Salinity and sediment stress: Metabolic responses in the leaf oyster, Isognomon ephippium

Anthropogenic change is shifting hydrological patterns and degrading water quality in estuarine systems, where extreme rainfall and floods impact salinity and suspended sediment loads. This study investigated sublethal effects of combined salinity and turbidity stress on the leaf oyster, Isognomon ephippium, a species of interest for shellfish reef restoration. Using an orthogonal design, oysters were exposed to ecologically realistic salinity (10, 20, 35 ppt) and turbidity (0, 20, 50, 100 NTU) for 14 days. Catalase (CAT) and glutathione S-transferase (GST) activity were assessed in gill and gonad tissue after 7 and 14 days, and fatty acid profiles compared in gonad tissue after 14 days. CAT activity showed a significant salinity × turbidity interaction, with the largest increase under combined hyposalinity and moderate turbidity (10 ppt/50 NTU). There was an interactive effect of salinity, turbidity, and exposure duration on GST activity, with early induction at hyposalinity and moderate-high turbidity (10 ppt/50-100 NTU) followed by sharp declines. Lipid concentration peaked at intermediate salinity (20 ppt), while polyunsaturated fatty acids (PUFAs) were greatest at 10 ppt. PUFA composition was affected by turbidity, with females showing stronger, more salinity-dependent shifts than males in monounsaturated fatty acids, saturated fatty acids, and omega 3:6 ratios. Collectively, these results demonstrate that I. ephippium is vulnerable to the combined stressors of salinity and sedimentation associated with intensified rainfall and flooding. Short-term exposure elicited oxidative stress responses that may constrain energy allocation, alter fatty acid biosynthesis, and depress lipid reserves, with implications for reproductive physiology, population resilience, and restoration site selection.