Thermal Tolerance with Depth : Comparison of a Mesophotic Scleractinian and an Antipatharian Species Subjected to Internal Waves in Mo’Orea, French Polynesia

As shallow water coral reefs are suffering from the increasing frequency of heat stress events, Mesophotic Coral Ecosystems (MCEs) were suggested as potential refuge, 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. However, ecophysiological data are currently lacking to understand the cause of lower susceptibility. To tackle that question, we first compared the metabolic responses (dark respiration, net photosynthesis and photosynthetic efficiency) of the depth-generalist scleractinian Pachyseris speciosa living at 25 and 85m depths to acute heat stress. Then, we tested the hypothesis whether the azooxanthellate antipatharian Stichopathes sp. is more resilient to heat stress than the scleractinian P. speciosa from the mesophotic environment (85m depth). Results revealed that mesophotic depth may provide a thermal refuge, not because they are protected from heat stress events occurring at shallower depths, but due to the higher thermal resilience of mesophotic colonies compared to shallow ones. We suggest that lower light levels and short-term cooling events at 85m, due to internal waves, contribute to this resilience. Unexpectedly, the azooxanthellate antipatharian Stichopathes sp. was more affected by heat stress than the scleractinian P. speciosa , despite living in the same environment (85m). This indicates that the refuge value of MCEs strongly differs according to taxa and, in particular, that cooling events generated by internal waves do not always induce adaptation/acclimatization to thermal stress. This further suggests that Stichopathes sp. is unlikely to dominate over other benthic taxa in future mesophotic landscapes in Mo’orea