Hidden in morphology, revealed by molecular genetics: synonymization of Gogatea burmanicus (Chatterji, 1940) with Gogatea serpentum (Gogate, 1932) (Digenea: Cyathocotylidae).

The classification of the digenean genus Gogatea Lutz, 1935 has been complicated for almost a century due to morphological variability and reliance on limited diagnostic traits. This study re-evaluates the taxonomic status of Gogatea serpentum (Gogate, 1932) and Gogatea burmanicus (Chatterji, 1940) using an integrative framework combining morphology and molecular phylogenetics. Trematodes were recovered from the gallbladder and intestine of the rainbow water snake (Enhydris enhydris) in southern Thailand. Morphological investigations included morphometrics, acetocarmine-stained preparations, scanning electron microscopy and multivariate analyses, while molecular analyses used mitochondrial COI and nuclear ITS2 and 28S rRNA markers. Both gonad-bearing and gonad-less individuals exhibited identical sequences across all markers, forming a strongly supported monophyletic group. Morphological variation was restricted to the presence or absence of gonads, with no separation detected by principal component analysis. These findings support the synonymization of G. burmanicus as a junior synonym of G. serpentum (following the original spelling by Gogate, 1932, as validated under the ICZN [International Code of Zoological Nomenclature]). The occurrence of gonad-less adults represents a biologically intriguing phenomenon, the causes of which remain unresolved but may involve developmental, host-related, or ecological factors. This study underscores the importance of combining molecular and morphological approaches for accurate delimitation of morphologically plastic digeneans. Updated morphological descriptions and molecular data for G. serpentum are provided, including morphometrics, staining profiles, scanning electron microscopy micrographs and genetic sequences. These findings refine the taxonomy of Gogatea, advance knowledge of helminth diversity in semi-aquatic snakes and support broader efforts in parasite systematics, host-parasite ecology and biodiversity monitoring in Southeast Asia.