Phylogeography of the marine benthic alga <i>Gracilaria salicornia</i> (Gracilariales, Rhodophyta) in Southeast Asia

Abstract Recent and future coastal habitat degradation and climate change can result in the loss of genetic diversity, especially in Southeast Asia. Despite its economic importance for the agar industry, the phylogeography of the intertidal red alga Gracilaria salicornia from Vietnam remains unexplored. Here we used 399 mitochondrial COI sequences, including 55 newly generated from Vietnam, to decipher the phylogeographic pattern of G. salicornia . The COI phylogeny and haplotype network revealed the presence of three haplogroups in G. salicornia . Haplogroup I, comprising 18 haplotypes, predominantly inhabited the west of the South China Sea, while haplogroup II, consisting of eight haplotypes from Japan and the Philippines, likely reflecting the influence of the Kuroshio Current. A single haplotype from the Philippines was placed in haplogroup III. G. salicornia exhibited high genetic diversity, with neutrality test, mismatch distribution, and Bayesian skyline plot suggesting a sudden demographic or range expansion during the mid-Pleistocene. The divergence of haplogroups I and II, occupying the eastern and western sides of the South China Sea, likely resulted from sea currents and glacial sea-level changes. Our study on historical phylogeography offers valuable insights that will encourage further research into the population history of tropical red algae in Southeast Asia, a region for its remarkable marine biodiversity.