Morphological Classification of the Sagittal Otoliths of Two Species of Sciaenidae Based on the Landmark Point Method

Larimichthys crocea and Larimichthys polyactis, two commercially and ecologically important sciaenid species, are often morphologically confused (especially at the juvenile stage or for incomplete specimens), leading to limitations in traditional morphological taxonomic methods for accurate identification. Otoliths, as stable hard tissues with species-specific morphological characteristics, serve as an ideal tool for species discrimination. To investigate the efficacy of landmark-based methods in extracting morphological information from different surfaces of sagittal otoliths, this study analyzed six surfaces (medial, lateral, dorsal, ventral, anterior, and posterior) of left otoliths from two sciaenid species using geometric morphometrics. We collected 487 sagittal otolith samples from sciaenids in the Zhoushan Islands of the East China Sea (Larimichthys polyactis: 277 specimens; Larimichthys crocea: 210 specimens). Landmark coordinates were extracted using tps-series software, and morphological differences were quantified through principal component analysis (PCA), discriminant analysis, and thin-plate spline visualizations. Key results include: relative warp PCA showed cumulative contributions of PC1 + PC2 at 52.48% (medial), 52.87% (lateral), 71.29% (dorsal), 63.7% (ventral), 64.8% (anterior), and 67.85% (posterior), effectively discriminating species with Type I/III landmarks demonstrating highest contributions; centroid size analysis revealed significantly larger values in L. crocea across all surfaces (most pronounced on medial surface: F = 183.450, p < 0.05); discriminant analysis achieved peak cross-validated success on the medial surface (98.6% for L. polyactis, 95.2% for L. crocea), with other surfaces ranging from 79.6–83.6%. This confirms that multi-surface landmark analysis effectively captures morphological divergence, with the medial surface providing optimal species discrimination. The established method provides a reliable supplementary tool for the taxonomy of L. crocea and L. polyactis, and offers scientific support for fisheries resource survey, population dynamic monitoring, and conservation of these sciaenid species.