Geometric Morphometric Analysis of Goatfish Scale Outlines: Spatial Gradient of Shape Variation

Ctenoid scales of teleost fishes are dermal bony structures that vary in shape both among species and across different body regions of a single individual. To investigate this variability, 824 scales were collected from multiple body regions of six goatfish specimens belonging to two species, Mullus barbatus and Mullus surmuletus (mean total length ± SE: 22.5 ± 0.9 cm). Scale shape was quantified using outline-based geometric morphometrics, with elliptical Fourier analysis followed by principal component analysis, in the R package Momocs. Results showed that scale shape overlapped extensively between species but varied substantially across body regions within individuals. Two broad scale morphological types were identified: short and wide versus elongated and narrow. In the two specimens examined for body curvature analysis, more elongated scales were associated with regions of higher curvature. These findings suggest that the primary axis of variation in scale shape may be more closely associated with anatomical factors related to body position, although the small sample size limits the generalizability of these discoveries. This study also highlights the potential of outline-based geometric morphometrics as an efficient and reproducible approach for quantifying scale-shape variation without relying on manually selected landmarks.