Thermal tolerance of black corals (Cnidaria: Hexacorallia: Antipatharia): A global comparative approach

Antipatharians or black corals are non-calcified colonial marine hexacorals that do not rely on algal symbionts for energy acquisition. For these reasons, they are present above and below the photic layer, with a high number of species described at mesophotic depths. They can form high-density aggregations referred to as black coral forests, serving as habitat, nursery or feeding area for a myriad of associated species, thereby creating biodiversity hotspots throughout the world oceans. Because black corals are threatened by anthropogenic activities (overharvesting, bottom trawling), they are listed under the CITES appendix II. Yet, the effects of another major threat, ocean warming, remained uninvestigated on this ectotherm taxon. Therefore, this thesis aimed at assessing the effects of increased seawater temperatures on antipatharians under laboratory-controlled conditions. As this was the first study of its kind, a global and comparative approach was used by considering species of various morphologies (branched and unbranched) and from different families (Antipathidae and Myriopathidae), climatic regions (Atlantic, Pacific and Indian Oceans, Mediterranean Sea) and depths (25 to 80m). Local bottom temperatures were recorded in each site and antipatharians were submitted in the laboratory to short/mid-term heat stress. Thermal performance curve for oxygen consumption were established for each studied species and their shapes (optimum temperature, width of the thermal window) were compared across species and sites. Other studied endpoints included tissue necrosis, healing capacity, release of bailout propagules as well as mucus and total antioxidant productions. Results showed that tolerance of antipatharians vary between species and sites. A large local annual temperature range was linked with a higher thermal tolerance evidenced by a wide thermal window and low or no impact on measured endpoints. On the contrary, a small annual temperature range went together with deleterious effects from a few degrees above the optimum temperature, in accordance with the climate variability hypothesis. In a sympatric context, unbranched species appeared more tolerant than branched ones. After this first analysis, phylogeny did not appear to influence thermal tolerance. So, temperature only appears as a factor limiting the upper depth distribution of antipatharians from sites with a small annual temperature range and other factors such as substrate availability, high hydrodynamism or interspecific competition are likely to contribute to the limitation of most populations to shallower depths. We concluded that antipatharians that are currently submitted to a large annual range of temperatures are unlikely to be impacted by ocean warming in comparison with those living in less heterogeneous thermal environments. In a sympatric context, unbranched species are likely more tolerant to ocean warming than branched ones.Doctorat en Sciences