If you've ever had the bad luck of scraping your skin against coral in the ocean, you know how painful it can be. That's because in addition to its hard and spiny architecture, corals also contain venomous stinging cells called nematocysts. Now imagine scraping your lips against some coral. Sounds pretty painful, right? The same holds true for fish, where only 128 species out of 6,000 that live in reefs dine on coral. To find out how the coral-eating fish manage it, researchers at James Cook University in Australia took a closer look at the extraordinary lips on a tubelip wrasse.

Using a scanning electron microscope, the researchers examined the lips of the tubelip wrasses and compared them to those on other wrasse species that don't feed on corals. The non-coral-eating fish had smooth lips and protruding teeth. The scans revealed something quite different about the tubelips though: They have fleshy lips that have a series of fins in them, similar to the fins beneath a mushroom cap. These lips also form a tube when the mouth is closed (thus their name), which completely covers their teeth. What's more, the researchers discovered within all the folds on their lips are mucus-secreting glands that coats the lips with mucus.

"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 theory is that the mucus protects the fish from the stinging cells in the coral while allowing them to get their meal by sealing their tube-like lips against the coral and delivering a strong suck. The researchers have compared the action to a kissing gesture.

"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."

"Our understanding of the functional role of fish lips is in its infancy, although the diversity of lips in reef fishes offers an exciting opportunity to explore the potential roles of lips in feeding," add the researchers in their study, which has been published in the journal Current Biology.

Source: Cell Press via Science Daily

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