Integrating physiological and molecular approaches to assess thermal tolerance in the Manila clam (Ruditapes...

Global climate change poses a significant threat to coastal aquaculture, with rising seawater temperatures considerably affecting the survival and distribution of economically important shellfish species. As a key aquaculture species, the Manila clam (Ruditapes philippinarum) faces the risk of mass mortality during summer heatwaves. Conventional methods for assessing thermal tolerance, such as the median lethal temperature (LT₅₀), are often destructive and inefficient in clams. In this study, we evaluated thermal tolerance by monitoring heart rate (HR) as an indicator of cardiac performance and physiological status across different clam strains, including a heat-tolerant strain and other selective strains, under three temperature conditions (20 °C, 26 °C, and 32 °C). The results revealed that the heat-tolerant strain exhibited markedly higher survival under 32 °C thermal stress, with a 49% survival rate over 504 h, compared to less than 10% in other groups. At 32 °C, the oxygen consumption rate of the heat-tolerant strain increased to 0.60 mg/(g·h), significantly higher than that of other groups. Concurrently, its ammonia excretion rate decreased to 0.22 mg/(g·h), which was significantly lower. Notably, the heat-tolerant strain also showed significantly elevated activities of antioxidant enzymes (SOD and GSH). Importantly, heart rate monitoring indicated that the heat-tolerant strain maintained stable cardiac performance under thermal stress, with heart rates increasing significantly and no arrhythmic events observed, whereas the control group displayed severe cardiac impairment. This study demonstrates that heart rate monitoring serves as an effective non-invasive indicator of thermal tolerance in the Manila clam. Its integration with physiological and cellular molecular analyses provides a comprehensive assessment framework. This multi-level approach offers a robust strategy for selective breeding of heat-tolerant strains in clam aquaculture and other aquatic species.