This study investigated the effects of reduced salinity on the heterotrophic capabilities of juvenile giant clams Tridacna gigas under hatchery conditions. Juvenile T. gigas were selected randomly and exposed to salinity levels of 18,25, and 35 (control)for 14 days. Clearance rate (CR)and absorption efficiency (AE) were determined on days 1,4, 7, 10, 12, and 14.The CR seems to be highly dependent on salinity—highest in the control, followed by salinity 25, and it was considerably reduced at salinity 18. There were significant differences in CR between treatments, except on day 14. Absorption efficiency was generally high, and was significantly different only on day 4. There was a weak negative correlation between AE and salinity, but no correlation between CR and AE. The exposure to reduced salinity resulted in epithelial hyperplasia and hypertrophy in the clam's ctenidial lamellae. Cells were closely packed and exhibited highly granulated nuclei with ruptured nuclear membrane. There were fusions and elongations of the branchial filaments with eroded frontal cilia. These structural changes could reduce feeding. However, an acclimatization response was observed in which increased CR was noted at the end of the exposure, indicating that, to some extent, giant clam juveniles are capable of acclimating to prolonged exposure to reduced salinity.
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Vol. 33 • No. 2