Shape and genetic patterns in Rhyssoplax olivacea (Spengler, 1797) from the eastern Mediterranean

Marine invertebrates with limited dispersal often exhibit population structuring shaped by historical isolation, local adaptation and/or habitat fragmentation. In this study, we examined the population dynamics of the chiton Rhyssoplax olivacea across five coastal regions of the Hellenic Seas, spanning both the Aegean and Ionian basins. We combined mitochondrial cyctochrome c oxidase subunit I sequencing with geometrics of three shell valves to assess spatial structuring and genotype–phenotype congruence. Genetic analysis revealed 24 unique haplotypes among 52 individuals and identified three distinct clusters: Ionio, Pagasitikos and a mixed Aegean group (Chalkidiki, Chios and Limnos). A similar clustering pattern was observed for the shape of the tail valve, suggesting a strong association with genetic structure. In contrast, the head and intermediate valves showed partial discordance, with Limnos morphologically aligning with Ionio despite genetic similarity to the Aegean group. These findings highlight the influence of both genetic divergence and phenotypic plasticity on shell morphology. The association between tail-valve shape and genetic clusters underscores the value of tail-valve shape as a phylogeographic marker, while the weaker correlation with the other valves suggests a potential environmentally mediated morphological variation. This integrative approach sheds light on local adaptation and emphasizes the need to consider both molecular and morphological data in marine biogeographic studies.