An update on the phylogeny of capillariid nematodes based on 18S rDNA sequences of Amphibiocapillaria tritonispunctati (Diesing, 1851) and four other species

The nematode family Capillariidae represents a taxonomically complex and understudied group of parasitic nematodes infecting a broad range of vertebrate hosts. Despite more than 300 described species, phylogenetic relationships within the family remain unresolved due to limited molecular data and ongoing taxonomic revisions. In this study, we generated new sequences of the 18S ribosomal RNA gene for <i>Amphibiocapillaria tritonispunctati</i> from the Danube crested newt, <i>Triturus dobrogicus</i>, and four additional capillariid species (<i>Aonchotheca annulosa</i>, <i>Baruscapillaria inflexa</i>, <i>Eucoleus</i> sp. 1 from the common starling <i>Sturnus vulgaris</i>, and <i>Eucoleus</i> sp. 2 from the black-headed gull <i>Chroicocephalus ridibundus</i>) from birds and rodents, expanding the molecular dataset for the group. Phylogenetic analyses using Bayesian inference and Maximum Likelihood methods revealed <i>A. tritonispunctati</i> as the earliest-diverging lineage within the Capillariidae, suggesting deep evolutionary divergence. Our results also supported the monophyly of <i>Eucoleus</i> and <i>Capillaria</i> and confirmed the distinctness of <i>Baruscapillaria</i>. Morphological examination of <i>A. tritonispunctati</i> corroborated its identification and highlighted the weight of diagnostic characters of the genus <i>Amphibiocapillaria</i>. Our findings underscore the need for broader molecular sampling and integrative taxonomy to clarify capillariid systematics and host-parasite relationships, particularly among nematodes from cold-blooded vertebrates.