Interactive effects of marine heatwave and fish predator cues on F1 generation
Results in the F1 generation mostly confirmed and strengthened major findings of severe MHW impacts on P. incisus performance which were observed in our previous study focusing on the interactive effects of MHW and FPC on the life history traits of P. incisus for an entire one generation (Truong et al., 2020). Indeed, MHW exposure increased mortality, lowered hatching success, and cumulative nauplii and faecals. MHW increased mortality, which may be a result of physiological dysfunctions such as the collapse of ATP synthesis (Harada, Healy, & Burton, 2019), the damage of macromolecules (Somero, 2010), the higher cellular oxygen demand than the capacity of oxygen delivery (Pörtner, 2010). The reduced performance of P. incisuswas particularly strong for reproductive success with ~30-60% reduced the size of egg clutches, the percentage of females produced hatched eggs, hatched nauplii from a clutch, and cumulative nauplii under direct exposure to MHW are widespread in tropical copepods (Doan et al., 2019; Nguyen et al., 2020). The lower reproductive outputs are generally related to reduced grazing, thereby energy intake. While we observed a lower cumulative faecals of F1 P. incisus , the correlation of cumulative faecals and nauplii was insignificant, but a positive correlation of these two parameters was observed in F2 P. incisus .
The FPC presence caused a small increase in females with successfully hatched eggs, the number of nauplii hatched per clutch of P. incisus , and the FPC effect was stronger at 30°C than at 34°C, which may be an adaptive response to predators. For example, predation can cause 50-75% mortality in marine copepods, and increased reproductive outputs are a general mechanism to compensate for consumptive mortality (Hirst & Kiorboe, 2002). Importantly, the FPC-induced increase in nauplii production only occurred at the control temperature, but not under MHW may result from the reduced grazing as indicated by the MHW-induced lower cumulative faecals and higher energy demand for metabolism (Low et al., 2018). Stronger MHW-induced reductions in cumulative nauplii and faecals were observed in FPC-exposed P. incisus in our previous study (Truong et al., 2020), in which we could quantify both parameters for an entire adult lifespan.