cave-dwelling and surface-dwelling morphotypes still exist within the same species” says Dr Sylvie Rétaux, a neuroscientist from the Paris-Saclay Institute of Neuroscience, France, who is fascinated by sonic cavefish. 

Retaux led the research study “Evolution of acoustic communication in blind cavefish” published by Nature Communications to determine whether this loss of sight was compensated by other senses.

“We wanted to test whether acoustic communication would exist and would have evolved with their adaptation to darkness” said Retaux. 

A cavefish recorded in the Pachon cave, located in the North of the Sierra de El Abra, North-East Mexico. Photo: Sylvie Rétaux.

The research team ventured to six of the thirty caves that host Astyanax mexicanus troglomorphic cavefish populations in San Luis Potosi and Tamaulipas, North-Eastern Mexico (Molino, Pachón, Los Sabinos, Tinaja, Chica, and Subterráneo); and a well in the village of Praxedis Guerrero, inhabited by the surface morphs. 

Reaching the cavefish proved to be no ordinary task, and an adventure in the making. The team traversed through mud, contended with the dark, and wore masks to protect against histoplasmosis, a disease of the lungs caused by fungus, all while carrying heavy backpacks full of electronic equipment. The researchers also had to contend with abseiling down a sheer 68-metre vertical rock face to reach the Molino study site. But once inside the caves, the acoustic environment was very calm says Retaux.

“It is a kind of dream for bio-acousticians. Almost no sonic pollution down there.”

The researchers working at pool number 1 in the Los Sabinos cave, located in the middle of the Sierra de El Abra. Photo: Jean-Louis Lacaille-Muzquiz

Both Astyanax morphs are known to produce the same repertoire of 6 sounds: clicks, serial clicks, sharp clicks, clocs, serial clocs and rumblings; and the anatomical structures used to make these noises are conserved between the morphs. “They scrape their teeth, use a series of small bones (the Weber apparatus), drum on the swim bladder or they can produce hydrodynamic waves with their fins” Retaux says.

The team discovered a sound made by wild cavefish when compared to surface morphs represented a different behaviour.

The sharp click by surface morphs is visually triggered in the presence of rivals and is used to establish and maintain hierarchy. However, when cavefish produced the same sound it was in response to detecting food odours while foraging. 

Unlike the surface morphs, cavefish don’t display schooling behaviour and are non-aggressive.

“It is unclear whether they ‘call’ other individuals to come and share food when they find some, but it is a possibility. In any case, communication about finding food is crucial for cavefish, much more beneficial than spending energy in aggressive behaviours” Retaux explains.

“Drastic environmental changes can induce significant evolution or shifts in behaviours and probably, in the brain, the neural circuits that govern these behaviours” Retaux says.

Even though morphological, physiological and behavioural changes occurred in cavefish, studies report Astyanax morphs breed with one another after flood waters wash surface morphs into the cave system. “A really interesting hypothesis is that one of the sounds of their repertoire, which again, is shared by the two forms, is used during courtship. In this case, a prediction is that the use and meaning of this particular sound has not changed, contrary to the sharp click that has evolved from an aggressive signal to a feeding signal” says Retaux.   

Both Astyanax morphs are the perfect model for researchers to reconstruct the evolutionary paths each morphotype has taken since their common ancestors were separated. The lateral line in cavefish is enhanced, an adaptation which has increased their sensitivity and response to living in confined zones with no light. “It is a sense that we humans do not have but that fishes and amphibians do have, which allows them to detect differences in the water pressure around them.”  

The only audible sounds in the caves, are water dripping or bats chirping and Retaux plans a return trip to record the acoustic environment. She wants to answer the ironic question Arthur Popper asked when he first discovered cavefish have exceptional hearing: what are cavefish listening to? 

Report by Gabrielle Ahern

Thank you to Dr Sylvie Rétaux, Directeur de Recherche CNRS Team DECA, France, for permission to publish her team’s field trip photos. More information about the cavefish research study and the team’s experiences is available here.