This video says about itself:
5 June 2017
(Inside Science) — With their sharp, stony skeletons and stinger-laden flesh, corals are well protected against most potential predators. Tubelip wrasses are one of the few fish to overcome these defenses, and now, scientists have discovered how they do it. The secret is in the lips.
Unlike their thin-lipped relatives, tubelip wrasses have big, fleshy lips that protrude in front of their teeth. These lips appear smooth to the naked eye. But in a study published today in Current Biology, scientists examined them for the first time under an electron microscope, and saw they are actually covered in parallel flaps like the gills of a mushroom. The lips produce huge amounts of mucus, and can close to form a sort of straw.
High-speed videos revealed what the strange lips are for.
“[T]ubelip wrasses feed using short sharp ‘kisses’ to suck mucus and occasionally tissue off the coral surface,” the authors write. The kisses make a smacking sound the researchers describe as “tuk.”
According to the researchers, the lip mucus probably serves a double purpose, forming a seal on the coral’s surface while shielding the fish from stinging barbs known as nematocysts. Thus protected, the fish are free to suck out coral’s nutritious goo.
With specialized lips, these fish dine on razor-sharp, stinging corals
June 5, 2017
Summary: More than 6,000 fish species that live on coral reefs, but only 128 are known to feed on corals. Now, researchers have discovered how at least one species of coral-feeding fish does it. They ‘kiss’ the flesh and mucus off the coral skeleton using protective, self-lubricating lips.
Of all the things an animal could eat, corals are arguably one of the toughest, thanks to their thin, mucus-covered flesh packed with venomous stinging cells spread over a razor-sharp skeleton. Perhaps that explains why of the more than 6,000 fish species that live on the reef, only 128 are known to feed on corals. Now, researchers reporting in Current Biology on June 5 have discovered how at least one species of coral-feeding fish does it. They “kiss” the flesh and mucus off the coral skeleton using protective, self-lubricating lips.
“The lips are like the gills of a mushroom but covered in slime,” says David Bellwood of James Cook University in Australia. “It is like having a running nose but having running lips instead.”
The researchers suggest that the mucus may facilitate suction while offering protection from corals’ stinging nematocysts.
Bellwood and the study’s first author, Víctor Huertas, recognized that the problem when eating corals would come as lips touched the surface. They wanted to find out exactly what was happening in that process. They used a scanning electron microscope to get extremely high-quality images that could capture the specialized lips of tubelip wrasses (Labropsis australis) in unprecedented detail.
Those images revealed remarkable differences between the lips of the tubelip wrasse and another wrasse species that doesn’t feed on corals. Wrasses that don’t eat corals have lips that are thin and smooth, with teeth that protrude slightly. By comparison, tubelip wrasses have lips that are fleshy and stick out, forming a tube when the mouth is closed that covers all the teeth.
The most prominent characteristic of the tubelip wrasse’s lips, they found, are numerous thin membranes arranged outward from the center like the gills of a mushroom. The mouth surface of tubelip wrasses also includes many folds loaded with highly productive mucus-secreting glands. In other words, their lips drip with slime.
High-speed video images of feeding tubelip wrasses showed that they briefly place their lips in contact with the coral prior to delivering a powerful suck. Rather than grabbing onto coral, they appear to seal the mouth over a small area, presumably to increase suction-feeding efficiency, the researchers report. The new evidence suggests the tubelip wrasses and their mucus-laden lips survive by feeding primarily on coral mucus. The findings open up a whole new way of looking at the nature of feeding in fishes, the researchers say.
“One always assumes that fishes feed using their teeth, but, like us, the lips can be an essential tool,” Bellwood says. “Imagine feeding without lips or cheeks; the same applies to fishes.”
On their quest to learn how the wrasses cope with the challenge of reef feeding, the researchers say the next step is to discover the “magic of the mucus.”
See also here.