A ligament influences the diversification of damselfishes (Pomacentridae)

The Pomacentridae (damselfishes) is one of the most successful families of reef-associated fishes (386 species). In 1981, Stiassny described a synapomorphic trait of the Pomacentridae: the cerato-mandibular ligament (CML) joining the hyoid bar (ceratohyal) to the internal part of the mandible (coronoid process of the articulo-angular). Here, we highlight that this ligament is lacking in at least 18 damselfish species from different subclades (i.e. Chrominae, Abudefdufinae and Pomacentrinae) and we explore the impact of its disappearance on lineage, morphological and ecological diversification through their evolutionary history. Using a time-calibrated phylogeny including 208 species, we found no support for different rates of lineage diversification between species that lack the CML and the others. Yet, the CML clearly governs a part of the morphological diversity in damselfishes. Indeed, the body and mandible shapes differ significantly between species without CML and the others. Evolutionary modeling of some phenotypic traits (i.e. body and mandible shape) mainly supports models with two rates of morphological diversification across the time-tree with the species without CML having a higher rate of morphological diversification than the others. Mapping the diet of all studied species illustrates that damselfishes lacking CML are highly zooplanktivorous species. Their shapes suggest they feed on planktonic copepods with a higher contribution of ram-feeding (i.e. predator movement towards prey) in comparison with the other planktivorous damselfishes. Finally, the CML could have promoted easy shifts among the three main trophic guilds in damselfishes (i.e. grazers, zooplanktivorous and omnivorous) during evolution but the disappearance of the CML allowed a functional specialization linked to prey-capture strategies. These results support the primary role of the CML in the evolution and diversification of pomacentrids.