Trophic specializations of damselfishes are tightly associated with reef habitats and social behaviours

peer reviewedDespite the increasing need to understand factors shaping community assembly, few studies have simultaneously explored the influence of niche-based and phylogenetic processes. Here, we investigate the relationships between diet, habitat and social behaviour in damselfishes (Pomacentridae) collected in 2014 at Moorea Island (17°30′S, 149°50′W), French Polynesia. Isotope ratios of carbon and nitrogen, in association with stomach contents, delineate three trophic groups: pelagic feeders consuming zooplankton, benthic feeders mainly grazing on algae and an intermediate group feeding on prey from the whole bentho-pelagic compartment. Sulphur isotope ratios indicate segregation between species of the outer reef mostly depending on oceanic input of zooplankton and the lagoonal species relying on locally produced resources or even on terrestrial supply. We demonstrate a tight association between dietary specializations, habitat characteristics and social behaviours, and these correlations are further confirmed by integrating the phylogeny of Pomacentridae. We also provide evidence of phylogenetic conservatism for the stomach content and the habitat–behaviour characters. However, the isotopic trait is evolutionarily more labile probably because it thoroughly depicts the ecological niche of species. To summarize, pelagic feeders (mainly from the Chrominae) usually form shoals in areas close to the open ocean at a maximum depth of 20 m. Benthic feeders (well represented in the Stegastinae) are ubiquitous, solitary and mostly territorial species found at various depths. The intermediate group includes gregarious species from three subfamilies that forage in the lagoon usually above 12 m depth. Overall, we give insight into processes that have structured the damselfish community in Moorea

Acclimatization of massive reef-building corals to consecutive heatwaves

Reef-building corals typically live close to the upper limits of their thermal tolerance and even small increases in summer water temperatures can lead to bleaching and mortality. Projections of coral reef futures based on forecasts of ocean temperatures indicate that by the end of this century, corals will experience their current thermal thresholds annually, which would lead to the widespread devastation of coral reef ecosystems. Here, we use skeletal cores of long-lived Porites corals collected from 14 reefs across the northern Great Barrier Reef, the Coral Sea, and New Caledonia to evaluate changes in their sensitivity to heat stress since 1815. High-density 'stress bands'-indicative of past bleaching-first appear during a strong pre-industrial El Nino event in 1877 but become significantly more frequent in the late twentieth and early twenty-first centuries in accordance with rising temperatures from anthropogenic global warming. However, the proportion of cores with stress bands declines following successive bleaching events in the twenty-first century despite increasing exposure to heat stress. Our findings demonstrate an increase in the thermal tolerance of reef-building corals and offer a glimmer of hope that at least some coral species can acclimatize fast enough to keep pace with global warming

Évaluation de la capacité adaptative des poissons demoiselles (Pomacentridae)

Assessing the multiple facets of adaptive capacity is a core issue as it reflects species' ability to cope with variations in environmental conditions. Within the threatened coral reefs, the Pomacentridae (damselfishes) represent a very suitable model to undertake such issue. Damselfishes are one of the most ecologically dominant reef fish families whose lineages repeatedly radiated across convergent trophic strategies. These strategies comprise (1) pelagic feeders (mainly picking zooplankton in the water column), (2) benthic feeders (essentially feeding on algae and vagile invertebrates on the benthos) and (3) an intermediate group (foraging in the entire bentho–pelagic compartment). To date, most studies compared the degree of vulnerability between specialists and generalists because the first ones are thought to have a higher risk of extinction. The tripartite trophic guild system in Pomacentridae provides the opportunity to step out of the binary "specialist–generalist" classification and to robustly grasp the adaptive capacity by focusing on fundamental units in assemblages. In the present thesis, I estimated the ecology of damselfishes by quantifying their eco–functional diversity (including trophic, habitat and behavioural diversity and also the ecosystem functions they performed) and by gauging their abiotic (i.e. species–environment relationship) and biotic (i.e species–species relationship) interactions. Ecological results revealed (a) the systematic presence of the three main trophic guilds in each assemblage, and (b) a consistent pattern of species eco–functional niches that are highly differentiated and evenly distributed in eco–functional spaces of similar size, but they provided (c) limited support for any degree of trophic plasticity (i.e. little evidence for any geographic variation in the diet of widespread species). The trophic guilds were also functionally dissimilar and displayed different levels of functional diversity. The intermediate group was the least functionally diverse guild with the lowest functional redundancy, while opposite findings were found for the pelagic–feeding guild. Then, I quantified the genetic diversity, which varied among trophic guilds and was associated with variation in their trophic ecology and habitat–behavioural traits (e.g. pelagic feeders have the lowest genetic and ecological diversity). Using a phylogenetic framework, I found that functional traits were evolutionarily labile and accumulated at a similar rate within the three trophic guilds. Transition rates among guilds were also biased, suggesting differences in the evolvability. The evaluation of all of these components indicated that some processes occurring at an "ecological" timescale and others at an "evolutionary" timescale had concordant effects (e.g. the coupling between levels of genetic and ecological diversity supported by the evolvability result for the intermediate group), although sometimes one type of process had more important effects (e.g. the principle of competitive exclusion probably shaped the high functional diversity of pelagic feeders). Overall, I demonstrated that the adaptive capacity of Pomacentridae is bounded and that trophic guilds are not equal in the face of environmental changes. The case of pelagic feeders illustrates a scenario wherein evolution and the right environmental conditions led to the success of a given group, but their persistence may be jeopardized by a low genetic diversity. The two other guilds possess more intrinsic assets (e.g. less restricted habitat and dietary requirements) to adapt. Given the uncertainty regarding the pace at which evolutionary processes will occur and the potential imperilment of evolvability to act as a source of future diversity, the ability to diversify along many niche axes associated with broad intrinsic characteristics (i.e. higher genetic diversity) may help organisms to sustain themselves in a long–term perspective and to maintain more ecosystem functions

