Morphological Variation of the Fiddler Crab <i>Tubuca paradussumieri</i> (Decapoda: Ocypodidae) Among Provinces in the Vietnamese Mekong Delta and a Classification Key for Genus <i>Tubuca</i>

Although Tubuca paradussumieri ​ (Decapoda: Ocypodidae) is an essential species in tropical mangrove ecosystems, little is known about how its intraspecific morphology varies depending on the habitat. Clarifying local adaptations, resource partitioning, and biodiversity patterns in vulnerable coastal ecosystems requires understanding these variances. From December 2023 to January 2024, 42 individuals (26 males and 16 females) were analyzed to capture sexual dimorphism and geographic variation in the morphological traits of T. paradussumieri across three Mekong Delta sites (Dam Doi–Ca Mau, Duyen Hai–Tra Vinh, and Cau Ngang–Tra Vinh). ANOVA was used to compare meristic ratios after measuring key morphological features (carapace width, manus, pollex, dactyl, and propodus lengths). Significant spatial divergence was found in the results: males from Ca Mau had larger claws (propodus length: 71.72 ± 8.63 mm) than those from Tra Vinh populations (24.75 ± 3.77 mm; p &lt; 0.001), which was probably caused by the different substrates (soft mudflats vs. sandy soils). The claw sizes of females were consistent across sites ( p &gt; 0.05), indicating selective pressures specific to sex. This study highlights T. paradussumieri ’s function as a bioindicator for habitat‐specific adaptations by offering the first quantitative proof of phenotypic plasticity in the Mekong Delta. To help with field identification and conservation, we also created a taxonomic key for Vietnamese Tubuca species. Our results highlight how crucial region‐specific morphological research is for guiding ecosystem management in mangrove ecosystems that are changing quickly.