show Abstracthide AbstractMesophotic coral reefs have been proposed as refugia for corals, providing shelter and larval propagules for shallow-water reefs that are disproportionately challenged by global climate change and local anthropogenic stressors. Yet, knowledge of the capacity of coral larvae to adjust to different depth environments is still limited. In this study, planulae of the reef-building coral Stylophora pistillata from 5-8 and 40-44 m depth in the Gulf of Aqaba were tested in a long-term in situ translocation experiment for their ability to settle and acclimate to reciprocal depth conditions. We assessed survival rates, photochemical, physiological and morphological characteristics, as well as gene expression variations in juveniles grown at different depths, comparing them to non-translocated adults, juveniles and planulae. We found high mortality rates among mesophotic-origin planulae, irrespective of translocation depth. Gene expression patterns suggested that deep planulae lacked settlement competency and experienced increased developmental stress upon release. Symbiont photochemical acclimation to depth occurred rapidly within 8 days, with symbiont populations showing changes in photochemical traits but no symbiont species shuffling between deep and shallow juveniles. In contrast, coral host physiological and morphological acclimation were less evident. We observed minimal overlap in gene expression patterns between different life stages and depths, indicating that gene expression significantly depends on life stage. The study also identified a set of DEGs associated with initial stress responses following translocation, lingering stress response, and environmental effects of depth. In conclusion, though our data reveal rapid symbiont acclimation, host acclimation to match deep coral phenotypes was incomplete within 60 days for planulae translocated to different depths. These results have implications for understanding the ecological significance of mesophotic reefs as potential larval sources in the face of environmental stressors. Overall design: We sampled coral planulae from shallow (5m) and mesophotic reefs (45m) in Eilat, Israel. Gene expression patterns of planulae and their parents from each depth were assessed with RNAseq (Illumina HTS). Planulae were placed in situ in a fully corssed reciprocal depth transplant design. Samples for RNAseq were again taken at 8 and 60 days old.