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.

Episode 3 - Noisemakers Series by Salty Wave Blue

Welcome to the podcast series – Noisemakers – presented by Salty Wave Blue. This episode features my interview with Dr. Sebastian Thomas (March 2017) from the University of Melbourne, who discusses why mangrove forests are considered the major players of blue carbon, amazing sounds from the noisemakers of the wild, quizzes to solve and some fascinating tales to follow from the rainforest to the reef.

Stock Media provided by Pond 5 and Monsoon Enterprises.

This podcast is dedicated to my dog, Scruffy, who loved mangrove forest walks.

Episode 3

Kissing coral in the Great Barrier Reef

Tube lip wrasses use mucus-coated lips to feed on the surface of corals. When they feed, these fishes close their mouths, push their fleshy lips against the coral, and suck off the coral’s mucus and flesh. These “kisses” are possible thanks to a protective coat of slime around their lips. Image courtesy of Victor Huertas and David Bellwood.

Small changes in any organism take millions of years and multiple generations to evolve and learning why the design and function of certain traits are successful is not as easy. Tube lip wrasses are a familiar sight in tropical coral reefs across the Indian and Pacific Oceans and recognised for their thick, fleshy, tube shaped lips. 

Intrigued by this conspicuous physical adaptation, fish biologists Victor Huertas and Professor David Bellwood from James Cook University decided to investigate further. “We wanted to see if this morphology in the lips of tube lip wrasses matched with the hypothesis they feed on coral mucus” Huertas says.

Damaged coral produces more mucus than healthy coral and observations in the field report tube lip wrasses preference for feeding in damaged coral areas. Coral mucus is not a nutritional source of food for fish and it is difficult to imagine how these wrasse species survive on it.

To the naked eye, the lips of Labropsis australis appear smooth but when magnified by scanning electron microscopy the images revealed the surface has numerous grooves similar to the underside of a mushroom with a reduced tooth. It is a remarkably different trait when contrasted to the lips of other reef fish and even those of typical non-coral feeding wrasse species, Coris gaimard, which have thin, smooth lips with a protruding tooth. “There are species of damsel fish that have larger than usual lips. But it was only in these tube lip wrasses, these fish that feed on coral, that we observed this new adaptation” says Huertas.

The mouth of a tube lip wrasse with self-lubricating lips. 

These lips enable the fish to ‘kiss’ mucus and flesh from the surface of corals. 

SEM image courtesy of Victor Huertas and David Bellwood.

It is normal for any fish to produce mucus from their skin, they’re slippery to hold onto when you catch one. So it was extraordinary when histology showed the mouth of L. australis contained a very high proportion of mucus-secreting goblet cells. “We noticed that among these groups there was a large number of mucus producing cells. Occasionally, you find goblet cells in the lips and the lip skin but it is quite rare. In this case, what we saw is a lot of them” says Huertas. “This was the eureka moment. We realised this is what enables the fishes to feed on coral”. 

In their paper “Mucus-secreting lips offer protection to suction-feeding corallivorous fishes” published in Current Biology early in 2017, the authors compared the grooved lips to tissues that usually line a fish’s gut. “The reason why we wanted to make the analogy is to highlight surfaces or tissues that specialise in either secreting or absorbing substances, generally tend to show this type of morphology” says Huertas.

How all these elements conspire together so successfully shows the devil in the design. High-speed videos recorded L. australis swim toward a coral with its closed mouth forming a tube to suck off coral mucus and flesh. The ‘kissing action’ or suction only lasts a brief 13.1 milliseconds and you can actually hear a short ‘tuk’ sound.

Tube lip wrasses use mucus-coated lips to feed on the surface of corals. When they feed, these fishes close their mouths, push their fleshy lips against the coral, and suck off the coral’s mucus and flesh. These “kisses” are possible thanks to a protective coat of slime around their lips. Gif image courtesy of Victor Huertas and David Bellwood.

It appears as though the fish suck up the coral mucus through their lips like a straw. The fish don’t appear to grab or hold any coral material and the lubricated lips enable the fish to latch onto the uneven surface and achieve a more efficient suction. “The problem with tube lip wrasses is they have to push their lips against the coral surface, so these lips become exposed all of a sudden to the coral they fancy” says Huertas.