Comparison of isotopic turnover dynamics in two different muscles of a coral reef fish during the settlement phase

The temporal variation in carbon and nitrogen isotopic compositions (noted as δ13C and δ15N) was investigated in the convict surgeonfish (Acanthurus triostegus) at Moorea (French Polynesia). Over a period of 24 days, juveniles were reared in aquaria and subjected to two different feeding treatments: granules or algae. The dynamics of δ13C and δ15N in two muscles (the adductor mandibulae complex and the epaxial musculature) having different functions were compared. At the end of experiments, a steady-state isotopic system in each muscle tissue was not reached. Especially for the algal treatment, we found different patterns of variation in isotopic compositions over time between the two muscles. The turnovers of δ13C showed opposite trends for each muscle but differences are mitigated by starvation and by the metamorphosis. Our study highlighted that the metabolism of coral reef fish may be subjected to catabolism or anabolism of non-protein precursors at settlement, inducing variation in isotopic compositions that are not linked to diet change

The mangrove forests as nursery habitats for the ichthyofauna of Mida Creek (Kenya, East Africa)

This study investigated the presumed nursery function of mangroves for the ichthyofauna in East African swamp forests, Mida Creek. The species and size composition of the fish fauna and their trophic interactions were studied for five stations. Samples were collected during 5-6 consecutive days close to spring tide in mid-July 2011. Fish were caught using different types of passive fishing gear, such as large and small fyke nets, gill nets and, additionally, beach seines. All individuals were identified to species level, measured and preserved for further analyses. Stomach content analysis was applied to provide information about their diet. Stable isotope analysis was used to detect whether the source of primary production for the higher trophic levels is mangrove-related or not. A total of 27 fish species was recorded in a catch of 938 teleost specimens. Our analyses showed that a majority of fish belonged to the zoobenthivorous/omnivorous trophic mode, since they were partially feeding on invertebrates and on various other food sources. Two species (Sphyraena barracuda and Synodus variegatus) exhibited a mixed diet, with a piscivorous preference. Results concerning the population structure suggested that the fish community of Mida mainly consists of transient species. Juveniles were numerically more abundant in the whole Creek than adults. These findings support the hypothesis of a nursery function of the mangrove forests, Mida Creek

Generalist feeding guilds in reef fishes: Macroevolutionary sink or future source of diversity?

Reef fishes have diversified into thousands of species that fill various types of ecological niches, contributing to the tremendous biodiversity of reef ecosystems. Yet, the dynamics of this diversity remain understudied, especially the evolutionary relation between the different functional traits, which relate to the life strategies and ecosystem roles of organisms. Here, we explore how one functional trait, the feeding habit, impacts the dynamics of species diversification and functional evolution. Using comparative phylogenetic methods in conjunction with taxonomic, trophic and functional datasets, we analyze the functional evolution of two major radiations of reef fishes, the wrasses (Labridae) and the damselfishes (Pomacentridae). We demonstrate that the feeding habit plays a key role in the evolution of the rest of functional diversity. The nature and the diversity of functional roles hosted by fish in reef ecosystems are tightly linked to their feeding attributes. Counter to a simple prediction of ecological opportunism, we found that “generalist” feeding guilds (i.e. those composed of species feeding on food sources from the whole bentho-pelagic compartment) do not show higher rates of functional diversification and do not necessarily display higher levels of functional diversity. Furthermore, in contrast to recent macroevolutionary studies on mammals and birds, we highlight that these “generalist” guilds of fishes represent the basis of future diversity and cannot be considered as evolutionary sinks or as "dead-ends". These findings clearly re-define our view on the ecological and evolutionary roles played by generalist feeding guilds