Deep heat: a comparison of water temperature, anemone bleaching, anemonefish density and reproduction between shallow and mesophotic reefs

French Polynesia is experiencing increasing coral bleaching events in shallow waters triggered by thermal anomalies and marine heatwaves linked to climate change, a trend that is replicated worldwide. As sea surface thermal anomalies are assumed to lessen with depth, mesophotic deep reefs have been hypothesized to act as refuges from anthropogenic and natural disturbances, the ‘deep reef refugia hypothesis’ (DRRH). However, evidence supporting the DRRH is either inconclusive or conflicting. We address this by investigating four assumptions of the DRRH focusing on the symbiotic association between anemones and anemonefish. First, we compare long-term temperature conditions between shallow (8 m) and mesophotic sites (50 m) on the island of Moorea from 2011–2020. Second, we compare the densities of the orange-fin anemonefish, Amphiprion chrysopterus between shallow and mesophotic (down to 60 m) reefs across three archipelagos in French Polynesia. Finally, we compare the percentage of anemone bleaching, as well as anemonefish reproduction, between shallow and mesophotic reefs. We found that the water column was well mixed in the cooler austral winter months with only a 0.19 °C difference in temperature between depths, but in the warmer summer months mixing was reduced resulting in a 0.71–1.03 °C temperature difference. However, during thermal anomalies, despite a time lag in warm surface waters reaching mesophotic reefs, there was ultimately a 1.0 °C increase in water temperature at both 8 and 50 m, pushing temperatures over bleaching thresholds at both depths. As such, anemone bleaching was observed in mesophotic reefs during these thermal anomalies, but was buffered compared to the percentage of bleaching in shallower waters, which was nearly five times greater. Our large-scale sampling across French Polynesia found orange-fin anemonefish, A. chrysopterus, in mesophotic zones in two high islands and one atoll across two archipelagos, extending its bathymetric limit to 60 m; however, orange-fin anemonefish densities were either similar to, or 25–92 times lower than in shallower zones. Three spawning events were observed at 50 m, which occurred at a similar frequency to spawning on shallower reefs at the same date. Our findings of thermal anomalies and bleaching in mesophotic reefs, coupled with mainly lower densities of anemonefish in mesophotic populations, suggest that mesophotic reefs show only a limited ability to provide refugia from anthropogenic and natural disturbances

Understanding ecological functionning of ecological habitats, and building New Indicators based on Genetic Tools to assess their GES (Good Environmental Status).

International audienceThe coralligenous is a typical Mediterranean marine habitat and hosts the highest concentration of biodiversity of this sea. Paradoxically, unlike Posidonia meadows, it is poorly studied. Like the famous tropical coral reefs, it is a complex biogenic habitat, based on encrusting red algae (numerous metazoan phyla also participate to bio-construction)(Ballesteros 2006). Concerning societal interest coralligenous habitats are known to be very attractive diving spots and but the importance of this ecosystems for fisheries The societal interest relies on the setup of innovative, cost-effective and accurate methods for biodiversity characterization and monitoring, based on genetic tools rigorously inter-calibrated with traditional taxonomy and photo-quadrate approaches. The aim of this project is to improve our understanding of its functioning and resilience capacity, and establish biodiversity and connectivity patterns among localities and ecological profiles, providing information for rationalizing the design of Marine Protected Area networks and monitoring methods. To reach this goal, synergy of interdisciplinarity will be used, combining population genetics, community ecology, microbial ecology and physical oceanography. This requests four tasks, partly using the same data, but different analysis methods. 1- Establishment of the taxonomic composition of the engineer corallinale algae. Several cryptic species in red algae were revealed by genetic markers (cf. B-4). Molecular tools (IMBE) and classical taxonomy (Dr Marc Verlaque, MIO) will be used 2- Establishment of the species composition for numerous small quadrates of coralligenous using meta-barcoding in different ecological profiles 3- Population structure and phylogeography of two selected taxa: a red alga Lithophyllum spp. and a bryozoan Myriapora truncata, on the CIGESMED sampling network, contextualized by ecological factors (cf above). 4- Synthesis and Comparisons: Propose new GES (good environmental status) indicators for the coralligenous and biodiversity management rules

Understanding ecological functionning of ecological habitats, and building New Indicators based on Genetic Tools to assess their GES (Good Environmental Status).

International audienceThe coralligenous is a typical Mediterranean marine habitat and hosts the highest concentration of biodiversity of this sea. Paradoxically, unlike Posidonia meadows, it is poorly studied. Like the famous tropical coral reefs, it is a complex biogenic habitat, based on encrusting red algae (numerous metazoan phyla also participate to bio-construction)(Ballesteros 2006). Concerning societal interest coralligenous habitats are known to be very attractive diving spots and but the importance of this ecosystems for fisheries The societal interest relies on the setup of innovative, cost-effective and accurate methods for biodiversity characterization and monitoring, based on genetic tools rigorously inter-calibrated with traditional taxonomy and photo-quadrate approaches. The aim of this project is to improve our understanding of its functioning and resilience capacity, and establish biodiversity and connectivity patterns among localities and ecological profiles, providing information for rationalizing the design of Marine Protected Area networks and monitoring methods. To reach this goal, synergy of interdisciplinarity will be used, combining population genetics, community ecology, microbial ecology and physical oceanography. This requests four tasks, partly using the same data, but different analysis methods. 1- Establishment of the taxonomic composition of the engineer corallinale algae. Several cryptic species in red algae were revealed by genetic markers (cf. B-4). Molecular tools (IMBE) and classical taxonomy (Dr Marc Verlaque, MIO) will be used 2- Establishment of the species composition for numerous small quadrates of coralligenous using meta-barcoding in different ecological profiles 3- Population structure and phylogeography of two selected taxa: a red alga Lithophyllum spp. and a bryozoan Myriapora truncata, on the CIGESMED sampling network, contextualized by ecological factors (cf above). 4- Synthesis and Comparisons: Propose new GES (good environmental status) indicators for the coralligenous and biodiversity management rules

Symbiotic status does not preclude hybridisation in Mediterranean octocorals

International audienceUnderstanding how species can form and remain isolated in the marine environment is still an active research area. Correctly delimiting species if also of interest for biodiversity conservation and for a wide range of biological studies. Here we study the differentiation and the possibility of hybridization among three temperate octocorals : Eunicella cavolini, E. singularis and E. verrucosa. These species can occur in sympatry and morphologically intermediate individuals have been observed. Among these three species, E. singularis is the only one known to show a mutualistic symbiosis with photosynthetic Symbiodiniaceae. As symbiotic relationships can be involved in species barriers, we tested here the possibility of hybridization between symbiotic and non-symbiotic Eunicella species. Through multivariate analyses and hybrid detection, we prove the existence of current gene flow between E. singularis and E. cavolini, with the observation of F1 and F2 hybrids and backcrosses. Demographic inferences indicate a scenario of secondary contact between these two species. Our data suggest an intermediate abundance of Symbiodiniaceae in the hybrids of the two species. We discuss the evolution of the Symbiodiniaceae / cnidarian symbiosis in the light of our results

Copernicus Marine Service Ocean State Report, Issue 3 Introduction

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