Pomacentridae

Study of the transformations linked to the settlement of Pomacentridae

The majority of coral reef fishes have a complex life cycle with a oceanic and pelagic larval phase followed by sedentary récifal phase for the youthful ones and the adults. In the hours or the days which follow the colonization of the reef, the larvae metamorphose themselves into juvenile which will become adults adapted to their ecological niche. This adaptation is often accompanied by changes in the way of life and the structures which ensure the realization of it. The study seeks to establish during the development the relations between these changes. It will be based primarily on the principles and the methods of functional morphology, ecomorphology, systematic and geometrical morphometry.

 

poma noirpoma blanc

 

 

The damselfishes represent one of the most successful radiations of coral reef fishes. More than 300 species are associated with coral reef environment. The aim of this project is to understand their evolutionary history and, then compare their diversification to other specious fish family as Labridae and Cichlidae.

 

Trophic diversity

We combine stomach contents and stable isotope analyses to determine the trophic diversity present in this family. Isotopic signatures reveal three main groups according to their foraging strategies: pelagic feeders, benthic feeders and an intermediate group.

fig1

 

Average (± SD) δ15N and δ13C ratios of thirteen pomacentrid species and of food items collected from the coral reef. Isotopic signatures of each benthic algae species are summarized in one mean using a SD reflecting their disparity. The groups A, B and C of fishes are respectively defined as the benthic feeders/foragers, the intermediate group and the pelagic feeders. Pomacentrid species belonging to the groups A, B or C and the potential primary food sources (zooplankton, zoobenthos and algae) are represented by different icons. POM is the particulate organic matter.

 

Stomach contents show that planktonic copepods and filamentous algae mainly represent the diets of pelagic feeders and benthic feeders, respectively. Two species of benthic feeders (Cheiloprion labiatus and Plectroglyphidodon johnstonianus) feed mainly on coral polyps. The intermediate position of the third group resulted from a partitioning of small planktonic prey, small vagile invertebrates and filamentous algae.

 

fig2

Summarized dietary composition of thirteen pomacentrid species. The six categories of preys (phytoplankton, benthic algae, sessile invertebrates, vagile invertebrates, zooplankton and detritus) are represented by different color patterns

 

Currently, we focus on diet variation within and among populations of damselfishes belonging to the intermediate group and benthic feeders. Most of these species appear as more “generalists” in comparison with “specialists” species feeding exclusively on planktonic copepods. Using stable isotopes and stomach contents, we explore individual-level diet specialization in several damselfishes testing the hypothesis that these more generalized populations are more heterogeneous. In parallel, we test if these generalized populations are also more morphologically variable.

Morphological diversity at the adult stage

In an ecomorphological perspective, geometric morphometrics were used to explore and quantify form variations in different functional units of the head skeleton (i.e. neurocranium, suspensorium, opercle, mandible and premaxilla) among several damselfishes belonging to each trophic group.

This method allowed us to reveal shape and size differences correlated to functional diversity both within and between trophic guilds. For example, shapes of herbivorous and corallivorous species are mainly related to improving the robustness of some skeletal parts (broad hyomandibular, short and high mandibles with long symphysis mandibulae).

 

fig3

 

Morphological diversification thorough their ontogeny

As the majority of coral reef fishes, damselfishes have a complex life-cycle with two distinct phases: (1) a dispersive pelagic larval phase and (2) a sedentary adult phase associated with the coral reef environment. The larval phase ends at reef settlement. In the pelagic environment, all pomacentrids larvae feed on planktonic copepods. On the other hand, their trophic diversity is higher at the adult stage (see above).

We tested the hypothesis that a higher trophic diversity at the adult stage corresponds to a higher disparity (i.e. the diversity of morphological form) level than at the larval stage.

 

As expected, the morphological disparity is higher at the adult stage for each studied skeletal unit (i.e. neurocranium, suspensorium, opercle, mandible, premaxilla) although the larval shapes are already species-specific at settlement. Post-settlement growth is highly allometric in damselfishes and we compared their ontogenetic trajectories using several geometric morphometric methods.

fig4

 

Ontogenetic transformations in neurocranium and mandible shape. These transformations, calculated by multivariate regression of shape on log-transformed centroid size, are depicted as deformation grids using the thin-plate spline. The larval and adult shapes are depicted in blue and red, respectively.

 

Generally speaking, the allometric transformations reveal an optimization of the suction feeding system in all studied species.