Patterns of Dorsal Fin Shape Variation in Commerson’s Dolphins Across Isolated Subantarctic Populations

Understanding morphological variation across geographic space is central to evolutionary and functional studies of marine vertebrates. Here, we examine variation in dorsal fin shape in the Commerson’s dolphin (Cephalorhynchus commersonii), a small coastal cetacean with a disjunct distribution across South America and the Kerguelen Islands. Dorsal fin shape was quantified using geometric morphometrics based on photographs, and comparisons were conducted across subspecies and populations. Principal Component Analysis revealed partial overlap in morphospace, with higher shape variability in the Kerguelen subspecies. Canonical Variate Analysis showed separation among geographic groups, although patterns of overlap remained. Differences in mean fin shape were observed among populations, particularly in the relative development of the free span and base of the fin. Kerguelen dolphins exhibited a relatively broader fin base and reduced free span, whereas South American populations generally showed a wider free span, with additional variation among regional populations. Morphological patterns did not fully correspond to population genetic structure, suggesting that multiple factors may contribute to phenotypic variation in this species. Similarities between geographically distant populations highlight the potential influence of shared environmental conditions. These results emphasize the value of dorsal fin morphology for investigating spatial variation in small cetaceans and provide a basis for future integrative studies combining ecological, physiological, and genetic data.