Huertas suggests the slime produced from their lips is a protective mechanism, which shields the fish from stinging nematocyst cells that might be accidentally eaten; and from any damage posed by the sharp coral surfaces. “If they didn’t have this mucus they would probably not be able to feed on corals” says Huertas.

Traditionally, it has been assumed tube lip wrasses fed on coral polyps like butterfly fish. “They do not inspect the coral surface very carefully. They pretty much go in there and start striking. If they were feeding on specific things that grow on the coral surface, like parasitic worms, you would expect to see the fish approach and then stop and inspect the surface, but that’s not what we saw” says Huertas.   

18 species out of the 600 wrasses in Family Labridae feed on coral in the Great Barrier Reef and judging by the population numbers of wrasses distributed across reefs in the Indo-Pacific region, the success of these slimy sucking lips is evident. Determining what triggered this unusual feeding trait is the pandora box the researchers are looking forward to opening.

“Tube lip wrasses have found a very creative way to overcome the corals defenses. How this mechanism happened in evolution? We really don’t know. But we know that these are the only group of fishes that have been able to evolve it. There could be others, but so far, this is the only one that we have found” Huertas says.

Story by Gabrielle Ahern

My interview with Victor Huertas will soon feature in the SaltyWaveBlue podcast series so stay tuned!

Coral reef fish – Butterflies of the oceans

Butterfly fish (Family Chaetodontidae) are one of the most conspicuous of the fish species inhabiting coral reefs worldwide, due to their beautiful colour, distinctive markings, morphology and interesting social behaviour. Butterfly fish species remain close to coral reefs all their lives as research shows these fish depend on coral species and other reef organisms for food, habitat and protection from predators. Generalist feeders (that rely on a variety of food sources in addition to coral species) are less common than butterfly fish that consume only corals and experience slower growth rates than their corallivorous counterparts.

Butterfly fish swimming in its coral reef habitat. Photo © Anje Ranneberg

Interestingly, some butterfly fish species have a preference for a particular species of coral so their habitats location might be linked to the type and amount of coral substrate present at various locations along the reef. The reefs exposure to the currents and the protection it provides against egg predators might also be reason’s influencing fish preference for a location over other sites. Studies have revealed some butterfly fish species have a home range they explore to forage for food.

As juveniles, butterfly fish have been observed to find a monogamous mate. This type of pairing early in development decreases the stress levels experienced by these fish, for example, they expend less energy defending their home range or competing with other butterfly fish to reproduce. The constraints that solitary fish or fish living in a harem experience, may play to their favour. Female territories have been observed close to the male, which allows the male to protect and maintain his territory, resulting in less energy being used, much like the monogamous pairs but with multiple chances for fertilising opportunities.

Coral reef habitat. Photo © Vincenzo Piazza 

Butterfly fish use complex forms of social and mating behaviour to exploit the ecological advantages available on the coral reef. Their courtship patterns are an interesting window to their survival. Some butterfly fish mate before sunset. This timing reflects their natural behavior but also increases the chances of egg survival from predation by other fish. Some rogue male butterfly fish are opportunistic and intrude on a spawning monogamous pair by attempting to fertilise the female’s eggs with their own gametes.

Once the eggs of butterfly fish hatch, larvae usually settle close to their native spawning grounds, but this can be a temporary strategy to avoid predators or to find food quickly while migrating to another site. Many view habitat destruction from storms and cyclones negatively, but in the marine environment they have a positive affect, because the amount of coral reef refuge for butterfly fish larvae to settle in increases.

Colour might also be a reason for the success of butterfly fish in coral reef environments. Butterfly fish species are well known for their beautiful markings, which might protect individuals from attack by their counterparts or assist mate recognition. Other cues butterfly fish use to identify their mates from other fish are the sounds they make, for example, some make grunting noises while others slap their tails. So a combination of colour and sound cues improves the success of mating, territorial protection and defence against predators. 

Butterfly fish are linked to coral reefs through their colour, morphology, social behavior, courtship, mating patterns and the sounds they make. Their dependence on coral species for food, protection and recruitment, highlights their potential as a coral reef health indicator species. Coral species are experiencing declines from the effects of warmer oceans, smothering from sediment, disturbance from infrastructure development and fishing pressures.

