Comparative analysis of size and shape variation in Tridacna maxima (Röding, 1798) along the central and northern Red Sea coasts of Saudi Arabia.

This study examines regional and genetic influences on shell morphology of the small giant clam Tridacna maxima from the central and northern Red Sea of Saudi Arabia using linear mixed model and geometric morphometrics. Shell height and length exhibit significant regional differences, with individuals from the North showing greater dimensions (height: F1,17 = 12.102, p = 0.003; length: F1,17 = 9.219, p = 0.007) than those from the Central region. No significant effects were observed for haplotype, side, or their interactions, suggesting that shell size variation reflects environmental rather than genetic drivers. Geometric morphometric analysis revealed that the first two principal components explained 44% of total shape variance, but discriminant analysis showed weak and nonsignificant group separation (Mahalanobis distance = 1.78-1.89; p = 0.999). The stability of shell form across haplotypes and regions indicates developmental canalization, with phenotypic plasticity influencing size under different environmental conditions. Larger shells in the North likely reflect more favourable hydrological and nutrient regimes, while smaller shells in the Central region may be linked to higher turbidity and anthropogenic stress. These findings highlight environmental modulation of clam growth in the Red Sea, supporting the view that regional habitat variability, rather than genetic differentiation, primarily shapes phenotypic diversity in T. maxima populations.