Of the 6,000 species of reef fish in the world, only 128 feed on corals. Although corals are abundant, it's easy to see why so many fish avoid them: They certainly don't provide an easy meal.
Coral is made up of small, sharp polyps covered in stinging cells called nematocysts that the coral uses to capture food. Those cells have a nasty, venomous sting.
But the tubelip wrasse is up to the coral-eating challenge with the help of an amazing mouth. Looking like Botox gone terribly wrong, the bee-stung lips of the tubelip wrasse (Labropsis australis) allows the fish to feed on coral, reports researchers in a new study published in the journal Current Biology. The fish repeatedly attacks the coral with a mucus-laden kiss.
The fish's fleshy lips protrude over its lips and are covered in numerous lamellae — thin, tightly layered folds of tissue resembling what you'd find under a mushroom cap. The researchers found the folds were packed with mucus-producing cells, which were critical to how the tubelip wrasses were able to feed.
Instead of grabbing or holding coral, the fish briefly touch the coral with their lips then give a powerful suck. The researchers say the lips appear to seal the mouth over a small area, likely to increase the efficiency of suction and feeding. The mucus helps with the suction seal and wards off the stinging of the coral.
"The highly modified lips of coral-feeding tubelip wrasses are fundamentally different from those of non-corallivorous wrasses and, to our knowledge, have not been reported previously," the researchers write.
"The structure of these lips strongly suggests that mucus secretion is the key factor that enables these fishes to feed on corals by providing protection, a seal for suction (the thick mucus and soft lips providing a seal on the uneven coral surface during suction) and, potentially, a means of mucus ingestion. Indeed, video observations reveal that tubelip wrasses feed using short sharp ‘kisses’ to suck mucus and occasionally tissue off the coral surface. Feeding strikes were often associated with an audible ‘tuk’."