Shape matters: Exploring morphological variation in

The non-indigenous mussel Mytella strigata is rapidly proliferating in Panguil Bay, Northern Mindanao, raising concerns about its impacts on biodiversity and coastal fisheries. Understanding its shell morphology is crucial, as shape variation can provide insights into its adaptability, dispersal potential, and interactions with the local environment. This study investigated shell morphological variation among individuals collected from the inner, middle, and outer zones of the bay, corresponding to a natural salinity gradient. Elliptic Fourier analysis, geometric morphometrics, and assessments of fluctuating asymmetry were employed to characterize shape differences and developmental instability. Shells from the inner zone exhibited dorsal expansion with ventral compression; middle zone shells showed generalized expansion except at the hinge; and outer zone shells displayed expansion with exceptions at the hinge and ventral regions. Consistent asymmetry between left and right valves, as well as pronounced individual-by-side variation, indicated the presence of fluctuating asymmetry across zones. Fourier analysis revealed distinct shape patterns: rounder and shorter shells in the outer zone, narrowly elongated shells in the middle, and broadly elongated shells in the inner zone. These morphological differences may indicate adaptive responses to environmental heterogeneity, supporting the invasive success of M. strigata and highlighting potential impacts on local ecosystems and fisheries.