Molecular Phylogeny, Species Delimitation, and Biogeography of the Varunid Crab Genus Metaplax (Crustacea, Varunidae)

ABSTRACT The Indo‐West Pacific genus Metaplax comprises 11 recognised species inhabiting intertidal mudflats, some adjacent to mangroves. To resolve long‐standing uncertainties, we analysed mitochondrial (COI, 16S) and nuclear (28S) markers. Phylogenetic analyses recovered Metaplax as monophyletic and resolved four well‐supported clades—the M. distincta , M. elegans , M. crenulata and M. indica clades—corroborated by morphology. COI barcoding delimited all 11 species, with intraspecific distances ≤ 2.01% and interspecific distances ≥ 5.76%, establishing a clear barcode gap. Molecular‐clock estimates date the split of Metaplax from other varunids to ca. 31.6 Ma, with subsequent diversification into four clades between 29.8 and 26.3 Ma. Ancestral‐range reconstructions support an Indian Ocean origin followed by stepwise dispersal into the western Pacific. These findings clarify interspecific relationships, confirm the utility of COI for Metaplax species delimitation, and suggest an Indian Ocean origin with later west‐to‐east expansion.