Green sea turtle swimming off the Hawaii Islands. Photo © Chris LaCroix

A combination of all these factors and pressures from the natural environment are creating extreme conditions coral species cannot withstand for long periods of time. Although present studies are working to identify tolerant species of coral that cope with more hostile environments, alternative methods can also be pursued, by protecting coral reefs through expanding marine parks and reducing the activities that are causing their decline both in the marine and terrestrial environments.

Research studies provide evidence of how coral reefs, other marine habitats and associated plants and animals are being affected. These studies have not just been completed in the last few years but over centuries. Considering the weight of evidence that is available to read; and the benefit provided by the natural environment, you have to wonder why the facts presented by scientific research to reduce negative activities are continually being ignored.

Butterfly fish are more than just a fish species swimming around coral reefs, they have developed different ways to communicate with one another, using colour, movement and sound, creating an unforgettable panorama of beauty on many levels.

Written by Gabrielle Ahern

Salty Wave Blue – Into all things ecology.

http://www.saltywave.blue

Follow @SaltyWaveBlue on @Instagram and @Twitter

If you would like to see images of  animals in their environment, especially butterfly fishes and coral species,  please take a look at my Pinterest site: 

https://www.pinterest.com/saltywave

WARM FOREST WALK

Forests survive in many types of environments all over the world, so it is no surprise that these same trees can thrive at the National Arboretum, Canberra, Australia. Someone recommended the National Arboretum as one of the more interesting places to see while I am here; so I thought I would take a look, as I love trees and was really curious. The National Arboretum was not what I expected.

Looking up to the tree tops at the Aboretum. Photo © Gabrielle Ahern

It was opened in February 2013, following much discussion about how the site would be managed. A devastating bushfire all but destroyed the area in 2001 and 2003. The Arboretum is the realisation of a design originally created by Walter and Marion Griffin in the early 1900’s. Many voices were productive in the development of this area, with the result being an incredible conglomeration of forests from various global locations. I was fortunate to arrive when a walking tour was leaving. The Guide was like a Pandora box of information. There are apparently 94 forests of rare, endangered and symbolic trees. The trees are in ‘forest’ terms only new and the Guide described how this place would look in one hundred to two hundred years. I can only but imagine the difference time will make to this site.

Walking through a forest of trees. Photo © Gabrielle Ahern

Naturally, it was one of the coldest days to visit with winter temperatures close to zero as we visited ‘warm trees’ wrapped with knitted scarves. The scarves were knitted by many people interested in attracting visitors to explore the Arboretum during the time of the year when trees are naked of leaves. The trees trunks are clothed in colorful displays and provide a stunning contrast to the bleak winter landscape.Trees, of course, grow into all sorts of shapes and sizes and smaller statured trees should not be ignored. The National Bonsai and Penjing Collection of Australia houses nearly 80 native and exotic trees all year round. It was an incredible walk through a forest of trees that are normally colossal but in miniature. 

The Arboretum. Photo © Gabrielle Ahern

Ironically, it houses the oldest tree at the Arboretum, which is over 130 years old. If you are interested in learning the art of bonsai, there are workshops and training courses available. The Arboretum is the place for studies into forest ecology, with the Australian National University managing research about the impact of climate change and trees reaction to changes in temperature and rainfall.The many trails are a fantastic diversion for walkers or cyclists to travel through the forests. The Village Centre caters for visitors with a café and restaurant, and if you would like a souvenir there is a gift shop. I bought some seeds.

Wattle tree in bloom. Photo © Gabrielle Ahern

A feature of the National Arboretum and a way to appreciate its growing forests and the Australian Alps is the view from Dairy Farmers Hill. You’ll notice a small forest of Aleppo Pine (Pinus halepensis). These trees were cultivated from seedlings of the commemorative Lone Pine planted at the Australian War Memorial in 1934 (a gift from the Turkish Embassy). Something hard to miss is the incredible iron sculpture of an eagle on its nest.

Photo © Gabrielle Ahern

I recommend the National Arboretum to anyone who loves trees and supports their conservation worldwide. Whatever your personal interests you walk away with a great sense of the forest.

