Abstract
Ascaridia compar (Schrank, 1790) is a nematode parasite of wild galliform birds that has long been surrounded by taxonomic uncertainty due to overlapping morphological features with several historically described species. In this study, we provide molecular evidence supporting the species identity and preliminary phylogenetic placement of A. compar isolated from western capercaillie (Tetrao urogallus) in Bosnia and Herzegovina. Morphological analysis of seven adult nematodes (six females, one male) revealed diagnostic traits consistent with assignment to the genus Ascaridia, however, because morphological characters overlap among several historically described grouseassociated taxa, species-level identification was subsequently supported by molecular analysis of the ITS1, cox1, and cox3 gene regions and comparison with homologous sequences available in GenBank. Phylogenetic analysis supported the identification of the capercaillie-derived specimens as A. compar, with high bootstrap support distinguishing them from A. galli, A. columbae, and A. nymphii. Pairwise distance analysis showed a 1–4% divergence from the Italian A. compar isolate, and significantly greater divergence from congeneric species. This study contributes preliminary molecular evidence relevant to a longstanding taxonomic ambiguity and supports the distinction of A. compar from currently compared congeners, although broader sampling across multiple hosts and populations is needed to confirm these patterns. The presented findings highlight the relevance of integrating morphological and molecular data in nematode systematics and point to the need for future studies on parasite dynamics in declining grouse populations.