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.

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

HERON ISLAND

The Great Barrier Reef has earned its place as one of the great wonders of the world. Visible from space, it is 2,300 km long and comprises of 2900 reefs and 900 islands to explore.

The Great Barrier Reef started to develop approximately 600,000 years ago and a combination of physical and biological factors made conditions suitable for the growth of many types of coral species (450 known hard coral and 150 soft coral species) over geological time.

Heron Island forms part of the Capricorn Bunker group of reefs located at the southern end of the Great Barrier Reef. The vegetated cay island was named after the bird species, the reef heron, recently renamed the eastern reef egret (Egretta sacra). It has taken 11,000 years to develop.

The reef around Heron Island is a type of shelf reef. Its location and exposure to the elements: waves, wind, ocean currents, climate and extreme weather events, have contributed to the type of coral, plant and animal species that inhabit the reef. Induced pressure from animal activity, for example, grazing and colonisation, play a part in its continual growth.

Heron Island attracts a population of 900 of the 1625 species of fish inhabiting the Great Barrier Reef, for example, parrotfish, butterfly fish, trevally, wrasse, triggerfish and sea perch.

Parrotfish are interesting to observe. Some researchers have made mistakes in the past, identifying species due to an anomaly not apparent in other fish species. For instance, the ability of parrotfish to change gender (sequential hermaphrodite) and colour (polychromatism) throughout their lives. Most species start as plain coloured, small females (initial phase) and at certain points in their lives, transform into bright coloured, large males (terminal phase).

The parrotfish diet consists of algae, scraped off coral by using specialised teeth. The coral remains are excreted from the fish as sand. Research studies estimates these fish produce 30% of the sand around reefs and are one of the marine animals that play an important role in maintaining coral reef health.

At night some parrotfish species have been observed to envelope themselves in a cocoon of mucous. Apparently, the cocoon masks their scent from reef predators, like the moray eel (Family Muraenidae).

Nests made by wedge-tailed shearwaters (Ardenna pacifica), bridled tern (Onychoprion anaethetus) and the black noddy (Anous minutus) bird species can be spotted all over the island. The nesting burrows of shearwaters, shelter chicks waiting for their parents to return with food. Other species that live and breed on the island all year round include the buff banded rail (Gallirallus philippensis), the eastern reef egret (Egretta sacra), bar shouldered dove (Geopelia humeralis), black-faced cuckoo-shrike (Coracina novaehollandiae), capricorn silver eye, sacred kingfisher (Todiramphus sanctus), white-bellied sea eagle (Haliaeetus leucogaster), the silver gull (Chroicocephalus novaehollandiae), and migratory and visiting bird species. Black noddies are the pretty black and white birds that nest in the pisonia trees (Pisonia grandis).

These forests produce a sticky sap, which can unfortunately, trap black noddies, supplementing the cycle of nutrients from guano and plant material. You can also find screw palms, casuarina, she oak and other plant and grass species as you wander around the island.

If you’re visiting Heron from November to January, loggerhead turtles (Caretta caretta) and green turtles (Chelonia mydas) make their way slowly onto beaches at night to lay their eggs. The turtle’s sensitivity to any artificial light (torches etc.) can distract or disorientate them. Moving too close to a nesting turtle might also cause them to panic, so it is recommended to maintain a safe distance of approximately 10 metres away. Just listening to them breathing in the darkness while they lay their eggs is an amazing experience.

Heron Island is but a small microcosm of the Great Barrier Reef, which forms the biggest tropical Marine Reserve and the largest protected World Heritage Area. It may have taken its time to develop into the extraordinary beauty it is today, but wasn’t it well worth the wait.

Please take a look at the University of Queensland’s Centre for Marine Studies link for a bird’s eye view of Heron Island.

http://www.science.uq.edu.au/content/facilities/hirs-virtual-tour/

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 marine life in coral reef environments, please take a look at my Pinterest site: https://www.pinterest.com/saltywave

Book

Photographic Field Guide: Birds of Australia. An Australian Museum / Reed New Holland Publication. Text by Jim Flegg. 2006. Second Edition.

