Special Lips Help Fish Feed on Sharp, Stinging Coral

A tubelip wrasse feeding on coral. The species has specialized lips to prevent the coral from cutting and stinging it. (Victor Huertas & David Bellwood)

(CN) – The razor-sharp skeleton and venomous stinging cells of corals make them one of the most challenging meals in nature, requiring hungry organisms to develop features that can bypass these biological defenses. Researchers have discovered how at least one fish species is able to feed amid such hazards – by “kissing” the mucus and flesh off of the coral skeleton using self-lubricating, protective lips.

“The lips are like the gills of a mushroom but covered in slime,” said David Bellwood of James Cook University in Australia, co-author of the new study published Monday in the journal Current Biology. “It is like having a running nose but having running lips instead.”

The team believes that the mucus may aid suction while offering protection from the corals’ poisonous cells, known as nematocysts.

Bellwood and lead author Victora Huertas realized that the greatest risk these fish face is posed when its lips touch a coral’s surface. In order to study this interaction, the researchers used a scanning electron microscope to get high-quality images that clearly depicted the specialized lips of tubelip wrasses in detail.

The images revealed differences between the lips of the tubelip wrasse and another wrasse species that doesn’t feed on corals. The tubelip wrasses feature fleshy lips that stick out, creating a tube when the mouth is closed which covers all teeth. Wrasses that don’t eat corals, meanwhile, have lips that are smooth and thin with teeth that barely protrude.

The scientists found that the most pronounced characteristic of the tubelip wrasse’s lips was the multiple thin membranes arranged outward from the center like the gills of a mushroom. Their lips drip with slime, and the mouth surface of the tubelip wrasses also has several folds loaded with highly-productive mucus-secreting glands.

Still images from high-speed video of feeding tubelip wrasses show they momentarily place their lips in contact with the coral before delivering a powerful suck. As opposed to grabbing onto the coral, they appear to use their mouth to seal a small area, likely to increase suction-feeding efficiency, according to the report.

The new findings also suggest the tubelip wrasses survive by feeding primarily on coral mucus. The team says the findings can be used to open up a new way of analyzing the nature of feeding in different fish species.

“One always assumes that fishes feed using their teeth, but, like us, the lips can be an essential tool,” Bellwood said. “Imagine feeding without lips or cheeks; the same applies to fishes.”

The tubelip wrasse is one of just 128 species that are known to feed on corals. More than 6,000 types of fish live on reefs.

Next, the team plans to study how the wrasses persevere and recover from the challenges of reef feeding, labeling the next step of their research the “magic of mucus.”

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