Stress responses of zooxanthellae in juvenile Tridacna gigas (Bivalvia, Cardiidae) exposed to reduced salinity

Storm events are common in tropical regions and predicted to increase in frequency and intensity attributed to climate change. The iconic giant clams thriving in shallow coral reef areas in the Indo-Pacific region could experience salinity fluctuations during these events characterized by extreme precipitation. Given the limited ecotoxicological studies on the effects of osmotic stress in tridacnids, this study investigated the acclimatization potential to different salinities i.e., 18, 25, and 35aEuro degrees (control) for 14 days in juvenile Tridacna gigas. Results showed significant decrease in the chlorophyll a concentrations in giant clams at 18aEuro degrees. This may be caused by the marked increase in number of degenerating zooxanthellae. Chlorophyll c and carotenoid concentrations were also significantly reduced at 18aEuro degrees. The zooxanthellae density decreased significantly on Day 1 in both low salinity treatments, although bleaching was not observed. Symbiont cell enlargement was also observed at both low salinity treatments, affecting pigment concentrations at 18aEuro degrees but not at 25aEuro degrees. This study shows that zooxanthellae in giant clams were affected at 18 and 25aEuro degrees, but exposure to the latter displayed acclimation response as indicated by recovery in the various measured parameters after 14 days of exposure.