Influence of sex and body size on selected morphometric traits of Metopograpsus frontalis (Miers, 1880) from Southern Vietnam

Abstract

A total of 167 specimens of Metopograpsus frontalis (105 males and 62 females) were collected monthly from intertidal mudflats in Ganh Hao, Ca Mau Province, Southern Vietnam, from July to October 2025. Selected morphometric traits, including carapace width from left to right (CW_L–R), carapace width from right to left (CW_R–L), manus width (MW2, MW3), pollex width (PW5), and dactyl width (DW7), were measured with a 0.01-mm vernier caliper. Generalized linear models were fitted to evaluate the effects of sex and body size, using Gaussian models for CW and Gamma models with a log link for MW, PW, and DW. We found no significant sex-related differences in CW_L–R and CW_R–L (p > 0.05), whereas MW2, MW3, PW5, and DW7 were significantly larger in males (p<0.001). Sex coefficients (β=0.156–0.257) corresponded to rate ratios (RR) of 1.17–1.29, indicating that male claws were 17–29% larger after adjusting for CW_L–R and CW_R–L. Both CW variables had strong positive effects (β=0.054–0.070; RR=1.06–1.07; and p<0.001). All Gamma models showed χ²/df<1, confirming good model fit without overdispersion. These results provide quantitative evidence of sex-related variation in morphometric traits of M. frontalis and establish a morphometric baseline for future ecological and evolutionary studies of mangrove crabs in the Mekong Delta.