Abstract
Marine amphipods are crustaceans that lack a larval phase and consequently have low dispersion rates. Despite that, these crustaceans present a remarkable ability to be transported by rafting on natural floating substrata, especially macroalgae, where they find shelter, food and a mating ground. The species Ampithoe marcuzzii is widely distributed throughout the western Atlantic Ocean. Here, it was used as a model to study seascape genomics and phylogeography in invertebrates with low dispersion capacities. We anticipated that the lineages would present isolation-by-distance patterns. However, surface currents and other abiotic variables could facilitate connectivity among distant sites. Based on mitochondrial and nuclear genes, SNPs, and environmental associations, we observed the presence of a species complex within A. marcuzzii, separating mainland and insular populations. Each species showed an independent evolutionary history, with a strong latitudinal population structure and evidence of isolation-by-distance and isolation-by-environment, characterizing the 'continent' species. Historical expansion and environmental variables were observed associated with the southeastern population, and ecological niche modeling corroborated the region as a paleorefuge. Conversely, populations from 'islands' presented complicated evolutionary histories, with closer localities genetically isolated and distant localities connected. These findings indicate that insular populations with low dispersion capacity might be more susceptible to spatial connectivity by floating substrata and to changes in surface currents. In contrast, mainland populations might be more vulnerable to local climate changes due to lack of gene flow.