Allometric Growth and Morphological Plasticity of the Fiddler Crab <i>Tubuca rhizophorae</i> (Ocypodidae: Decapoda) in the Mekong Delta, Vietnam

This study examines allometric scaling of five morphological traits in the fiddler crab Tubuca rhizophorae from the Mekong Delta, Vietnam, based on 760 individuals (493 males, 267 females) collected monthly from intertidal mudflats in Bac Lieu and Ca Mau between March 2024 and February 2025. We analyzed how left–right carapace width (CWlr), right–left carapace width (CWrl), abdominal width (AW), and abdominal length (AL) scale with body size (carapace width, CW). Using log‐log linear mixed‐effects models, we evaluated scaling relationships across sex and season. Isometry in log–log space was explicitly defined as b = 1 and assessed via 95% confidence intervals (CIs) of group‐specific slopes, while fixed‐effect t ‐tests tested whether slopes differed from 0. Both CWlr and CWrl generally showed negative allometry in the dry season and weak or undetectable scaling in the wet season, with no consistent sex effects. The AW exhibited sex differences in intercepts and seasonal differences in slopes, whereas AL exhibited a three‐way interaction (body size × sex × season) with the clearest size dependence in dry‐season males. Most slopes were &lt; 1, and many CIs overlapped 0, indicating slow or undetectable scaling rather than isometry. Model fit was moderate for carapace traits and lower for abdominal traits; temporal (month/season) effects exceeded site effects. These results show sex‐ and season‐specific variation in allometric slopes, while interpretations about plasticity or adaptation remain tentative without direct environmental and reproductive measurements.