The evolution of the damselfishes: Phylogenetics, biomechanics, and development of a diverse coral reef fish family

Some of the central questions in the evolution of coral reef ecosystems concern the historical patterns of structure, function and ecology in the clades of fishes that have inhabited reefs during much of their history. This study employed multiple phylogenetic, morphological, biomechanical, and developmental methods to explore the trophic evolution of the damselfishes (Perciformes: Pomacentridae), which are one of the most abundant groups of fishes on all coral reefs. By utilizing the collections of the Field Museum of Natural History and collecting large numbers of specimens during a series of expeditions, it was possible to obtain morphological and genetic data from all 29 damselfish genera and over 100 species. This data was used to perform morphological, biomechanical and phylogenetic analyses that permitted detailed descriptions of pomacentrid trophic evolution. Trophic states have evolved very freely amongst the Pomacentridae, as has the evolution of five biomechanical characters that are associated with feeding performance. The repeated evolution of planktivory has played a major role in diversification of damselfish head shapes, and appears to have involved similar morphological and biomechanical changes regardless of the ancestral morphology. The transition from a non-planktivorous to a highly planktivorous state is likely to occur in two very different evolutionary stages. Although the 16 most recently evolved damselfish genera constitue a monophyletic lineage that is an important part of the Indo-Pacific reef fish biodiversity, these fishes represent a case where a radiation in species number has not been accompanied by the evolution of unique morphologies. Both phylogeographic and molecular approaches were utilized in order to determine the time at which this lineage diverged, since its origin marks an important change in the biogeographic patterns of the Pomacentridae. As the radiation of damselfish form required a radiation in patterns of morphogenesis, a comparative developmental approach to studying the formation of jaw shape was included in this project. RNA in situ hybridization of the regulatory gene Dlx-2a was used to label the initial formation of the first pharyngeal arch. The cells within this arch develop into the first set of functional jaws that a fish will use for feeding. The development of this arch between first formation and first feeding was compared between the damselfish, Pomacentrus philippinus, and the zebrafish, Danio rerio, and found to be highly divergent. These results suggest that it is possible that important aspects of jaw formation are determined during the earliest stages of the morphogenesis of these structures.