Bivalve aquaculture is an important activity able to provide healthy food and has a low carbon footprint. T his activity is threatened by climate extreme events such as marine heatwaves (MHWs), which are increasing their frequency, duration and intensity every year. In shellfish farming the scope for contrasting the impact of MHWs is limited. H eat-priming may be an effective solution to mitigate the negative consequences of these events. Priming is a plastic response of the phenotype triggered by non-lethal stress stimuli, which might help reducing the impact of a subsequent lethal stress. In this work we assessed whether heat-priming in Manila clams for 7 days at 30 ˚ C could increase survival to a lethal HW even a relatively long time (two weeks) after the end of the priming period and show long term (> one month) protective effects .
Half of a hatchery-produced population was subjected to priming ( Primed, P), while the remaining animals were kept at normal summer conditions (Naïve, N) for 7 days. A fter a recovery phase of 15 days, for each group half of the animals were exposed to a simulated HW (Heat wave challenged, H) , while the remaining were not (Controls, C), in a 2x2 full factorial design ( four groups: PH, PC, NH, NC; Fig. A). Mortality was recorded daily. Immediately after the HW, clam burrowing behaviour was tested, while 15 days after the end of the thermal challenge, the animal antioxidant activity, digestive gland transcriptome and microbiome were evaluated.
A higher survival rate in PH clams (compared to NH) was found after HW (Fig. B) . A t the behavioural level, 64% of PH clams were able to fully hide in the sand, while only 20% of NH clams did so. At the transcriptomic level we found upregulation of HSPs expression and metabolic pathways in PH clams. Beneficial bacterial taxa were more abundant in PH clams while families associated with detrimental effects were more abundant in NH clams. S ignatures of putative protective changes were evident 38 days after priming in PC clams (primed, but not exposed to HW). Those changes included higher antioxidant activity and upregulation of metabolic pathways (in contrast with NC clams). Overall, all the evidence suggests long-term protective effects of priming and its potential as a mitigation strategy to alleviate the negative consequences of MHW on this important aquaculture species.