Phylogeny, taxonomy and intraspecific variation of the rocky-intertidal trematode Opecoelus adsphaericus (Opecoelidae: Opecoelinae), a parasite of Clinocottus analis (Perciformes: Cottidae) from Baja California

The trematode <i>Opecoelus adsphaericus</i> is a common parasite of <i>Clinocottus analis</i> in rocky intertidal biotopes of southern California (USA) and northwestern Baja California (Mexico). Since the original description of <i>O</i>. <i>adsphaericus</i>, morphological and molecular data have been lacking. This study aims to characterize the morphological and molecular variation of this trematode to support future inter- and intra-specific comparisons in the genus <i>Opecoelus</i>. Over a year (August 2021-September 2022), ten sampling events were carried out to obtain biological material for morphometric and molecular analyses of <i>O</i>. <i>adsphaericus</i> at El Mosquito (n = 9) and Playa Piedra Blanca (n = 1), Baja California. Morphometric data were obtained from 44 specimens, along with Scanning Electron Microscopy (SEM) images. Intra-specific morphological variation was quantified using Principal Component Analysis (PCA). Eight nuclear 28S gene sequences were obtained for phylogenetic analysis (Bayesian Inference and Maximum Likelihood), while 27 mitochondrial Cytochrome C Oxidase subunit I gene (COI) sequences were generated to characterize population genetic variation. The phylogenetic analyses placed <i>O</i>. <i>adsphaericus</i> within the Opecoelinae clade of Opecoelidae, as a sister group to <i>Opegaster</i>. This relationship suggests that <i>Opegaster</i> may be synonymous with <i>Opecoelus</i>, although broader taxon sampling is required to test reciprocal monophyly and clarify generic boundaries. The PCA revealed marked morphological polymorphism in <i>O</i>. <i>adsphaericus</i>. Our morphological analysis detected morphometric characters among <i>O</i>. <i>caballeroi</i>, <i>O</i>. <i>cameroni</i>, and <i>O</i>. <i>pacificus</i> that largely overlap with those reported in this study and with the original description of <i>O</i>. <i>adsphaericus</i>, suggesting that these taxa may represent junior synonyms of <i>O</i>. <i>adsphaericus</i>. Based on the analyzed barcode dataset, <i>O</i>. <i>adsphaericus</i> shows high intraspecific genetic variation at both study locations. Future research on populations along the Baja California Peninsula and California may reveal greater genetic and morphological diversity.