Migration-Driven Phylogeographic Homogeneity in the Red Macroalga Halymenia durvillei (Halymeniales, Rhodophyta) Along the Andaman Coast of Thailand

The Thai-Malay Peninsula is an important marine biodiversity hotspot at the boundary between the Indian and Pacific Oceans. Yet, phylogeographic understanding of macroalgae along the Andaman Sea coast of Thailand remains limited relative to other benthic species. Here, by sequencing mitochondrial cox1 and chloroplast rbcL sequences from 333 specimens of the red seaweed Halymenia durvillei across 20 sites, we found low genetic diversity (11 and 4 haplotypes, respectively) and a lack of pronounced population structure. A single dominant haplotype per marker was restricted to Phuket Island and Phang Nga. Analysis of variance (AMOVA) revealed no significant genetic differentiation between the Myanmar Shelf and the Malacca Strait Shelf, with most of the variation (90.28–98.63%) partitioned within populations. Gene-flow estimates revealed significantly asymmetric migration among populations, with most sites (i.e., Ranong, Phangnga, and Phuket) functioning as both sources and sinks of migrants, broadly consistent with the dispersal mediated by ocean currents during the northeast monsoon season. Bayesian Skyline Plot analysis revealed a sharp demographic contraction during the early middle Miocene, followed by a progressive expansion since the early Pleistocene. The presence of endemic haplotypes in the northern Malacca Strait suggests this area likely served as a marine refugium during glacial maximal low stands. In brief, phylogeographic homogeneity of H. durvillei along the Andaman Sea reflects the combined effects of glacial bottlenecks during glacial sea-level fluctuations and postglacial recolonization driven by ocean circulations. These results highlight populations with unique genetic diversity (e.g., Tankhen Bay, Phuket, and Lam Kaen Beach, Phangnga) as candidates for in situ protection and integration into protected area networks under climate warming.