A new perspective on biogeographic barrier in the flathead grey mullet (Pisces: Mugilidae) from the northwest Pacific

The flathead grey mullet, Mugil cephalus has a global distribution in tropical, subtropical and temperate seas worldwide. Previous studies proposed that globally distributed M. cephalus comprises at least 14 different lineages, and divided into two or three groups in the northwest Pacific. Therefore, we hypothesized that there may exist at least two lineages in Korea.

The aim of this study was to determine the number of lineages of flathead grey mullet in Korea, and detail their biogeographic boundaries, and taxonomic implications using various molecular markers.

Two partial mitochondrial DNA (mtDNA) gene sequences (COI and 16S rRNA) and nine microsatellite loci were analyzed in 260 individuals from ten locations.

Phylogenetic trees from two partial mtDNA gene sequences revealed two distinct lineages of flathead grey mullet (P distance = 0.016-0.021). Lineage 1 (L1) consisted of mullets from all locations except for the Pacific coasts of Japan, while lineage 2 (L2) consisted of mullets from the Jeju Island, Shanghai, and the Pacific coasts of Japan. The STRUCTURE analyses of msat data also revealed two distinct groups.

This study was the first to reveal the coexistence of two lineages of flathead grey mullet in Korea. L2 was confined to the Jeju Island in Korea, implying a biogeographic boundary between the two lineages. The allopatric distribution and genetic heterogeneity between lineages may be affected by an ancient geographic isolation during the glacial period and maintained due to adaptation to oceanic characteristics. Collectively, the two lineages may be comprised of distinct species.