Genomic evidence of local connectivity in the brooding Antarctic isopod Spinoserolis beddardi (Calman, 1920), a key component of coastal benthic ecosystems

The Antarctic coastal environment, influenced by a dynamic glacial and oceanographic history, hosts diverse benthic invertebrate communities. However, how these communities evolve across ecologically heterogeneous coastal habitats remains poorly understood. Population genomic structure in this extreme region is particularly relevant because pelagic larval stages are not common among polar benthic invertebrates, raising questions about how genomic homogeneity is maintained among nearby populations. To address this, we investigated the population genomic structure of the crustacean isopod Spinoserolis beddardi, a widespread brooding species in the South Shetland Islands, using 13,928 genome-wide SNPs generated through ddRAD-seq. A total of 75 individuals were sampled from three ecologically distinct sites on Livingston Island, covering different sedimentary and hydrological conditions. Multivariate and Bayesian clustering approaches revealed weak and mixed genetic signals, while pairwise FST values were close to zero and not significant. These results indicate an absence of detectable genomic structure at this fine scale, consistent with local homogeneity among sites. Demographic analyses suggested a long-term population expansion followed by a gradual decline toward the present, but this pattern was not corroborated by genomic differentiation, pointing instead to a lack of detectable genomic differentiation at the local scale. Taken together, our results suggest that local genetic homogeneity in S. beddardi may reflect local connectivity and/or shared demographic processes. This study underscores the importance of integrated genomic approaches at microregional scales to understand how Antarctic benthic invertebrates maintain genomic homogeneity and ecological connectivity across heterogeneous coastal habitats.