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 have discovered that at least one species of coral-feeding fish ‘kisses’ 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 the corals' stinging nematocysts.
Wrasses that don't eat corals have lips that are thin and smooth, with teeth that protrude slightly. By comparison, Tubelip wrasses, Labropsis australis, 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 also includes many folds loaded with highly productive mucus-secreting glands. In other words, their lips drip with slime.
The wrasses feed by briefly placing their lips in contact with the coral prior to delivering a powerful suck, appearing to seal the mouth over a small area. The new evidence suggests that Tubelip wrasses survive by feeding primarily on coral mucus.