"It is like having a running nose but having running lips instead." This is the eloquent description marine biologist David Bellwood has offered up of the curious reef-dwelling tubelip wrasse and it’s incredible ability to withstand the pain of feeding on stinging coral.
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Coral reefs are highly endangered as a result of warming climes. But some species also pose a threat to the marine life that attempts to consume it and any diver that gets too close. They have hard, sharp exoskeletons and in some instances sting prey, thanks to nematocyst cells emitted by polyp tentacles covering some species. They are also covered in sticky mucus. Despite this less that enticing menu, a handful of fish species still decide to dice with death and feed on these corals – around 128 of the more than 6,000 species that live on reefs attempt this according to Bellwood and his team at James Cook University in Australia. Now, they think they know how this is possible.
“Of all coral feeders, tubelip wrasses appear to have modified their mouth the most to meet this dietary challenge,” the team writes in Current Biology. They attest that little is known about the role of fish lips but, in the case of the tubelip wrasse, it’s clear they play a vital role. Its lips are described as “the most distinctive” of all wrasse species, and are “highly modified” for sucking from the surface of coral – the team studied electron micrographs (images taken using an electron microscope) of different species before coming to this conclusion. While other fish lips are thin and smooth with teeth that point outwards, the wrasse’s lips are “fleshy, protruding, and form a tube when the mouth is closed, covering all of the teeth”.
"The lips are like the gills of a mushroom but covered in slime," Bellwood explains. The team speculates that this slime, essentially mucus, may act as a barrier to the stinging nematocysts and help them form a suction lock. In fact, it all seems to come down to the fish’s sucking technique – high-speed video shows the lips barely touch the surface of the coral before the suction begins.
The fish additionally has a highly developed lining along the lip epithelia, filled with mucus-secreting cells. “This convoluted epithelium, lined with goblet cells that secrete mucus onto the external surface of the lip, bears more resemblance to the epithelium of a fish gut than to the lips of other reef-dwelling fishes. These grooves increase the lip surface area in a manner comparable to the gills of mushrooms and toadstools.”
The fish hook onto a small area of the coral in this way, perhaps only consuming the equally mucus-laden coral surface. The fact the fish spend most of their time near damaged coral, which produces more mucus, backs this theory up. “Video observations reveal that tubelip wrasses feed using short sharp ‘kisses’ to suck mucus and occasionally tissue off the coral surface," write the paper authors.
The team will be following up by investigating exactly how the mucus aids the feeding process. But for now, the study may have opened up a whole new world of marine biology study: the importance of fish lips. "One always assumes that fishes feed using their teeth, but, like us, the lips can be an essential tool. Imagine feeding without lips or cheeks; the same applies to fishes," Bellwood said.
This article was originally published by WIRED UK
