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The farmer fish
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Bruno Frédérich and Damien Olivier also demonstrate that this ligament acts as a constraint to a morphological evolution of the jaws and prevents, for example, an adaptation for maximizing the capture of zooplankton. “It is important to note that the zooplanktivorous species have several strategies for capturing prey. One of these consists of creating a protrusion of the mouth. The upper jaw can slide forward. The mouth then forms an actual tube which enables the fish to capture its prey without approaching its body which would otherwise create disturbances in the water alerting the prey which could then escape. We have observed that those damselfishes which have lost the cerato-mandibular ligament have morphological adaptations for capturing zooplankton. In other words, the disappearance of the ligament has made hyperspecialization possible for zooplanktivory, explains Bruno Frédérich.

How did they draw this conclusion? In this publication, the authors have divided the 124 species into two groups according to the presence or non-presence of the cerato-mandibular ligament. By means of methods of evolutionary modelling in molecular biology coupled with fossil data, they were able to trace their phylogeny (see evolutionary tree above). Once this time-tree was completed they were able to test a whole series of theories. “We wanted to verify whether the disappearance of the ligament modified the rate of evolution of the groups studied. In the first instance, we looked at whether the ligament influenced the speed of speciation and therefore the multiplication of species. But there was no evolutionary burst, an explosion of species within the different lineages which lost their ligament. This would have been plausible”. On the other hand, while carefully looking at the rate of morphological diversification for the body and the two mouth pieces: the upper jaw (premaxilla) and the mandible, they observed an important difference. The morphology of the species without the ligament evolved very rapidly, that is to say up to three times faster than the basic morphological variation of the other species, to optimize zooplankton hunting. This discussion is based on ecomorphological theory but the researchers will tend to demonstrate their hypotheses in a future publication. 

3D graph Damselfish

The articles of Bruno Frédérich and Damien Olivier have two different approaches. One deals with comparative methods in morphology and phylogeny and embraces a large number of species whereas the other studies the behavior and functional morphology of only one species. Finally, both have the same objective; that of understanding the diversification within the same family of fish, observing the causes and consequences of the appearance of certain phenotypes and analyzing the way the morphology of fish has been a determining factor for their evolutionary success.

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