Spatial variation in biometric parameters and condition index of Modiolus capax in contrasting coastal environments in the Gulf of California

The biometric parameters and organismal condition of marine bivalves are strongly influenced by environmental conditions such as temperature, food availability, and anthropogenic disturbance. Using biometric parameters, the condition index (CI), and weight-length relationships (WLR), this study evaluated spatial variation in the organismal condition of Modiolus capax at coastal sites with contrasting environmental conditions in the Gulf of California, Mexico. Mussels from the Santa Rosalía (SR) mining port, a site historically impacted by mining activities and characterized by lower temperatures and higher food availability (chlorophyll-a), exhibited larger shell sizes and greater visceral mass, along with negative allometric growth patterns associated with shell elongation. In contrast, mussels from the La Paz Bay sites with higher temperatures and lower food availability, and with different types and intensities of anthropogenic influence (thermoelectric plant and phosphorite mine), were consistently smaller and lighter. Biometric parameters, CI, and WLR revealed clear spatial variation, indicating marked differences in morphological responses and somatic growth patterns among sites. However, at SR, shell elongation altered growth allometry and skewed condition-index interpretations based on shell length, artificially lowering CI values despite high visceral mass. These differences indicate that local environmental conditions strongly modulate biometric parameters across sites and suggest that morphological responses may mask organismal condition when conventional indices are applied without accounting for shell shape variation associated with potentially stressful conditions. Overall, this study highlights the value and limitations of biometric and condition indices as integrative indicators of population dynamics, providing a baseline to integrate future biometric assessments with physiological and biochemical biomarkers in historically impacted coastal systems.