Physiological Effects of Salinity on the Osmotic Properties and Oxidative Stress Responses of the Razor Clam Solen regularis in Don Hoi Lot, Thailand

Estuaries are diverse coastal ecosystems that act as transitional zones between freshwater and seawater. The Don Hoi Lot tidal flat, located in the upper Gulf of Thailand, is one of Thailand's most important estuarine ecosystems. Nonetheless, the Don Hoi Lot area faces increasing environmental pressures due to human activities and natural changes. One of the most prominent species well-known in this area is the razor clam Solen regularis. This study investigated the effects of salinity fluctuations on the osmotic properties and antioxidant enzyme activities of the five tissues: foot, mantle, adductor muscle, gill, and digestive gland from the razor clam S. regularis collected from the Don Hoi Lot tidal flat. Razor clams were exposed to a range of salinity levels (0–35 PSU) for 7 days. The results indicated that the hemolymph osmolality of S. regularis increased with increasing salinity, demonstrating an osmoconforming pattern. Salinity changes significantly affected the activities of antioxidant enzymes (SOD, CAT, and GPX) in various tissues. In most tissues, SOD and CAT activities increased at higher salinities (30–35 PSU), while GPX activity generally increased across all tissues with increasing salinity. GST activity was not significantly affected by salinity changes. These findings suggest that S. regularis can activate antioxidant defense systems to reduce oxidative stress caused by salinity fluctuations. This study provides valuable insights into the physiological responses of S. regularis to environmental salinity changes, which can inform conservation efforts for this ecologically important species in estuarine ecosystems.