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

The classification of the digenean genus <i>Gogatea</i> 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 <i>Gogatea serpentum</i> (Gogate, 1932) and <i>Gogatea burmanicus</i> (Chatterji, 1940) using an integrative framework combining morphology and molecular phylogenetics. Trematodes were recovered from the gallbladder and intestine of the rainbow water snake (<i>Enhydris enhydris</i>) 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 <i>G. burmanicus</i> as a junior synonym of <i>G. serpentum</i> (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 <i>G. serpentum</i> are provided, including morphometrics, staining profiles, scanning electron microscopy micrographs and genetic sequences. These findings refine the taxonomy of <i>Gogatea</i>, 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.