Comparative analysis of claw morphology, handedness, and brachychely in Austruca annulipes and Austruca perplexa (Decapoda: Ocypodidae) in Ca Mau, Vietnam

This study compares external morphology, claw handedness, and relative claw exaggeration in two sympatric fiddler crab species, Austruca annulipes and Austruca perplexa , collected from Ca Mau Province, Vietnam. A total of 206 adult males were measured across 19 morphometric traits, including carapace size, claw elements, and walking leg proportions. Pearson’s correlation analysis was used to identify and remove highly collinear variables (| r | > 0.85) and redundant paired measures, resulting in six representative traits for further analysis. Morphological variation was examined using covariance (ANCOVA) and multivariate analysis of covariance with log-transformed carapace width (LogCWLR) as a covariate to control body size. Fourteen traits differed significantly between the two species in unadjusted comparisons, but size-corrected ANCOVA revealed significant interspecific differences in only MW2 and PL4. Principal Component Analysis of size-corrected residuals indicated broad morphological overlap, with Dim1 driven primarily by MW2 and PL4 and Dim2 by MW3 and ML1. Claw-handedness analysis revealed a significant right-biased pattern in A. annulipes , whereas A. perplexa exhibited a near-equal distribution. The brachychely index, calculated from deviations from allometric expectations, was significantly higher in A. annulipes , suggesting greater major claw exaggeration. These results indicate that claw development and handedness patterns may be subject to species-specific selective pressures rather than being conserved across the genus. This study provides a detailed morphological baseline for understanding functional traits and behavioral evolution in Austruca fiddler crabs. Clinical trial : This study does not involve any clinical trial.