The Role of Zooxanthellae in the Growth of the Giant Clam Tridacna maxima in Outdoor Aquaculture System, Red Sea, Egypt

The Tridacninae subfamily of the giant clams is vital for the biological functioning of coral reefs in the Red Sea. Despite a rise in the commercial mariculture of the giant clams in developing countries, there is still limited understanding of the factors that influence the growth rate of Tridacna species. This study aimed to address this gap by investigating the factors affecting the growth rate of Tridacna maxima under cultured conditions, including seasonal fluctuations, temperature, mantle colors, zooxanthellae density, and light energy absorption. Our findings indicate that all these factors influence the growth patterns of this species. Monthly and seasonal growth rate fluctuations correlate directly with temperature variations. Specifically, extreme winter and summer temperatures are associated with reduced growth rates, while milder temperatures in spring and fall correspond to higher growth rates. Additionally, the mantle color suggests a potential three-way interaction between growth rate, symbiont color, and zooxanthellae density. Individuals with brown mantles exhibited higher growth rates and greater zooxanthellae density. The number of zooxanthellae varied significantly among individuals of different colors, with brown individuals having much higher concentrations compared to blue-brown and blue individuals. Understanding the complex relationships among environmental conditions, symbiotic interactions, and growth dynamics is crucial for developing effective conservation and management strategies to protect T. maxima populations.