Thermal and chemical priming for resilience: a mitigation strategy against marine heatwaves

The Manila clam (Ruditapes philippinarum) is a bivalve species with a broad native and introduced distribution holding significant economic and ecological importance. Climate change, characterized by a rise in average global temperatures, is one of the major global issues. In particular, marine heatwaves — defined as prolonged periods of abnormally high seawater temperatures and closely linked to anthropogenic climate change — have been shown to impact marine life at multiple biological scales, including individual organisms, species, populations, and ecosystem services. These extreme events have been linked to alarming mass mortality events in bivalve populations. In this context, the development of effective mitigation strategies is urgently needed. One promising approach is priming, described as the controlled exposure to a sublethal stress to enhance an organism's ability to cope with future environmental challenges. This study focuses on testing different priming strategies on Manila clam. Both thermal and chemical sub-lethal stresses were used as priming agents. Clams were divided into four groups that underwent the following treatments, with a duration of 7 days: no treatment (control), heat priming, chemical priming (hydrogen peroxide), and the combination of the two. After a period of resting (7 days), clams experienced a 7 days challenge during which they were exposed to daily 34°C heatwaves. The clams' digestive glands were sampled prior to priming, after priming, and after challenge. Clam performance was evaluated using the following methods: survival monitoring over the following 14 days, hepatosomatic index (HSI) and condition index (CI), burrowing test, and transcriptional/biomarker analysis of samples. To evaluate the effectiveness of priming as a mitigation strategy, the same treatment was applied to a second batch of clams for field testing. This approach aimed to assess the animals' responses to summer heatwaves under natural environmental conditions, thereby validating the potential of priming in realistic settings.