Exploring the symbiotic resilience of boring giant clams and zooxanthellae during heatwave conditions: An investigation into strategic countermeasures

Global warming has significantly increased the frequency of marine heatwaves, causing severe and long-term impacts on tropical marine ecosystems and coral reef systems. As a key component of coral reefs, giant clams (Tridacna spp.) and their symbiotic algae are critical to ecosystem health, yet their complex regulatory mechanisms under environmental stress remain poorly understood. Our study revealed that while temperature fluctuations minimally affected the growth rates of giant clams, they significantly reduced survival probabilities. Physiological indicators demonstrated that giant clams mitigate oxidative stress caused by heatwaves through the regulation of antioxidant enzymes, such as catalase (CAT), glutathione reductase (GSH), and acid phosphatase (ACP), which play a key role in immune regulation and stress mitigation. For symbionts, with the elastic change of biomass, photosynthetic efficiency and chlorophyll content show dynamic changes, which is consistent with the pattern of symbionts' loss or death. Transcriptomic analysis further revealed distinct regulatory pathways for both giant clams and zooxanthellae under heatwave conditions, including nutrient metabolism, transport, energy distribution, and immunomodulatory responses. Notably, lipid metabolism genes were down-regulated in host cells, suggesting reduced metabolic activity, while the MAPK (Mitogen-Activated Protein Kinase pathway) pathway was highly activated in clams, indicating enhanced immune responses to temperature stress. Conversely, the ABC (ATP-binding cassette transporter) transporters were significantly upregulated in zooxanthellae, likely facilitating stress-related material transport. These findings highlight the need for integrated strategies targeting transcriptomic adjustments across symbiotic partners under stress-specific conditions, offering valuable insights into enhancing Tridacna resilience and addressing the broader impacts of climate change on marine ecosystems.