The discovery of <i>Paratestophis gelicolus</i> gen. nov., sp. nov. from the rainbow water snake, <i>Enhydris enhydris</i> , in Thailand, with systematic update of Echinochasmidae Odhner, 1910

A new genus and species of trematode, <i>Paratestophis gelicolus</i> gen. nov., sp. nov., is described from the large intestine of the rainbow water snake, <i>Enhydris enhydris</i>, collected from several provinces in southern Thailand. Morphological analyses reveal distinct characteristics that differentiate <i>P. gelicolus</i> gen. nov., sp. nov. from related echinochasmid taxa, specifically its elongated bottle-shaped body, presence of 22 collar spines, parallel testes and parasitism of snakes-features not observed in other echinochasmid genera. Multi-marker phylogenetic analyses (28S rRNA, 18S rRNA, ITS2 and <i>COI</i>) strongly support its taxonomic placement within Echinochasmidae while confirming its genetic distinction from known genera such as <i>Echinochasmus, Stephanoprora</i>, and <i>Microparyphium</i>, thereby warranting the establishment of <i>Paratestophis</i> gen. nov. The species exhibited a 24% prevalence of infection (25/106) in <i>E. enhydris</i>, and was found co-infecting with four other helminths, including <i>Tanqua siamensis, Encyclometra bungara</i>, and two additional trematode species currently under examination, all occupy distinct ecological niches. Principal Component Analysis based on 19 morphological characters revealed morphological homogeneity among the specimens. This study represents the first record of a new genus and species within Echinochasmidae infecting snakes, and provides an updated systematic framework for the family, including a revised key to genera. The findings emphasise the need for further research into parasite taxonomy, host specificity and evolutionary relationships in Southeast Asian ecosystems.