Slimy lips

Southern tubelip feeds on corals by kissing

Labropsis autralis feeding on corals

As one of only a few fish species, the tubelip wrasse Labropsis australis is able to feed on corals. Specialised lips protect the fish from being hurt, as Victor Huertas and David Bellwood show.

Lips of Labropsis australis bear lamellaeThe tubelip wrasse Labropsis australis, or Southern tubelip, looks like a normal fish. But it appears to have highly modified lips, as Victor Huertas and David Bellwood reveal after making a high-resolution image of the fish’s mouth. The lips form a protruding tube when the mouth is closed; they are thick and fleshy, bear lamellae much like a mushroom, and are covered with a thick layer of mucus, secreted by mucous glands.

That’s noteworthy, as most wrasses, the group of fish species to which Labropsis australis belongs, have thin, smooth lips that are neither slimy nor protruding.

The remarkable lips facilitate un unusual diet, the researchers found out. Living on the Great Barrier Reef off the north coast of Australia, Labropsis australis feeds on hard corals – and that’s not easy, because the corals have a sharp skeleton covered by a layer of tissue with venomous stinging nematocysts, like jellyfish have. No wonder that most fishes don’t touch them. But Labropsis australis seems not to care.

The biologists recorded the fish’s behaviour with a high-speed camera to see how it managed to feed on corals. Analyzing the footage, they saw how the fish approaches its meal, closes its mouth, pushes its fleshy lips against the coral, sealing them over a small area, and rapidly sucks off some of the coral’s mucus and flesh. This sucking is accompanied by an audible ‘tuk’; it’s just like kissing.

Mucus is the key factor that enables these fish to feed on corals, the authors suppose. The thick mucus layer prevents the sharp edges and nematocysts of the coral from damaging the fish.

Willy van Strien

Photos: © Victor Huertas and David Bellwood
Large: Southern tubelip Labropsis australis
Small: Image of the lips of Labropsis australis

The kissing tubelip wrasse on a video made by Victor Huertas and David Bellwoo

Source:
Huertas, V. & D.R. Bellwood, 2017. Mucus-secreting lips offer protection to suction-feeding corallivorous fishes. Current Biology 27: R399–R407. Doi: 10.1016/j.cub.2017.04.056

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