Phylogeography of the Littoral Isopod <i>Ligia persica</i> Challenges the Tabula Rasa Hypothesis: Evidence for Pleistocene Refugia in the Persian Gulf

ABSTRACT Aim There are two hypotheses regarding the life history of the Persian Gulf's biota. The Tabula Rasa hypothesis posits that the Gulf was entirely devoid of marine life during the Last Glacial Maximum (LGM), either completely dry or containing only freshwater lakes. In contrast, estuarine and swamp‐associated taxa could have survived within the Gulf during the LGM and undergo peripheral speciation. The present study reconstructs the phylogeography of the littoral isopod Ligia persica to evaluate whether populations within the Persian Gulf originated from post‐LGM colonisation from the Gulf of Oman or persisted around the within‐basin water bodies during glacial periods. Location Persian Gulf and Gulf of Oman. Taxon Ligia persica . Methods We sequenced three mitochondrial markers (COX1, 12S, 16S) and reconstructed phylogenies using Bayesian inference and maximum likelihood. Phylogeographic structure was evaluated through haplotype networks, AMOVA, and Mantel test. Demographic dynamics were inferred from neutrality tests and mismatch distributions. Results Our analyses recovered five distinct lineages of L. persica , four of which are endemic to the Persian Gulf. Lineages 1 and 2 are restricted to Bushehr, Lineage 3 in Parsian, and Lineage 4 in Kish Island. Lineage 5 is distributed widely in most localities from the Gulf of Oman to the Persian Gulf but is absent in Bushehr. Molecular clock estimates and genetic distances indicate that these divergences occurred during the mid‐to‐late Pleistocene. The analyses revealed high genetic differentiation among localities and low genetic diversity within lineages and localities. Main Conclusion We infer that Lineage 5 expanded from the Gulf of Oman into the Persian Gulf after post‐LGM sea‐level rise, overlapping spatially with endemic Persian Gulf lineages, but failing to extend as far west as Bushehr. We further suggest that Lineages 1 and 2 originated in the Western Lake, whereas Lineages 3 and 4 evolved in the Central Lake. These divergences among these lineages likely occurred during major Pleistocene glacial cycles, when the lakes became isolated from one another and from adjacent open marine waters. This biogeographic pattern challenges the Tabula Rasa hypothesis and indicates that marine life persisted within the Persian Gulf during Pleistocene glaciations, including the LGM. The pronounced genetic structure and demographic patterns observed in L. persica reflect the combined effects of LGM‐associated bottlenecks, inbreeding, and the species' limited dispersal capacity.