Environmental fingerprints of cultivation site and depth: Variation in shell shape and coloration of the green mussel (Perna viridis) from the southeast coast of the Bay of Bengal, Bangladesh

Understanding how environmental gradients influence bivalve shell characteristics is essential for developing ecological knowledge, strengthening consumer preferences, and increasing market value. This study employed geometric morphometrics and digital color profiling to assess the impact of cultivation sites (Khurushkul, Moheshkhali, and Chowfaldandi) and depths (0.5, 1.0, and 1.5 m) in raft-based mariculture on the shell morphology and coloration of the green mussel Perna viridis along the southeastern coast of the Bay of Bengal, Bangladesh. At each site, a total of 150 mussels were collected directly from raft-culture socks, with 50 individuals sampled from each of the three depths, resulting in a total sample size of 450 mussels. Morphometric variation was assessed by geometric techniques, and coloring was measured using RGB, HSV, and CIELAB models under standardized imaging conditions. Environmental and trophic conditions varied across sites and depths, producing distinct phenotypic patterns in mussels. Khurushkul individuals were more elongated with the most saturated and brilliant green hues of coloration, Chowfaldandi mussels were compact and appeared pale and chalky in color, and Moheshkhali populations showed intermediate traits. Multivariate analyses supported strong spatial separation with >90 % classification accuracy. Depth gradients further shaped morphology and color: shallow mussels were larger and greener, mid-depth individuals exhibited a more vivid brownish tone, and deeper mussels were smaller, dorsally compressed, and weakly pigmented, reflecting reduced light, food availability, and hydrodynamic differences. These findings indicate that site-specific hydrographic conditions, associated food availability, and vertical gradients collectively shaped mussel phenotype, underscoring the adaptive plasticity of P. viridis to local ecological conditions. These findings also provide compelling evidence that P. viridis exhibits pronounced adaptive plasticity, enabling its phenotype to mirror localized ecological conditions. Ecologically, the study demonstrates that shell traits serve as sensitive integrators of environmental variability, making them powerful bioindicators for detecting habitat quality, trophic shifts, and environmental stress in tropical coastal systems. From a practical perspective, the phenotypic indicators identified in this study can inform site selection, optimize culture depth, and improve broodstock and farming management for green mussel mariculture. Overall, the study provides a framework for integrating shell-based phenotypic traits into aquaculture planning and coastal environmental monitoring in the Bay of Bengal region.