Survival of coral juveniles at mesophotic depth during the massive bleaching event

Abstract

As shallow coral communities face increasing massive bleaching events, deeper environments may provide alternative sites for coral recruitment due to reduced seawater temperature and light intensity. However, the deeper depths' role as alternative settlement habitats for shallow coral larvae remains unclear. This study examined whether juvenile of shallow corals can survive in deeper depth environments under ocean warming. The larvae from two depth generalist corals, Galaxea fascicularis and Seriatopora hystrix, were obtained from shallow colonies and settled on settlement plates. The plates were attached to PVC pyramid frames and transplanted at different depths (5, 20 and 40 m). Their survival was monitored monthly with in situ pictures. No individuals survived at 5 m depth, whereas some juveniles of both species persisted at mesophotic depth, suggesting successful recruitment under thermal stress. The presence of a relatively greater number of survivors at deeper depths suggests that MCEs may provide partial refuge, although further long-term and multi-site studies are needed to confirm this potential.