Search

Coral reefs of Clipperton. — Since its discovery in 1705, the near atoll of Clipperton has repeatedly caught scientists’ attention by its remoteness in the East Pacific and its particular lagoon. A brief account of the island’s geomorphology and of its living communities is given within the local oceanographic context. A short history of scientific expeditions and of human settlement along with the interests of France, Mexico and United States for this small island is presented. This information is completed with an account of the present legal status of the island (French Natural Public Domain) and its stakes. Outer slope coral reefs are described along with flora and fauna inventories completed during the last expedition on Clipperton in 2004-2005. Reef communities from two sites on the outer slope are described. Markers have been installed so that these two sites may constitute future monitoring stations to investigate the health status of this coral reef ecosytem, in the context of regional and global monitoring networks., Depuis sa découverte en 1705, le presqu’atoll de Clipperton, à l’extrémité Est-Pacifique, retient l’intérêt des scientifiques par son isolement biogéographique et son lagon aux caractéristiques environnementales et morphologiques si particulières. Une brève description géomorphologique de l’île et de ses peuplements est donnée avec le contexte océanographique dans lequel elle se situe. Un rapide historique des expéditions de recherche et des occupations humaines sur Clipperton avec les intérêts de la France, du Mexique et des États-Unis pour cette petite île, se termine par un exposé sur son statut juridique actuel (domaine public naturel de la France) et les enjeux qui se rattachent à Clipperton. Les récifs coralliens des pentes externes de l’île sont décrits conjointement aux inventaires floristique et faunistique actualisés en 2004-2005. Les peuplements de deux stations sur les pentes externes récifales de l’île sont détaillés. Balisées sur le terrain, elles constituent les futures stations de surveillance du milieu corallien de Clipperton dans le cadre du suivi de l’état de santé des récifs au plan régional et mondial., Salvat Bernard,Adjeroud Mehdi,Charpy Loïc. Les récifs coralliens de Clipperton. In: Revue d'Écologie (La Terre et La Vie), tome 63, n°1-2, 2008. Les récifs coralliens de l’outre - mer français : suivi et état des lieux / French overseas coral reefs: monitoring and status / Los arrecifes de coral del ultramar francés: seguimiento y estatuto. pp. 179-187.

International audience

This study provides a baseline describing natural small scale variability of Symbiodinium density in the sentinel coral Acropora globiceps during the summer, under non-bleaching conditions. Spatial scales investigated range from the colony scale (1–10 cm, i.e. among branches of the same colony) to the reef scale (1–10 km, i.e. among stations distributed over several locations and depths), at Moorea Island, French Polynesia. The coral–Symbiodinium symbiosis is a key process in scleractinian coral physiology, and Symbiodinium density provides an easy-to-measure and inexpensive biomarker of this symbiosis health. Spatial variability of three major environmental factors: light intensity, sedimentation and water motion was also assessed to evaluate their potential link with Symbiodinium density. Density of Symbiodinium did not significantly differ within colonies or among colonies within a station. However, a marked depth gradient was observed, showing increasing density with increasing depth and decreasing light intensity. These observations provide an interesting reference for forthcoming comparisons with disturbed conditions, such as bleaching events.

International audience

International audience

As the effects of climate change have become increasingly visible over the past three decades, coral reefs have suffered from a number of natural and anthropogenic disturbances that have caused a critical decline in coral populations. Among these disturbances are coral diseases, which have appeared with increasing frequency and severity, often in correlation with increases in water temperature. Although the crucial role played by Vibrio species in coral disease has been widely documented, the scientific community does not yet fully understand the infection process of Vibrio or its impact on coral physiology and immunology. Here, we investigated the physiological and transcriptomic responses of a major reef-building coral, Pocillopora damicornis, when exposed to a specific pathogen (Vibrio coralliilyticus) under virulent (increasing water temperature) and non-virulent (constant low temperature) conditions. The infection process was examined by electron microscopy and quantitative reverse-transcription PCR, and coral health was monitored by visual observations and measurements of zooxanthellar density. The results obtained suggest that coral tissue invasion occurs upon increasing water temperature only. Transcriptomic variations were investigated using a suppression–subtractive–hybridization approach, and the expression levels of six candidate immune-related genes were examined during bacterial exposure. These genes correspond to three lectin-like molecules putatively involved in the recognition of pathogens, two metal-binding proteins putatively involved in antibacterial response and one cystein protease inhibitor. The transcription patterns of these selected genes provide new insights into the responses of coral colonies to virulent versus non-virulent bacteria.