These links provide some great views, photographs, videos and information about Heron Island and the Great Barrier Reef.

Reef Biosearch

http://www.greatbarrierreefs.com.au/parrotfish/

National Geographic

http://animals.nationalgeographic.com.au/animals/fish/parrot-fish/

Onboard – The Tourism Operators Handbook for the Great Barrier Reef

http://onboard.gbrmpa.gov.au/home/marine_park/what_makes_the_reef_special

Heron Island – Great Barrier Reef

http://www.heronisland.com/Turtles-and-Birds.aspx

Caitlin Seaview Survey – An underwater view of the coral reefs around Heron Island

http://catlinseaviewsurvey.com/panorama/?url=http%3A%2F%2Fembed.panedia.com%2Fvst%2Fuwe%2Fvstplayeruwe%3Fmulti%3Dpypmmntf%26map%3Dtqvqslkj%26start%3D600463&call_type=internal&start_url=surveys/great-barrier-reef-and-coral-sea/heron-island

GHOST NETS

Commercial fishing boats have negatively impacted wild fisheries with nets that don’t discriminate between size class and the reproductive maturity of fish species. Most boats are equipped with technology that target a school of fish, with one catch potentially wiping out the next generation, leaving some populations depleted or completely destroyed. Recovery takes a while, and the disappearance of key stone species over time affects the collective ecology and abundance of marine animals and plants. Each year approximately 640,000 tonnes of fishing gear and nets are discarded by the fishing industry. Is it game, set and match in the world’s oceans?

Conservation of the Reef along the east coast of Australia with the Great Barrier Reef Marine Park has made a huge difference to the abundance of tropical reef fish and other marine life. ‘No take’ and ‘take’ zones guide recreational and commercial fishers to places where fish and crabs etc., can be legally caught.

The ‘no take’ zones allow animals to breed without fishing pressures, which in turn increases the numbers of wild fish available in ‘take zones’. Marine Parks around Australia’s coastline and worldwide, continue to positively affect marine life abundance and diversity, supported by community awareness campaigns.

More people are investing their expertise and knowledge into aquaculture as a sustainable alternative to the wild caught fisheries. There are a range of methods used to breed fish, crustaceans, shellfish and algae for restaurants and the markets, in fresh water or saltwater aquaria/ponds. Similar to any competition, aquaculture has encountered its fair share of criticism; but improved technologies have made it more commercially viable.

Nets abandoned by fishing crews, continue to make a big haul at sea though, fatally trapping whales, dugongs, fish, crocodiles, sea birds, sharks, seals and dolphins, as they drift along the oceans currents. Recently, a spate of shark attacks along beaches prompted the introduction of drum lines in New South Wales to curb the risk to surfers and swimmers. Drones equipped with cameras are currently being trialled to alert authorities if shark activity is observed along beaches. This method combined with drum lines is considerably less threatening to marine species, compared to the use of nets.

Turtles and seabirds recurrently swallow or are entangled by fishing line, tackle, plastic debris and nets, despite efforts led by community groups like Clean Up Australia to remove marine debris and rubbish in general. One of the hotspots for discarded fishing nets is the Gulf of Carpentaria in Australia’s north. Seven species of marine turtle are listed as threatened, including the Hawksbill, Green, Olive Ridley, and Flatback turtles. Ghost nets are the common cause of death.

Ghost nets are a serious environmental hazard, not just to animals, but also to anyone going out on a boat. A net can jam an engine. If you spot an injured animal, please contact a wildlife rescue association or a veterinary clinic for assistance; and safely throw discarded nets or tackle into a bin. It’s just one thing to do, but if we all do it, this deadly game out at sea might be over for good.

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 like turtles, rays, whales and seals in their marine environment, please take a look at my Pinterest site: https://www.pinterest.com/saltywave