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.
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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.
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!