The farmer fish
1/6/15

By slamming their oral jaws shut, damselfishes produce noise and use it to defend their territory or to attract partners during courtship displays. For most of these species, the mechanics of this action are linked to the cerato-mandibular ligament. Located in the mouth, it enables the jaws to slam shut rapidly thereby producing sound. This ligament does not only have an acoustic function. It enables the fish to select algae with surgical precision and to feed on the cultivated algae. Some species have therefore been able to adopt “farming” behavior and occupy a unique position in coral ecosystems and reefs throughout the world. This morphological particularity is therefore directly behind this ecological behavior that is unique in fishes. Two studies conducted at the Functional and Evolutionary Morphology Laboratory of the University of Liege have just revealed how a morphological trait was able to shape the evolution of damselfishes.

Poisson demoiselleDuring dives along coral reefs, an attentive eye can observe small parcels of algae that are around one meter squared in size. These are small carefully arranged fields that look like well-manicured lawns. As soon as the diver approaches, the damselfishes try to hunt him away by slamming their jaws shut in a very deliberate way. These “farmer” fish are the owners of such parcels and spend many hours tending to their food store. They remove algae that are not to their liking and cultivate those they prefer. No intruders are tolerated and their tiny size in comparison with the divers does not seem to dampen their protective ardor. Even though this behavior has been known for a long time in damselfish, it is still not understood why they are capable of it (Read the article : The evolution of damselfishes).

The sound produced which in this instance is an actual defense reflex, is emitted by means of a slamming of the jaws and is perfectly audible to humans. It is produced by means of the cerato-mandibular ligament which links hyoid bar to the mandibles, and which enables the jaws to  very quickly. One of the aims was to know whether this same slamming of the jaws was produced during the trimming of algae. The other was to understand whether this ligament completely or partly influenced the diversification of damselfishes during their evolution. In order to do this, two studies were jointly conducted; one was morpho-functional and behavioral studying a “farmer” species of damselfishes(1), and the other using phylogeny and comparing morphology of 124 species (2). Among these, it was noticed that some do not have this ligament and have adopted a diet consisting mainly of zooplankton.  

The ligament that has been known for thirty years

The presence of this ligament in damselfishes has been known since 1981. But at that time, nobody really knew what it was used for. In 2007, Eric Parmentier, a researcher at the Functional Evolutionary Morphology Laboratory of the University of Liege, discovered its function in the production of clownfish (these belong to the family of damselfishes) and had an intuition that this slamming noise was initially the result of a mechanism at work during feeding. Following these observations, Damien Olivier, a PhD student in the same laboratory, supervised by Bruno Frédérich, a postdoctoral researcher at the F.R.S.-FNRS, extended the research to the entire family of damselfishes (Pomacentridae). He attempted to understand how the biomechanics of this oral ligament could influence their feeding, their diet and therefore their ecological role within coral reefs. 

(1) Damien Olivier, Bruno Frédérich, Milton Spanopoulos-Zarco, Eduardo F Balart and Eric Parmentier (2014) The cerato-mandibular ligament: a key functional trait for grazing in damselfishes (Pomacentridae). Frontiers in Zoology, 11: 63.
(2) B. Frédérich, D. Olivier, G. Litsios, M.E. Alfaro and E. Parmentier. 2014. Trait decoupling promotes evolutionary diversification of the trophic and acoustic system of damselfishes

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