Hypoxia as a physiological cue and a pathological stress for coral larvae.

Ocean deoxygenation events are intensifying worldwide and can rapidly drive adult corals into a state of metabolic crisis and bleaching-induced mortality, but whether coral larvae are driven into a similar metabolic crisis remains untested. We experimentally exposed apo-symbiotic coral larvae of Acropora selago to deoxygenation stress with subsequent reoxygenation aligned to their night-day light cycle, and followed their gene expression using RNA-Seq. After 12 hours of deoxygenation stress (~2 mg O2 L-1 ), coral planulae demonstrated a low expression of HIF-targeted hypoxia response genes along with a significantly high expression of PHD2 (a promoter of HIFα proteasomal degradation), similar to corresponding adult samples. Despite exhibiting a consistent swimming phenotype compared to control samples, the differential gene expression observed in planulae exposed to deoxygenation-reoxygenation suggests a disruption of pathways involved in developmental regulation, mitochondrial activity, lipid metabolism, and O2 -sensitive epigenetic regulators. Importantly, we found that treated larvae exhibited a disruption in the expression of HIF-targeted and conserved developmental regulators, e.g., Homeobox HOX, corroborating how changes in external oxygen levels can affect animal growth. We discuss how the observed deoxygenation responses may be indicative of a possible acclimation response or alternatively may imply negative latent impacts for coral larval success