Asymmetric depth acclimation and plasticity limit the refugial potential of mesophotic Porites astreoides

Mesophotic coral ecosystems have been proposed as climate refugia for shallow reefs, yet the capacity of mesophotic corals to persist across depth gradients remains unresolved. We conducted a 7-month reciprocal transplantation of the Caribbean coral Porites astreoides between shallow (10 m) and mesophotic (40 m) reefs to assess physiological, skeletal, and transcriptomic plasticity. Shallow colonies exhibited higher metabolic activity and skeletal extension, whereas mesophotic colonies showed reduced protein content, slower extension, and elevated expression of skeletal organic matrix genes. Transplant responses were asymmetric: shallow-to-deep corals acclimated through coordinated physiological and transcriptional adjustments, while deep-to-shallow transplants experienced elevated mortality and limited plasticity. SNP-based analyses suggest incomplete isolation across depth, while reciprocal transplant responses reflect greater phenotypic plasticity in shallow corals and indications of depth-specific specialization in mesophotic corals. Our findings indicate that shallow populations harbor greater acclimatory capacity, whereas mesophotic corals show constrained upward resilience, challenging the generality of deep reefs as refugia under rapid environmental change.