Morphometric analysis of selected claw traits in Tubuca forcipata (Ocypodidae) from Southern Vietnam

Abstract

A total of 233 specimens of the fiddler crab Tubuca forcipata (189 males and 44 females) were collected monthly from intertidal mudflats in Dam Doi and Hoa Binh, Ca Mau Province, Southern Vietnam, from July to October 2025. Selected morphometric traits, comprising the carapace width left-to-right (CW_L–R), the carapace width right-to-left (CW_R–L), the manus widths (MW2 and MW3), the pollex width (PW5), and the dactyl width (DW7), were measured to 0.01 mm. Generalized linear models with a gamma distribution and log link were used for trait comparisons; cheliped traits were analyzed with sex and the carapace width (CW_L–R or CW_R–L) as predictors. The carapace width did not differ by sex (p > 0.05), whereas MW2, MW3, PW5, and DW7 were significantly larger in males after adjusting for the carapace width (β_sex = 0.156–0.257; rate ratios (RR) = 1.17–1.29; p < 0.001). The carapace width had strong positive effects on all cheliped traits (β_CW = 0.054–0.070; RR ≈ 1.06–1.07; p < 0.001). Model diagnostics showed χ²/df < 1, indicating no overdispersion. These results demonstrate pronounced sex-linked functional dimorphism in the chelae of T. forcipata despite similar overall body size, providing a quantitative morphometric baseline for ecological and evolutionary studies of mangrove crabs in the Mekong Delta.