Bipartite life cycle of coral reef fishes promotes increasing shape disparity of the head skeleton during ontogeny.

Disparity is a major theme in the paleobiological literature. Most studies of disparity have focused on its temporal dynamics over a geological time scale and generally show that phenotypic disparity decreases or remains stable in numerous groups. Surprisingly, the relationship between ontogeny and disparity has received little attention. The ontogenetic dynamics of shape disparity are influenced by diverse interactions such as developmental constraints and/or environmental factors.
Coral reefs have an extraordinary diversity of fishes. As the majority of demersal coral reef fishes, the damselfishes (Pomacentridae) have a bipartite life cycle. During the pelagic dispersion phase, all larvae feed on planktonic preys. On the other hand, juveniles and adults associated to the coral reef environment show a high diversity of diets. We test the hypothesis that this ontogenetic change in habitat and diet is related to an increase of shape disparity in head skeletal units (neurocranium, suspensorium, opercle, mandible and premaxilla). We also compare ontogenies of shape among eight species of damselfishes to identify the evolutionary changes in developmental processes modifying shape disparity.
By comparing levels of disparity between the eight species at three developmental stages, at the end of the larval phase (reef settlement), at 60 mm SL, and at maximum adult body size, we found that disparity increases significantly during ontogeny. At the end of the larval stage, all skeletal units are already species-specific. The ontogenies of shape are highly divergent among species. At least, evolutionary changes affected three parameters of ontogenetic trajectories of shape in this group: (1) the allometric pattern (the direction in which the vector representing the ontogeny of shape point), (2) the amount of change undergone during the juvenile and adult phases and (3) the rate of shape changes. The interactions among environmental constraints and the three developmental parameters are responsible for increasing disparity.