Outbreaks of Acanthaster planci (COTS) in French Polynesia were observed in 1969 and between 1980-1984 where concentrations as high as 110 ind/hectare were observed at Tahiti and Moorea in 1984. Subsequently, outer reef slopes have recovered their preoutbreak percentage coral cover. According to a general survey carried out at the end of 2006, COTS have increased in abundance on islands of both the Society (Tahiti, Moorea, Bora Bora, Raiatea, Huahine) and the Austral archipelagos (Rurutu), but not for the Tuamotu atolls (Manihi, Rangiroa, Tikehau, Fakarava). In order to evaluate the distribution and abundance of COTS at an island scale, a « manta tow » was developed to survey the outer reef slope along the 95 km perimeter of Moorea. This method consists of evaluating feeding marks and after establishing the correlation between feeding marks and COTS abundance, the density of COTS is calculated. Results from 2006-2007 show a mosaic distribution of COTS around Moorea. The outer slope of Moorea, between 10 and 30 m in depth was divided into sectors 250 m long, parallel with the reef crest. Only a few sectors (10 out of 395), mainly on the North and East coasts, show COTS concentrations above 17 ind, which is lower than in 1984. Other sectors show lower concentrations, of which 117 sectors had less than 2 COTS. A comparison of COTS densities between November 2006 and April 2007 does not indicate that the COTS outbreak is progressing. Nevertheless on some sectors with COTS, coral cover, mainly Acropora and Pocillopora, has been reduced by up to 50 %. Future surveys are planned to monitor this event and its consequences

This study provides the first detailed quantitative data on the sexual reproduction, larval development and settlement competency periods of broadcast spawning Acropora reef corals at Moorea, French Polynesia. From 2002 to 2004 the timing and mode of sexual reproduction for Acropora reef corals was examined at four sites on the outer reef slope and reef lagoon. To determine extent of spawning synchrony within and among species, colonies of Acropora spp. were tagged and repeatedly sampled, or collected for spawning in flow through outdoor aquaria, during September through November. Spawning was observed in aquaria and inferred in the field from the disappearance of mature gametes for 13 species of Acropora, between 6-9 nights after the full moon in September, October and November. Following spawning in November 2004, post fertilisation samples were taken from A. nasuta, A. striata and A. lutkeni to compare the morphological stages of embryo and larval development from early cleavage to ciliated planula larvae. Larval development and settlement competency periods were studied using A. retusa and A. striata planulae. Larval attachment onto settlement tiles occurred within 3-4 DAS (days after spawning) in both species. Peak periods of larval metamorphosis and permanent settlement occurred between 4-10 DAS for A. retusa and 5-10 DAS for A. striata. The early development and initial rapid attachment patterns of A. retusa and A. striata larvae indicate that some planulae may be retained close to their natal reef. However, the extended settlement competency periods observed in A. striata (31 DAS) and A. retusa (26 DAS) suggest that some planulae also have the potential for dispersal away from their natal reef before settling. This information is important for understanding and predicting the likely sources of larval supply to Moorea, and examining the potential likelihood of reef interconnectivity via larval dispersal in French Polynesia

Bleaching in corals can be attributed to loss of endosymbiotic zooxanthellae and/or loss of photosynthetic pigments within zooxanthellae. This major disturbance of the reef ecosystem is principally induced by increases in water temperature. Since the beginning of the 80's and the onset of global climate change, this phenomenon has been occurring at increasing rates and scales, and with increasing severity. In this study, we focused on finding early regulated genes involved in bleaching. In aquaria, one set of Pocillopora damicornis nubbins was subjected to a gradual seawater temperature increase from 28°C to 32°C over 15 days, and a second control set remained at constant temperature (28°C). Bleaching was monitored by measuring zooxanthellae density. The mRNA differentially expressed between the stressed state (sampled just before the onset of bleaching) and the non stressed state (control) were isolated from the nubbins by Suppression Subtractive Hybridization. The corresponding cDNA were sequenced and confronted to sequence databases to obtain gene similarities. Finally, transcription rates of the most interesting genes were conducted by Q-PCR. Two particularly interesting candidate genes showed an important decrease in their transcription rates following thermal stress and before zooxanthellae loss. These two genes show similarities with genes involved in host/symbiont and host/parasite models. The implication of these molecular actors suggests a possible role of recognition mechanisms between the host and its symbiont, in the breakdown of the symbiosis during the bleaching phenomenon. Experiments such as RACE-PCR, in situ hybridization and immunohistochemistry are currently underway to confirm our hypotheses