Written by Gabrielle Ahern

Salty Wave Blue – Into all things ecology.

http://www.saltywave.blue

Follow @SaltyWaveBlue on @Instagram and @Twitter

If you would like to see images of forests, especially trees, plants & flowers, please take a look at my Pinterest site: https://www.pinterest.com/saltywave

Reference

Aboretum – The magazine for the National Aboretum Canberra Issue 2, Spring 2013 

Website Link

National Aboretum: www.nationalaboretum.act.gov.au/home

Fun Fact

Did you know the leaves of the monkey puzzle tree (Araucaria auracana) may remain on the tree for thirty years.

SAND DUNES

North Stradbroke Island may be a sand dune island but it harbours an interesting, if not unique array of plant ecosystems. North Stradbroke Island is located 40 kilometres east of Brisbane, approximately 270 36’ 31. 32” S 1530 26’ 40. 12” E and enjoys a subtropical coastal climate.

The dune islands slowly formed over thousands of years, as layers of sand were gradually deposited by both changes in sea level and strong winds from the South East. Some of the corals that are found underneathe the fresh water lakes on North Stradbroke Island can be dated between 119 to 132 thousand years ago. It is second to Fraser Island as the largest sand island in the world, 38km long and 11 km wide. It is also the largest sand dune island in a chain that forms a protective barrier around Moreton Bay.

In the geological past, North Stradbroke Island was connected to the mainland via land bridges. These bridges enabled the spread of many plant species. Over time, sea level rise gradually isolated the island. The plants spread out and diversified, forced to adapt to the islands unique geomorphology and climate. Today, the island harbours approximately 450 plant species.

North Stradbroke Island represents a range of ecosystems. For example, there are about 6 sand systems, some dating back as far as 120 thousand years ago. Sand dunes form when wind blown sand is trapped by vegetation to form ridges. These ridges protect the small gullies below. Beach Spinifex (Spinifex sericeus), Pig Face (Carpobrotus glaucescens) and Beach Primrose (Oenothera drummondii) are species common to the outer edge of a dune community and play an important role in stabilising the sand.

Beach spinifex are perfectly suited to the dynamic environment of a dune, their structure and function have been shaped by its harsh conditions. But you might wonder how any plant could survive in such a nutrient poor environment like a sand dune. The reason? Its symbiotic relationship with a fungi species. These fungi grow all over the root system of the Spinifex. They fix the nutrients (except nitrogen) covering small grains of sand and pass these nutrients onto the Spinifex. This relationship is similar to coral species’ partnership with zooxanthellae (a microscopic algae that shares the products of photosynthesis, sugar, with its coral host).

Coastal Wattle (Acacia sophorae) and Coastal Sheoak (Casuarina equisetifolia) grow in dense communities and are protected by the sand ridges that form along the dunes. Coastal Wattle is an Australian native spreading shrub, considered to be a pioneer species that populates coastal sand dunes. Its success is based on its association with a nitrogen fixing soil bacteria called Rhizobium. The bacteria are present in root nodules, a formation on the roots initiated by the bacteria. The Rhizobium fix the atmospheric nitrogen present in sediment and make the nitrogen available to the shrub in a form the plant can absorb. In exchange, the Rhizobium receives the products of photosynthesis, sugar, from the plant. Coastal Wattle usually grows to a height of 3 metres and can spread out to between 10 to 15 metres.

Coastal Sheoak also fix nitrogen in the sediment through its symbiotic association with a species of fungi called Mycorrhizal. Sheoak grow quite densely in dune communities and probably owe their success to their ability to use leaf litter to their advantage quickly.

Sand dune ecosystems like the ones located on North Stradbroke Island are an amazing example of how the plants exposure to the elements has affected their adaptations to survive. The dune communities highlight their fragility and emphasise how important plants are to the health of marine and terrestrial ecosystems.

Written by Gabrielle Ahern

Salty Wave Blue – Into all things ecology.

http://www.saltywave.blue

Follow @SaltyWaveBlue on @Instagram and @Twitter

If you would like to see images of sand dunes and tropical islands, please take a look at my Pinterest site: https://www.pinterest.com/saltywave

References

Stradbroke Island – Official Website

http://stradbrokeisland.com

Australian Plants – A Simple Botany of Wattles                  

http://anpsa.org.au/APOL8/dec97-1.html

Australian National Herbarium – Australian Fungi 

http://www.cpbr.gov.au/fungi/mycorrhiza.html