Morphometric variation and geographic structuring in Contracaecum australe (Nematoda: Anisakidae) across cormorant hosts in South America

Contracaecum australe is one of the most common anisakid nematodes parasitizing cormorants throughout South America, yet information on the geographic structuring of morphometric variation remains limited. In this study, adult specimens of C. australe were recovered from five Nannopterum brasilianum and one Leucocarbo magellanicus from Buenos Aires Province and Patagonia, Argentina, and compared with published morphometric data from other South American localities. Morphological identification was based on light and scanning electron microscopy. Morphometric variation among populations was evaluated using univariate analyses and Principal Component Analysis (PCA). All specimens conformed to the diagnostic morphological characters currently recognized for C. australe . PCA revealed morphometric variation among some geographically distinct populations, particularly those from Córdoba (Argentina) and Rio Grande do Sul (Brazil), whereas specimens from Chile, northern Brazil, and the Argentine coast showed substantial overlap in multivariate space. Additional PCA analyses excluding specimens recovered from L. magellanicus yielded comparable morphometric patterns, suggesting that inclusion of specimens from L. magellanicus did not substantially influence the overall morphometric pattern. Scanning electron microscopy revealed subtle variation in the morphology of the interlabia, lips, and caudal papillae among populations, although these differences did not alter species identification. The present study provides the first confirmed record of C. australe in L. magellanicus and expands knowledge of the species’ morphometric variability across South America. Although the observed patterns are interpreted as morphometric variation within the nominal species, the absence of molecular data precludes evaluation of whether they reflect population structuring, phenotypic plasticity, or cryptic diversity. These results provide a baseline for future integrative studies combining detailed morphometric analyses with mitochondrial and nuclear molecular markers to clarify the population structure and taxonomic status of C. australe throughout its distribution.