Morphological plasticity in a reef-building coral is context-dependent and trades off with resistance to thermal stress

In sessile, long-lived organisms, such as reef-building corals, adaptive phenotypic plasticity may play a key role in mitigating negative fitness impacts arising from rapidly shifting environmental conditions under climate change. However, plasticity is often context-dependent and can have difficult-to-detect costs and limits, resulting in effects that can be adaptive, neutral, or even maladaptive. Here we show that morphological plasticity in the foundational Caribbean reef-builder, Acropora cervicornis, is neutral under ambient temperature conditions, but trades off with thermal stress tolerance during a marine heatwave in the field. While morphological plasticity was previously identified as adaptive in this system, we expand our understanding of plasticity by demonstrating neutral to maladaptive effects depending on the environmental context. These contrasting results are strengthened by our finding that plasticity changes over time, underlining the importance of considering the temporal dynamics of plasticity in long-lived organisms such as scleractinian corals. Our results reveal the nuanced role of plasticity in this climate sentinel species by demonstrating context-dependent costs under real-world thermal stress, revealing a possible constraint on the evolution of plasticity in a changing climate.