Diversity and distribution of cryptic species within the Mugil cephalus species complex in Vietnam

Mugil cephalus sensu lato is a globally distributed complex of cryptic species whose distribution range and evolutionary history remains largely unknown. In the North West (NW) Pacific three species have been identified genetically among fish described morphologically as M. cephalus. Their distribution ranges are largely parapatric and has been proposed to mirror different thermal preferences. To date, few samples have been analyzed from South China Sea, which limits inferences on the evolutionary history of the species complex. We sampled fish identified morphologically as M. cephalus along Vietnamese shores and characterized them using the sequence polymorphism of two mitochondrial genes, the cytochrome oxidase I and cytochrome b. This demonstrated that all three species described in the NW Pacific are present in both northern and southern Vietnamese waters. Although the difference in species abundance reflects those observed in the NW Pacific, no phylogeographic pattern was revealed. In addition, no population structure was observed whatever the species or the distribution range considered, which indicates a significant level of gene flow that maintains genetic homogeneity of the three species. It is also conceivable that each species experienced a recent population expansion from a single ancestral population. Finally we suggest that if the cold waters of the NW Pacific present a physiologic challenge leading to the almost parapatric distribution of the three species, then it is likely that the warm surface temperatures of the South China Sea negate this barrier.