Genomic, morphological, and physiological insights into coral acclimation along the depth gradient following an in situ reciprocal transplantation of planulae

Abstract

Mesophotic 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. For mesophotic reefs to be a viable refuge, firstly, deep origin larvae must survive on shallow reefs and, secondly, the two environments must be physically connected. This study tested the first condition. Planulae of the reef-building coral Stylophora pistillata from 5 to 8 and 40–44 m depth in the Gulf of Aqaba were tested in a long-term reciprocal transplantation experiment for their ability to settle and acclimate to depth in situ. We assessed survival rates, photochemical, physiological, and morphological characteristics, as well as gene expression variation in juveniles grown at different depths, comparing them to non-transplanted adults, juveniles, and planulae. We found high mortality rates among all mesophotic-origin planulae, irrespective of depth. Gene expression patterns suggested that deep planulae lacked settlement competency and experienced increased developmental stress upon release. For surviving shallow origin juveniles, symbiont photochemical acclimation to depth occurred rapidly within 8 days, with symbiont communities showing changes in photochemical traits but no algal symbiont shuffling. Yet, coral host physiological and morphological acclimation towards deep coral phenotypes was incomplete within 60 days. Gene expression was influenced by both life stage and depth. A set of differentially expressed genes (DEGs) associated with initial stress responses following transplantation, latent stress response, and environmental effects of depth was identified. This study therefore refutes the Deep Reef Refugia Hypothesis, as the potential for mesophotic origin S. pistillata planulae to recruit to the shallow reef is low, however potential remains for shallow planulae to survive at mesophotic depths.