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!

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

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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