Species-specific retention vs. recovery of coral thermal tolerance following nursery propagation

Thermal screening of coral source material is likely crucial to enhancing long-term restoration success under ocean warming. It is unclear, however, whether reef-based donor colonies retain their thermal tolerance in a nursery environment. Here, we used CBASS acute thermal assays to compare standardized thermal tolerance thresholds (ED50s) of donor colonies from Acropora cytherea and Acropora florida from two sites in Pulau Lang Tengah, Malaysia to their 'nursery propagules' reared in a common garden coral nursery over 365 days. CBASS assays of reef-based donors and their nursery counterparts were conducted in parallel and over two seasons to assess retention of thermal tolerance following nursery rearing. After 6 months, average ED50s of A. cytherea nursery corals were significantly lower compared to their reef-based donor colonies, but such difference disappeared after 365 days. By comparison, no such differences were measurable for A. florida and thermal tolerances were retained. Further, we did not observe trade-offs between growth and thermal thresholds for either species. Based on our findings, in situ thermal tolerance differences are likely adaptive and, consequently, either retained or recovered in longer-term restoration settings. Our findings further imply that thermal screening should be conducted prior to nursery propagation to avoid selection based on long-term acclimation artifacts.