Ecological and morphological diversity

The morphology of the oral jaws of damselfish is strongly related to each kind of diet. Therefore it differs according to the trophic behavior described above: the zooplanktivorous, grazing and intermediary species. We can thus classify the different species of damselfishes into three distinct types based on the shape of two skeletal units forming the oral jaws: the mandibles and premaxillaries.

In order to do this, the muscles of specimens were digested and the skeleton was colored with alizarin red. The mandibles and premaxillary bones were carefully dissected and photographed under a binocular stereomicroscope. Then the morphology of these two pieces was quantified by geometric morphometrics, a somewhat sophisticated technique of shape analysis. This method, which is based on the capture of shapes by means of landmarks, makes it possible to rigorously compare these morphological units between species and between the three trophic groups. Therefore the shape of mandibles and premaxillaries varies between two extremes, distinguishing the two functional groups of fish which are the grazing and planktivorous fish. The former have stronger jaws and the latter have longer and thinner jaws.

In order to determine whether this ecomorphological diversity is the result of numerous convergences, where each subclade (i.e. the sub-families described above) would show a similar ecological and morphological diversity, various approaches were used.

For example, the illustration of ecological traits (feeding and farming behavior) on the phylogenetic tree reveals many convergences. As for determining which of the trophic behaviors is found in the ancestral species, “I would like to know, but the statistics did not allow this determination”, declares the researcher.
 
Other methods compare the level of diversity of the shapes of mandible and premaxilla among subclades. We find overwhelming proof of convergence in the phenotypical traits of the Pomacentridae. Thus, the morphological disparity is similar among the major subclades and is so for both units of the oral jaws: some species of the different sub-families show very similar morphologies and the diversity within a clade is higher than when we compare one clade to another. In addition, the way in which the two units of the oral jaw varies when we go from one extreme to another (the phenotypical trajectories) is identical from one sub-family to another, only the extent of the morphological transformations varies.