Geographic Patterns of Head Morphology in Syngnathus typhle Across Marine Regions

Geographic patterns in head morphology can reveal important aspects of population structure and phenotypic differentiation in marine species. We investigated these patterns in the broad-nosed pipefish Syngnathus typhle Linnaeus, 1758, a coastal species widely distributed across distinct marine regions (Baltic, North, East Atlantic, and Mediterranean seas). Using landmark-based geometric morphometrics, we quantified head morphology variation in 241 live, wild-caught adults. Analyses revealed consistent geographic patterns in snout elongation, head depth, and eye position. Multivariate comparisons showed significant morphological separation among marine regions, with high classification success based solely on head morphology. These results indicate that S. typhle populations are spatially structured and not morphologically homogeneous across their range. Our findings demonstrate the utility of geometric morphometrics to detect subtle geographic variation in morphologically conservative taxa such as pipefish and provide a quantitative baseline for future research on phenotypic diversity and conservation management. Importantly, the study is based on non-invasive monitoring protocols using live animals.