Featured Image Credit: ryanphotogenic.com
By Eva Gruber
Coral reefs are one of the most biologically diverse habitats on our blue planet, and an excellent place to look for some of the most incredible examples of evolution towards weird and wonderful forms. One such example is the tubelip wrasse (Labropsis australis) which makes its living by feeding on stinging corals in the tropical western Pacific and Indian Oceans.
While corals may look helpless, most of these animals actually possess an impressive defense system exactly to try to prevent it from being eaten by hungry organisms – of which there are a lot in the ocean. Corals defend themselves from predation by having specialized stinging cells called nematocysts. These cells, located in the outer fleshy layers covering the rigid calcified “skeleton”, eject a stinging, harpoon-like flagellum upon arousal. Nematocysts are capable of delivering a painful sting, and some species even contain toxins that can immobilize or even kill. These cells also play an important role in securing food for the corals themselves.
Therefore, marine biologists have long wondered how coral-eating fish are able to feed on stinging corals with impunity. To answer this question, David Belwood and colleague Victor Huertas at James Cook University in Queensland, Australia, used an electron scanning microscope to closely analyze the feeding apparatus’ (mouth and lips) on certain fish. The fish chosen to study in detail were the tubelip wrasses, a coral-eating species, and a non-coral-eating wrasse relative.
What they found was that while the underlying boney structures of the fish were similar, the lips varied wildly. Tubelip wrasse have a special arrangement to the tissues of their lips – instead of smoth, narrow lips without membranes (as in non-coral-eating wrasse), the tubelip had numerous thin membranes arranged outward from the center. The tubelips wrasse lips were also loaded with mucous-secreting cells which helped produce a sort of lubricant against the corals’ stinging cells.
Finally, the researchers examined feeding behavior of the tubelips wrasse. It feeds by approaching corals, briefly placing its lips against the corals flesh, and producing a sudden and powerful suction, or “kiss” often accompanied by an audible sound. This showed that rather than grabbing and nipping at the corals flesh, the fish were creating a seal around a small area in order to more effectively suck off coral mucus flesh. The fleshy lips of the wrasse help to create a protective layer between the corals stinging cells and the fish while also facilitating feeding.