The sounds cavefish make

Scientists have discovered one of the ways cavefish have adapted to subterranean living is by using sound differently to surface fish morphs.

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

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

25,000 years ago, the ancestors of A. 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.

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. 

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 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.”

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” she says.

Even though morphological, physiological and behavioural changes occurred in cavefish, studies report Astyanax morphs breed with one another when 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?

Thank you to Dr. Sylvie Retaux for permission to publish her field trip photos.

Eye for Adventure

The ocean is there to be discovered and Steve Brady is an intrepid diver who has explored many underwater locations with a salty tale to tell.

“We’ve got it all and there’s just so much diversity of marine life in and around Australia. It’s amazing” Brady says. “This is probably a funny thing to say but if I had gills I’d be a very happy boy.”

With a litany of dive experiences under his belt, Brady wanted others to learn what the ocean has to offer. Informed by research from marine biologists and feedback from pro divers, he developed a calendar featuring marine wildlife events throughout the year, and uses the calendar to schedule dive trips at all levels of experience from beginner to advanced around Australia and beyond.

“Finding where you can go to see certain marine life is a very big thing for people and I’m the same. I want to see mola molas, I want to see mantas, so I want to go to a place that has the best chance at the best time to see them” says Brady.

Brady has an eye for adventure and despite the unpredictable nature of the sea, he is not deterred from making new discoveries. He recalls an incredible rendezvous with a manta ray after travelling out to an open ocean destination via spotter plane in West Australia.

“In West Australia we went to the township of Coral Bay. We specifically went out on a trip where they have spotter planes, and you aren’t on a reef. You’re in open water. Sandy bottoms, maybe twenty metres of water and you’re snorkeling with them. They’re just down there, just swimming around, very acrobatic, very inquisitive. Just to have one of them come gliding over the top of you, that’s very special” says Brady.

Australian dive sites are world renowned for their pristine waters, thanks to clean up projects led by conservation organisations and social media groups. Despite awareness campaigns and changes to the community’s attitude toward disposing rubbish, microplastics are severely impacting the health of marine life globally.

When marine organisms filter seawater for their food and to absorb oxygen, they vicariously take in these tiny microplastic pieces. This plastic can become lodged in digestive and respiratory tracts, causing animals to suffer health problems or worse, mortality. In the ocean, it's not just small filter feeding species like corals and clams that are being negatively affected, the larger pelagic animals, whales and sharks, are also being hit hard by this invisible threat. 

“Plastic is everywhere, even in places we can’t see” says Ian Kiernan, founder of Clean Up Australia, an organisation that coordinates community clean up events of the environment every year in Australia and worldwide.

“Even if you aren’t really concerned about the health of marine creatures, you might pause when you know that you cannot escape that easily because they are in our drinking water” Kiernan says.

To decrease the problem of microplastics in our water, Kiernan recommends consumers buy clothes made with natural fabrics instead of synthetic material and avoid purchasing products like cosmetics, soaps, shampoos and detergents that do not clearly state they are free of microplastics.

Another way of overcoming obstacles to the continued health of the marine world is through ecotourism, which is offering coastal communities the benefit of financial return, by showing visitors how beautiful preserved natural environments are.

The Great Barrier Reef along Queensland’s coast and Ningaloo Reef off Exmouth in West Australia, both World Heritage listed areas, are prime examples of ecotourism’s success.

Divers share their once in a lifetime experiences when they dive with whales, sharks and dolphins through social media, attracting a lot of attention which ultimately turns marine species into major tourism draw cards overnight. “People’s expectations are all different and everyone enjoys diving around the coral. But when people are fortunate enough to see species like mantas that would be their trip for the rest of their lives” says Brady.

Spectacular Coral Sea locations lying beyond the Great Barrier Reef are a prize ready for the taking. “You’re talking thousands of metres of water, massive drop offs, just awesome visibility and you will get large pelagic marine life out there. If you like sharks, that’s the place to go” says Brady.

Another exciting dive destination is the S.S. Yongala shipwreck, which sunk in 1911 off Alva Beach, Ayr, in North Queensland. “It’s a feeding station for mantas, bull rays, groupers and sharks. It’s safe to go learning to dive because you don’t have strong currents. You’ve got pristine visibility. You’ve got nice corals. I would promote it as one of the best places to learn to dive” says Brady.

The Ribbon Reefs are part of the Great Barrier Reef, which extend from Port Douglas all the way up passed Lizard Island. 

“In June, July each year, you get the migration of the Minke whale coming through and that’s very special. That’s really a highlight for me in terms of marine species” says Brady.

And for the nouveau diver who stumbles upon a manta ray or a whale shark for the first time? 

“You just let them be in their natural environment, you let them be the inquisitive one and you’re in for a great experience” says Brady.

Report by Gabrielle Ahern

Steve Brady manages ‘Dive In Australia’ located in Cairns (https://diveinaustralia.com.au) a travel agency matching dive companies to divers looking for their ideal wildlife encounter.

All images © Steve Brady - Dive In Australia - www.diveinaustralia.com.au

My interview with Steve Brady will feature in a future podcast episode of the Noisemaker series. So stay tuned.