Conclusions and perspectives
There is a great concern that the hyperdiverse tropical ecosystems may be collapsed under the cumulative impact of multiple stressors (Barlow et al., 2018; Dinh, 2019; Worm et al., 2006). Among others, MHWs are becoming a new but severe threat, which sometimes causes massive and widespread mortality across taxa (Garrabou et al., 2009; 2019). However, we know much less about how MHW may affect key grazers and intermediate prey in the coastal marine ecosystems such as copepods, particularly in an ecologically relevant context with biotic interactions (Truong et al., 2020). Our results contribute to this by highlighting the severe direct effect of MHWs on reproduction and grazing P. incisus , a common copepods species in the tropical coastal ecosystems of Southeast Asia. Strong mortality together with reduced reproductive success are important mechanisms underlying the reduced copepod abundance under MHWs (Evans, Lea, Hindell, & Swadling, 2020). Consequently, MHWs may substantially reduce the secondary biomass production of the zooplankton, which fuels resources and energy to the vast majority of other marine predators such as corals, crustaceans, and fish (Arimitsu et al., 2021; Chew et al., 2012). Most importantly, our results suggest that P. incisus may evolve in a rapid transgenerational acclimation to MHW, which fully ameliorated its lethal effect in the second generation. The thermal acclimation comes with a cost of reduced reproduction, and grazing may be a crucial and widespread mechanism for invertebrates coping with the transient effects of heatwaves (Cavieres et al., 2020; Dinh et al., 2020; Doan et al., 2019). Interestingly, fish predator cues played a minor role in shaping both direct and transgenerational effects of MHWs, highlighting the dominant MHW effects on P. incisus . Given the widespread and severe effects of MHWs on coastal invertebrates, the dominant effect of MHWs is likely widespread with implications for ecological risk assessment under the rapidly changing environment of the tropical coastal ecosystems.