Historical biogeography of the Mugil cephalus species complex and its rapid global colonization.

Our understanding of speciation processes in marine environments remains very limited and the role of different reproductive barriers are still debated. While physical barriers were considered important drivers causing reproductive isolation, recent studies highlight the importance of climatic and hydrological changes creating unsuitable habitat conditions as factors promoting population isolation. Although speciation in marine fish has been investigated from different perspectives, these studies often have a limited geographical extant. Therefore, data on speciation within widely distributed species are largely lacking. Species complexes offer valuable opportunities to study the initial stages of speciation. Herein we study speciation within the Mugil cephalus species complex (MCSC) which presents a unique opportunity due to its circumglobal distribution. We used a whole-genome shotgun analysis approach to identify SNPs among the 16 species within the MCSC. We inferred the phylogenetic relationships within the species complex followed by a time-calibration analysis. Subsequently, we estimated the ancestral ranges within the species complex to explore their biogeographical history. Herein, we present a fully resolved and well-supported phylogeny of the MCSC. Its origin is dated at around 3.79 Ma after which two main clades emerged: one comprising all West Atlantic and East Pacific species and the other all East Atlantic and Indo-Pacific species. Rapid dispersal following an initial founder colonization from the West to the East Atlantic led to the population of all major realms worldwide in less than 2 Myr. Physical and climatic barriers heavily impacted the ancestral distribution ranges within the MCSC and triggered the onset of speciation.