Predator cues increase negative effects of a simulated marine heatwave on tropical zooplankton

Mangroves and lagoons are spawning and nursery grounds of marine fish. These ecosystems are increasingly exposed to episodes of extreme temperatures from marine heatwaves (MHWs). However, it is unknown how MHW effects may interact with those of predators to affect the fitness of prey species in these ecosystems. To address the issue, we exposed the calanoid copepod Pseudodiaptomus incisus to a simulated MHW and with/without the presence of fish predator cues (FPC). The size at maturity, clutch size, hatching success, lifetime nauplii and faecal pellet production, and the adult lifespan were determined. All fitness parameters of P. incisus were lower under MHW. Remarkably, nauplii production was reduced 80%, and adult lifespan was shortened 50%, respectively. Overall, FPC increased the size at maturity and hatching at both temperatures. FPC also increased clutch size, nauplii and faecal pellet production in the control temperature, but decreased all three parameters under MHW. These results suggest that FPC may increase the vulnerability of P. incisus to MHWs by lowering reproductive success and reducing grazing rate. Our study sheds light on how interactive effects between biotic and abiotic factors may shape the vulnerability of marine copepods in mangroves and lagoons of tropical regions.