Rapid shift in benthic assemblages following coral bleaching at an upper mesophotic habitat in Taiwan

Mesophotic coral ecosystems (MCEs; typically, 30–150 m depths) have traditionally been considered potential refuges for shallow-water organisms, but recent evidence suggests that this role is context-dependent. Here, we document a singular habitat at the upper mesophotic depth (- 30 m) in Xiaoliuqiu, Taiwan, and assess the changes in the benthic assemblage one year after a heatwave affected the reefs. In the habitat studied, abundant branching depth-specialist corals were found free-living and thriving on the sandy-rubble bottom amidst dense filamentous turf algae. In 2022, 63.6% of the corals were observed bleached, which was associated with severe heat stress affecting shallow reefs. The dominant coral at the time, Acropora tenella, suffered the most from the bleaching, with only 9.7% of its population remaining healthy. After one year, there was a noticeable shift in dominance from A. tenella to the previously cryptic Anacropora spp. without an obvious change in coral cover. We hypothesize that this rapid shift is driven by Anacropora spp., benefiting from the dense canopy provided by A. tenella. This suggests that the composition of understory organisms may play an important role in the resilience of some reefs affected by disturbance. The characteristics of this habitat, which consists mainly of deep-water specialists, and its susceptibility to stressors indicate that this habitat is unlikely to serve as a refuge for most shallow-water taxa. Our findings reinforce that the effectiveness of MCEs as refuges is not universal, and emphasize the importance of incorporating these unique habitats into conservation strategies and gaining a deeper understanding of their functions before they are lost.

Methods
Full details on sample collection and analyses are available in the published manuscript.