Coral reefs worldwide are threatened by climate change and anthropogenic development. At depths below 30 m, zooxanthellate corals can still be found and these ecosystems are referred to as Mesophotic Coral Ecosystems (MCEs). The Deep Reef Refugia Hypothesis suggests that due to the relative isolation by depth, MCEs may act as refuge for shallow corals and source of larvae to reseed shallow coral reefs. Evidence supporting this hypothesis was found in Japan for example with the discovery, in 2013, of the coral Seriatopora hystrix at 40 m depth, while this species had been locally extinct in nearby shallow reefs following massive bleaching events. However, this finding occurred nearly a decade after the first massive bleaching event and little is known on how massive bleaching events affect mesophotic corals.
In the summer 2016 massive bleaching affected the Yaeyama Islands. At this time, we investigated the corals from the surface down to 35 m depth, in order to evaluate if deeper parts of the reef indeed act as a refuge. We compared the data obtained to data obtained at the same sites in the years before bleaching. Here we will present these results and discuss their implication for coral reef conservation.