At a fundamental level, spatial variation in the abundance and community structure of scleractinian corals must be influenced by patterns of larval settlement. However, spatial variation in early post-settlement mortality has the potential to greatly distort patterns established at settlement, and the relative influence of variation in settlement rates and post-settlement processes on spatial dynamics of coral assemblages remains largely unknown. In this study, spatial variation in mortality of recruits (< 3 months of age) and juvenile corals (< 5 cm in diameter; ~1-4 years of age) was quantified in three depth zones at each of three locations and round Moorea, French Polynesia. Mortality of coral recruits was extremely variable and particularly high (50 % in 7 days), and is associated with the abundance of parrotfishes (Fam. Scaridae). Juvenile mortality was less intense (up to 40 % in 14 months), but also shows strong spatial variability, corresponding with variation in abundance of butterflyfishes (Fam. Chaetodontidae). Spatial variability in early post-settlement mortality partly accounts for apparent differences in the abundance of coral recruits versus juveniles corals, highlighting the potential importance of post-settlement processes as a driver of spatial variation in the dynamics of coral assemblages. Moreover, variation in early-post settlement mortality of corals appears to be influenced mainly by local densities of Scaridae and Chaetodontidae, which each affect a different stage in the life-cycle (recruits versus juvenile corals, respectively).

Temporal variability in coral community structure on the outer reef at Tiahura, Moorea (French Polynesia) was investigated between 1991 and 2006 to test for the effects of major disturbances. During the study, the reef was impacted by one cyclone (1991) and four bleaching events (1991, 1994, 2002, 2003). The cyclone and bleaching of 1991 had the greatest impact, and caused a rapid decline in coral cover from 51.0 % in 1991 to 24.2 % in 1992. In contrast, the three successive bleaching events had little effect on coral cover, even though the thermal anomalies causing these events were similar to the conditions leading to the 1991 bleaching. By 2001, coral cover returned to the 'pre-disturbance' levels of early 1991, but the trajectories of change differed among genera. Acropora was affected by the disturbances of 1991, but subsequently showed a high rate of recovery; Montipora was affected by most disturbances and showed no sign of recovery; Pocillopora was affected by the disturbances of 1991, and showed a partial recovery by 1995; and Porites was not affected by any disturbances, but increased in cover throughout the study. Our results demonstrate that large-scale disturbances are not always associated with a phase-shift from coral- to algal-dominated communities, but instead show that coral cover can recover rapidly after dramatic declines. To our knowledge, our study is the first contemporary analysis to demonstrate rapid recovery of coral cover, despite the effects of ongoing disturbances, although importantly this trend was achieved through a shift in the relative abundance of coral genera. Thus, coral communities at Tiahura appear to be characterized by ecological resilience to disturbances in terms of coral cover, but fragility in terms of generic composition

