Timing of marine heatwave exposure across developmental stages modulates the copper effects on fitness traits in tropical copepods

Tropical coastal ecosystems are increasingly exposed to multiple anthropogenic stressors, including marine heatwaves (MHWs) and metal pollution. Although the interactive effects of warming and contaminants on marine organisms are well documented, the role of stressor timing relative to developmental stages in shaping organismal performance remains poorly understood. Here, we investigated how the timing of MHW exposure interacts with copper (Cu) contamination across the life cycle of the tropical copepod Pseudodiaptomus annandalei . Copepods were reared under four thermal regimes (control at 30°C; early-stage MHW (ES-MHW); mid-stage MHW (MS-MHW); and constant MHW across all developmental stages at 34°C (ADS-MHW) crossed with two Cu concentrations (0 and 20 µg L⁻¹), and we quantified survival, size at maturity, and multiple reproductive traits and faecal pellet production. MHW and Cu interactively affected most life-history traits, with both the magnitude and direction of responses depending on the developmental stage at which thermal stress occurred. Constant MHW exposure reduced reproductive performance despite maintaining adult body size, whereas short-term MHWs during early and mid-development generated pronounced carry-over effects on adult traits. Cu reduced somatic size but produced variable effects on reproduction and feeding-related activity, which were amplified or attenuated depending on MHW timing. These results reveal that the temporal dynamics of MHW exposure relative to life-history stages critically modulate Cu effects in tropical zooplankton, which can have cascading effects on tropical coastal food web under the cumulative impact of climate change and pollutants.