A change of attitude during experiments

A first article summarizes part of the thesis of Damien Olivier. In this study, the young researcher focusses on a damselfish species, Stegastes rectifraenum. A “farmer” fish that rarely wanders far from reef substrate and which ardently defends its ‘plot’ of algae. “It was an ideal candidate because I wanted to understand to what extent the presence of this cerato-mandibular ligament influenced the feeding habits of the damselfishes”.

For this purpose, he went to Mexico where he captured several specimens and also took pieces of rock with algae, their favorite food. He placed them in an aquarium, recreated their environment and filmed them by using a high-speed camera (up to 1000 images per second). For the first time, a researcher analysed in detail the movement of the damselfishes’ head when it was eating or defending its territory. The movement is identical in both cases. The ligament allows the fish to quickly close the mouth in order to capture filamentous algae without doing much damage to the substrate that supports the crop of algae.

 “It is a high-precision work”, explains the young doctoral researcher. “Some of these algae are very small, around one millimeter in size. The fish must therefore act with precision in order to avoid ingesting undesirable elements or damaging the cultivations”.

During a second phase, without damaging the specimens, he cut the cerato-mandibular ligament and put them back in the aquarium. “They were then incapable of carrying out the observed movement. It was therefore a mechanism that was determined by the presence of the ligament. They no longer emitted any noise and did not compensate with the adductor mandibulae muscles which are normally used by other fish to close the mouth. But the most astonishing thing was that they no longer ate any algae either. They did not even try. It was as though they knew they would be unable to grasp it. However, they were volunteers and they were hungry. When I placed other types of food that can be found in the water column, like zooplankton, they ate what I gave them without hesitation. This was where the link between the presence of the ligament and the handling of algae was observed. I had suspected that it was an important factor but I didn’t expect it to be indispensable to this extent”.

The absence of the ligament discovered by chance in some species

While all damselfishes studied to date produced sound, not all of them have the ligament in question. This was an evolutionary particularity which the two biologists were not expecting and which they did not observe by chance, as a prelude to the research for the thesis of the young doctoral student. “For his thesis work”, explains Bruno Frédérich, “Damien firstly dived in the Mediterranean Sea to capture specimens of Chromis chromis. It is one of the species of damselfish that doesn’t live in a coral environment. While dissecting them, he did not observe this famous cerato-mandibular ligament. At the outset, we thought that it was a handling error and then we had to recognize that this species was lacking the ligament”. While the PhD student was beginning his thesis, Bruno Frédérich was in the United States in order to gather as much data as possible on the jaws of a maximum of damselfish species. It was a real godsend. Damien Olivier’s observation brought a new dimension to the two research programs conducted simultaneously. If the ligament was necessary in order for the fish to feed, why did some of these fish lose that trait during the course of their evolution? Bruno Frédérich scoured museums, laboratories and aquariums. Each time he dissected a fish, he noted the presence or absence of this ligament. For the 124 species studied, 19 did not have the ligament. 

“We therefore began the phylogenetic studies and tried to find out if the common ancestor of these 124 species was lacking the ligament. During their evolution, at three different times at least, some lineages lost the ligament. What is remarkable is that they continue to produce noise but they do so with the cheek muscles”, says Bruno Frédérich.