Phylogeographical ‘breaks’ often occur at biogeographical province boundaries, suggesting common causative factors. Cape Hatteras, North Carolina, on the US Atlantic coast, separates water masses of sharply differing temperature and divides the Virginian and Carolinian provinces, but has not often been detected as a phylogeographical break. We studied the black sea bass, Centropristis striata (Linnaeus, 1758), a species with philopatric adults but long-lived planktonic larvae. An earlier study of only two Atlantic populations, and lower-resolution mitochondrial RFLP markers, showed strong separation between the Gulf of Mexico and the Atlantic, but not across Cape Hatteras.
Over 300 adults were collected from populations from the Gulf of Mexico, the US south Atlantic (North Carolina south of Cape Hatteras, the east coast of Florida, and South Carolina), and the US mid-Atlantic (North Carolina north of Cape Hatteras, Virginia, Delaware, New Jersey and Massachusetts).
We sequenced the mitochondrial control region, analysed DNA sequence and haplotype frequency differentiation and haplotype networks, and performed coalescent analyses of population sizes and migration rates.
Sequences formed three clusters – Gulf of Mexico, south Atlantic and mid-Atlantic – the Atlantic clusters being divided at Cape Hatteras. Analyses of molecular variance (AMOVA) showed that 31% of sequence variation was between populations on opposite sides of Cape Hatteras, while only 5% was between populations on the same side, yet migrate analysis detected asymmetric migration across the Cape, greater from north to south than in the opposite direction. A transitional population with roughly equal frequencies of south and mid-Atlantic haplotypes occupied a narrow stretch between Cape Hatteras and the Virginia border.
Strong barriers to gene flow exist between the Gulf of Mexico and the Atlantic, and across Cape Hatteras. Adult migratory and spawning behaviours that separate mid-Atlantic and south Atlantic populations, limited larval exchange, and selection imposed by thermal gradients are each potentially responsible. In addition, these data indicate that fishery stocks of black sea bass north and south of Cape Hatteras are genetically distinct, supporting their separate management.