Adventures with Blue Whales and Blue Waters

My book review - Adventures with Blue Whales and Blue Waters - featuring The Secret Life of Whales by marine biologist, Micheline Jenner; and The Oceans, A Deep History by paleoceanographer, Eelco Rohling - was published by Cosmos Magazine Winter 79 Edition.

I recommend both books, which are very different stories, but written by scientists who share a great respect and love of the ocean and its wildlife. Fantastic stories.

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!

COLOUR OF CLAMS

Clams and their beautiful coloured mantles are just one of the many amazing marine animals that attract millions of visitors each year to the Great Barrier Reef. I studied clam populations on reefs around Heron Island as an undergraduate Marine Studies student at the University of Queensland. It was a great opportunity to observe the variation in mantle colour across reef locations.

Clams belong to Class Bivalvia, Family Tridacnae, and range in size from 15 cm long in Tridacna crocea to more than one metre long in Tridacna gigas. Similar to corals, zooxanthellae (Symbiodinium microadriactum) live inside a clam’s tissues. These symbionts can also be found in gorgonians, sea anenomes and jellyfish. These zooxanthellae are the reason why clam behaviour is often compared to plants.

Just like plants, clams need sunlight. In the mantle, clams possess what are known as, eyespots or iridophores. These pigment cells can sense light conditions and trigger a response in the clam. For instance, if light is poor, the clam improves conditions for the zooxanthellae to photosynthesise by extending its mantle out or using its shell to reflect sunlight back onto the mantle, by holding its valves wide apart. If sunlight is too intense, the iridophores also protect the clam’s zooxanthellae from harmful ultraviolet radiation by reflecting light away.

Mantle colour of clam species is a poorly researched topic. The notion that mantle pigmentation in clams differs in clams across a coral reef has not been researched widely. Interestingly, studies have shown clams are sensitive to the blue (450nm), blue-green (490nm) and ultraviolet (360nm) colours in the spectrum. Light quality on the reef may affect mantle colour, with clams expressing different pigments, or perhaps a clam’s genetic adaptation to a particular reef habitat influences the type of zooxanthellae inhabiting its mantle tissues. I noted a trend in colour for different reef zones during my fieldwork and related this data to previous reports, which have suggested clams might share symbionts with other invertebrate species in a particular zone.

Further investigation into the mantle colour of clams by applying molecular biology might reveal why particular colour patterns occur. Research could also concentrate on how clams can adapt to changing light or temperature conditions by genetic variability.

Clams are quite a stable organism and can adapt to changes in their habitat over time. However, fishing pressure has dramatically reduced populations of clams in the pacific region, leading to the extinction of several species. Conservation methods that use aquaculture assist the remediation of wild clam populations on the reef and supply aquariums with different species.

Clams are interesting to observe in their natural environment. During my research, I noted variations in colour mantle between clam populations across the reef zones and between the two sites on Heron Island. While your exploring the reef, it is definitely a feature worth looking out for.

For more information, check out the world-wide-web.

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 beautiful images of clams in their coral reef habitat, please take a look at my Pinterest site: https://www.pinterest.com/saltywave