Zooxanthellae Densities within the Fluted Giant Clam Tridacna squamosa (Lamarck, 1819) from the Southern Coast of Egypt’s Red Sea

Giant clams are large bivalves predominantly found in tropical and subtropical seas. They are the largest bivalves with the fastest growth rates, thanks to their symbiotic relationship with phototrophic dinoflagellates (zooxanthellae). This study examined the dynamics of the symbiotic relationship between Tridacnid hosts and their dinoflagellate symbionts, as well as the environmental factors influencing this association. Zooxanthellae were extracted from the mantles of Tridacna squamosa samples monthly collected from January to December 2021 in the southern Red Sea. The mean density of zooxanthellae was calculated as the overall mean count of zooxanthellae per gram of wet weight of the T. squamosa mantle, expressed as ± SD. This density was then analyzed alongside various environmental conditions and biological characteristics of the studied clams to assess the factors influencing the density of the symbiotic zooxanthellae within T. squamosa. The study revealed that the densities of zooxanthellae ranged from 3.21 x 10⁵ to 48.19 x 10⁵ cells per gram of mantle, observed in July and December, respectively. Maximum density corresponded with the shortest exposure to sunlight, measured at 10 hours and 42 minutes during winter, with a mean temperature of 24.8°C. Conversely, the lowest density was associated with the longest light exposure period, recorded at 13 hours and 39 minutes in summer. These results underscore the complex nature of the symbiotic relationship between zooxanthellae and T. squamosa in the Red Sea. The interaction between environmental variables and host characteristics shapes zooxanthellae densities within these giant clams across reef ecosystems. This highlights the need for interdisciplinary research collaborations and holistic conservation efforts to maintain the delicate balance between giant clams and their symbiotic zooxanthellae in the face of ongoing environmental and anthropogenic stressors.