Higher daily temperature range at depth is linked with higher thermotolerance in antipatharians from the canary islands.

Sensitivity to ocean warming is generally expected to be lower in populations from more heterogeneous thermal environments, owing to greater phenotypic plasticity and/or genotype selection. While resilience of benthic populations from thermally fluctuating environments has been investigated at a variety of spatial scales, this has received limited attention across depths and has remained unresolved for Antipatharian corals, key habitat-forming species across a wide bathymetric range in all of the world oceans. In this study, we aimed at addressing the thermal sensitivity of Antipatharian corals across depths characterized by different levels of temperature fluctuations. We used an acute ramping experimental approach to compare the thermal sensitivity of colonies of (1) the branched Antipatharian Antipathella wollastoni (Gray, 1857) from two distinct depths (25 and 40 m) in Gran Canaria (Canary Islands, Spain); and of (2) unbranched mesophotic (80 m) Stichopathes species, from Lanzarote (Canary Islands, Spain; S. gracilis (Gray, 1857)), and Stichopathes sp. clade C from Mo'orea, French Polynesia. Results showed that the daily temperature range in Gran Canaria was larger at mesophotic depths (3.9 °C vs. 2.8 °C at 40 and 25 m, respectively) and this coincided with lower thermal sensitivity in mesophotic colonies of A. wollastoni. Second, S. gracilis from Lanzarote showed a lower thermal sensitivity than the previously studied Stichopathes sp. clade C from Mo'orea (French Polynesia) inhabiting a less variable habitat. These results are in line with the climate variability hypothesis, which states that populations under more variable thermal conditions have a lower sensitivity to warming than those from more stable environments, as they have adapted/acclimated to these higher levels of temperature fluctuations.

Thermal performance with depth: Comparison of a mesophotic scleractinian and an antipatharian species subjected to internal waves in Mo'orea, French Polynesia.

Local thermal environment has a strong influence on the physiology of marine ectotherms. This is particularly relevant for tropical organisms living close to their thermal optimum, well exemplified by the increasing frequency of bleaching occurrence in shallow-water corals. Mesophotic Coral Ecosystems (MCEs) were suggested as potential oases, especially when they are submitted to internal waves inducing short-term cooling events. Indeed, probability of bleaching occurrence in scleractinians was reported to decrease with depth in Mo'orea as temperature variability increases. However, ecophysiological data are currently lacking to understand the cause of lower susceptibility/increased plasticity of deeper corals. A growing interest has been devoted the last decade to MCEs, but our understanding of the physiological performance of benthic organisms living in this environment remains relatively unexplored. To tackle that question, we first compared the metabolic responses (dark respiration, net photosynthesis and photosynthetic efficiency) of the depth-generalist scleractinian Pachyseris speciosa from two heterogeneous thermal environment (25 and 85 m depths) to acute heat stress to determine if the local thermal environment could predict coral response to warming. Then, we tested the thermal performance of two sympatric species (the scleractinian P. speciosa and the antipatharian Stichopathes sp.) to determine if there are inter-species differences in performances in species experiencing identical levels of temperature variability, at mesophotic depths (85 m). Results revealed broader thermal performances in the mesophotic P. speciosa compared to mid-depth ones, and constrained performances in the mesophotic antipatharian compared to the scleractinian species. We hypothesize that the high fluctuations in temperature due to internal waves in deeper areas contribute to the broader thermal performances of mesophotic P. speciosa. However, the constrained performances of the mesophotic antipatharian compared to P. speciosa suggests that other processes than the symbiosis with zooxanthellae also influence thermal performances of these mesophotic organisms. Our results supported that Stichopathes sp. lives close to its thermal optimum, suggesting a (relatively) cold thermal specialist strategy. In this context, composition of MCEs in the future is unlikely to shift to antipatharian-dominated landscape and will remain coral-dominated landscape.

Two decades of seawater acidification experiments on tropical scleractinian corals: Overview, meta-analysis and perspectives.

Ocean acidification has emerged as a major concern in the last fifteen years and studies on the impacts of seawater acidification on marine organisms have multiplied accordingly. This review aimed at synthesizing the literature on the effects of seawater acidification on tropical scleractinians under laboratory-controlled conditions. We identified 141 articles (published between 1999 and 2021) and separated endpoints into 22 biological categories to identify global trends for mitigation and gaps in knowledge and research priorities for future investigators. The relative number of affected endpoints increased with pH intensity (particularly for endpoints associated to calcification and reproduction). When exposed to pH 7.6-7.8 (compared to higher pH), 49% of endpoints were affected. The diversity in experimental designs prevented deciphering the modulating role of coral life stages, genera or duration of exposure. Finally, important bias in research efforts included most experiments on adult corals (68.5%), in 27 out of 150 (18%) coral ecoregions and exclusively from shallow-waters.

Thermal stress responses of the antipatharian Stichopathes sp. from the mesophotic reef of Mo'orea, French Polynesia.

Antipatharians, also called black corals, are present in almost all oceans of the world, until extreme depths. In several regions, they aggregate in higher densities to form black coral beds that support diverse animal communities and create biodiversity hotspots. These recently discovered ecosystems are currently threatened by fishing activities and illegal harvesting for commercial purposes. Despite this, studies dedicated to the physiology of antipatharians are scarce and their responses to global change stressors have remained hardly explored since recently. Here, we present the first study on the physiological responses of a mesophotic antipatharian Stichopathes sp. (70-90 m) to thermal stress through a 16-d laboratory exposure (from 26 to 30.5 °C). Oxygen consumption measurements allowed identifying the physiological tipping point of Stichopathes sp. (Topt = 28.3 °C; 2.7 °C above mean ambient condition). Our results follow theoretical predictions as performances start to decrease beyond Topt, with lowered oxygen consumption rates, impairment of the healing capacities, increased probability of tissue necrosis and stress responses activated as a function of temperature (i.e. increase in mucocyte density and total antioxidant capacity). Altogether, our work indicates that Stichopathes sp. lives at suboptimal performances during the coldest months of the year, but also that it is likely to have low acclimatization capacity and a narrow thermal breadth.

Deep-sea sharks: Relation between the liver's buoyancy and red aerobic muscle volumes, a new approach.

Shark's buoyancy depends on two types of force: (i) the hydrostatic force which is mainly provided by their liver filled with low density lipids and (ii) the hydrodynamic force which is provided by the morphology of their body and fins. Shallow-water shark species are usually negatively buoyant, whereas deep-sea shark species have been suggested to display neutral buoyancy. It has been suggested that species that are close to the neutrality would have less red aerobic muscle fibers. Here, we investigated several liver features (the hepatosomatic index, the oil content and the lipid composition) playing a major role regarding the buoyancy of three deep-sea shark species (Etmopterus molleri, Etmopterus spinax and Isistius brasiliensis) and one shallow-water counterpart (Galeus melastomus). We used FT-Raman and FT-MIR spectroscopy to qualify/quantify the lipid composition of their liver. Our results showed that most deep-sea shark species studied have liver features providing more buoyancy than their shallow-water counterparts, appart from E. molleri which shows liver's features that resemble more shallow-water shark species (e.g. G. melastomus). Finally, data regarding liver features of several deep-sea shark species from the literature were added and the red aerobic muscle distribution/proportion of nine species was measured, to reveal how these parameters might be related. Our results showed that sharks characterized by a liver providing more hydrostatic force possess proportionally less red aerobic muscles than sharks having a liver that contributes less to their buoyancy. Therefore, our results i.e. deep-sea shark displaying less red aerobic muscle with a liver providing more buoyancy, support low metabolic rates hence slow swimming speed.