Monitoring of French Polynesia coral reefs and their recent development.-French Polynesia, consisting of 118 islands in the centre of the Pacific Ocean, has more than 15 000 km(2) of reefs and lagoons managed by the local government. Tourism and pearl culture are the two main economic resources of the country. Polynesian coral reefs are extremely diverse and are among those for which we have thorough knowledge. The exploitation of local resources has been recorded for multiple decades and includes : coral materials, fishing, harvest and export of mother-of-pearl molluscs, pearl production, and ornamental fish. All over the country, many monitoring programmes have been launched to measure the health of reefs and the natural and anthropogenic perturbations that they suffer : hurricanes and seismic events, water quality, health of benthic and fish communities, pearl oyster pathology and radiobiology. These data, collected over the last few decades, allowed to define the relative importance of natural and anthropogenic degradation on reefs and lagoons, and to explain the present status of reefs at different spatial scales. Devastating hurricanes are rare (1903-1906, 1982-1983 and occasionally at other times), but they may annihilate outer slope coral communities on some islands. Bleaching events with considerable coral mortality at different geographical scales occurred mainly in 1991, 1994 and 2003. Outbreaks of Acanthaster destroyed numerous reefs (lagoons and outer slopes) from 1978-1982 and a new demographic wave began in 2006 at many Society islands. Eutrophication events only occurred occasionally and only in some lagoons. Whereas natural catastrophic events degrade the coral reef ecosystem across many islands, at the archipelago or even regional scale, anthropogenic degradation is limited to a few Society Islands, occurring rarely on atolls and not at all on those (one third) which are uninhabited. The main causes of reef degradation in some areas of Tahiti and Moorea include the embankment of fringing zones, coral mining, overfishing, absence of urban sewage treatment and the development of leisure and tourism activities. Because of its large geographical extent, one may conclude that major reef degradation in French Polynesia is caused by catastrophic natural events. On the other hand, anthropogenic degradation is more localized. Unfortunately, the synergistic effects of these causes of degradation prevent reefs from recovering. Optimum coral cover on French Polynesian outer reef slopes is between 50-60%. After a major destructive impact (hurricane, bleaching, Acanthaster) a reef is reduced to less than 10% coral cover, however if no more major disturbance events occur a reef will recover in about 12 years. Most of the 15 000 km(2) of reefs and lagoons in French Polynesia are in good health, and along with their neighbouring reefs in East and Central Pacific they are considered as the least degraded reefs worldwide and at a low risk of becoming degraded in the few next decades. However, we are more and more anxious about the future of reefs in the world particularly because present simulations predict that major impacts of climate change would include : elevation of sea surface temperatures, increase in the strength of hurricanes and acidification of seawater which will affect the formation of coral structures., La Polynésie française, 118 îles au coeur du Pacifique, possède une surface de plus de 15 000 km2 de récifs et lagons gérés par le gouvernement polynésien. Le tourisme et la perliculture représentent les deux ressources économiques majeures du Pays. Les formations récifales très diversifiées sont parmi les mieux connues. Plusieurs suivis d'exploitation des ressources sont opérationnels depuis des décennies : granulats coralliens, pêche pour l'alimentation, collecte et exportation de mollusques nacriers, production de perles, poissons d'ornement. À l'échelle du Pays de très nombreux programmes de surveillance de l'état des récifs et des perturbations qu'ils subissent, naturelles et anthropiques, ont été mis en place : perturbations cycloniques et sismiques, qualité des eaux, état de santé des peuplements benthiques et ichtyologiques, pathologie des nacres, radiobiologie. Toutes ces données recueillies au fi l des décennies ont permis d'établir l'importance relative des dégradations naturelles et anthropiques sur les récifs et lagons polynésiens et d'expliquer leur état de santé actuel en considérant différentes échelles spatiales. Les périodes cycloniques dévastatrices pour les récifs sont rares (1903-1906, 1982-1983 et épisodiquement) mais les cyclones ont parfois anéanti les communautés coralliennes de pentes externes dans certaines îles. Les blanchissements suivis de mortalités importantes à des échelles spatiales diverses, ont été surtout ceux de 1991, 1994 et 2003. Les explosions démographiques d'Acanthaster ont détruit de nombreux récifs (lagons et pentes externes) en 1978-1982 et une nouvelle pullulation s'amplifie depuis 2006 dans plusieurs îles de la Société. Les crises dystrophiques n'ont perturbé qu'épisodiquement certains lagons. [...]

La Polynésie française, 118 îles au coeur du Pacifi que, possède une surface de plus de 15 000 km2 de récifs et lagons gérés par le gouvernement polynésien. Le tourisme et la perliculture représentent les deux ressources économiques majeures du Pays. Les formations récifales très diversifi ées sont parmi les mieux connues. Plusieurs suivis d'exploitation des ressources sont opérationnels depuis des décennies : granulats coralliens, pêche pour l'alimentation, collecte et exportation de mollusques nacriers, production de perles, poissons d'ornement. À l'échelle du Pays de très nombreux programmes de surveillance de l'état des récifs et des perturbations qu'ils subissent, naturelles et anthropiques, ont été mis en place : perturbations cycloniques et sismiques, qualité des eaux, état de santé des peuplements benthiques et ichtyologiques, pathologie des nacres, radiobiologie. Toutes ces données recueillies au fi l des décennies ont permis d'établir l'importance relative des dégradations naturelles et anthropiques sur les récifs et lagons polynésiens et d'expliquer leur état de santé actuel en considérant différentes échelles spatiales. Les périodes cycloniques dévastatrices pour les récifs sont rares (1903-1906, 1982-1983 et épisodiquement) mais les cyclones ont parfois anéanti les communautés coralliennes de pentes externes dans certaines îles. Les blanchissements suivis de mortalités importantes à des échelles spatiales diverses, ont été surtout ceux de 1991, 1994 et 2003. Les explosions démographiques d'Acanthaster ont détruit de nombreux récifs (lagons et pentes externes) en 1978-1982 et une nouvelle pullulation s'amplifi e depuis 2006 dans plusieurs îles de la Société. Les crises dystrophiques n'ont perturbé qu'épisodiquement certains lagons. Si les événements naturels précédents dégradent les récifs à l'échelle de plusieurs îles, d'archipel ou du Pays, les dégradations anthropiques sont limitées à quelques îles peuplées de la Société, plus exceptionnellement dans les atolls et encore moins dans un tiers d'entre eux qui sont inhabités. Les remblais en zone frangeante, les extractions de matériaux coralliens, la surpêche, l'absence de réseaux d'assainissement des eaux usées urbaines et le développement d'activités de loisir et du tourisme sont les causes essentielles de la dégradation des communautés coralliennes du lagon dans certains secteurs de Tahiti et de Moorea. Ainsi apparaît-il clairement que les dégradations majeures des récifs en Polynésie sont occasionnées par des phénomènes naturels compte tenu de leur étendue géographique. En revanche les dégradations anthropiques sont géographiquement plus localisées. Malheureusement la synergie des deux causes de dégradation ne facilite pas la récupération des récifs. Il est établi qu'une pente externe avec un recouvrement corallien de 50-60 % est à son optimum. Une dégradation majeure (cyclone, blanchissement, Acanthaster) réduit ce recouvrement à moins de 10 %. La communauté met une douzaine d'années pour revenir au recouvrement optimum si aucune autre perturbation importante ne survient. La très large majorité des 15 000 km2 de récifs et lagons de Polynésie française sont en bonne santé. Avec leurs voisins du Pacifi que Est et Central, ces formations coralliennes sont considérées comme les moins dégradées au monde et à faible risque de dégradation dans les prochaines décennies. Toutefois les inquiétudes sont grandissantes sur l'avenir des récifs dans le monde entier si l'on se réfère aux prédictions de changement climatique où les impacts majeurs tiendraient à l'augmentation des températures océaniques, à un renforcement des cyclones et à l'acidifi cation des eaux perturbant le métabolisme de calcifi cation des coraux.

International audience; Dispersal may be a critical factor in the ability of reef-building corals to recover after major disturbances. We studied patterns of geographical structure using four microsatellite markers in seven South Pacific populations of Pocillopora meandrina, a major coral species from Polynesia. Variation within populations showed evidence of heterozygote deficiency. Genetic differentiation between populations was detected at a large scale (2000 km) between the Tonga and the Society Islands. Within the Society Islands, four of the five studied populations from Bora Bora, Moorea and Tahiti were not significantly different from each other. Unexpectedly, one of the three populations surveyed in Moorea was genetically different from the other two populations of this island (that were 5 and 10 km apart), and from the populations of the other two surveyed islands in this archipelago. We cannot rule out the possibility that this pattern is an equilibrium state, whereby short-range dispersal is locally more differentiating than long-range dispersal, as has been suggested by similar patterns reported in other studies. An alternative explanation that is globally consistent with all observations is that this is the signature of a large-scale destruction event, as for instance a bleaching event, followed by the recent restoration of populations by new colonists.