Historical population dynamics of red snapper (Lutjanus campechanus) in the northern Gulf of Mexico

A total of 313 young-of-the-year red snapper (Lutjanus campechanus) belonging to the 1999 year class were sampled from three geographic regions in the northern Gulf of Mexico and assayed for haplotype variation in mitochondrial (mt)DNA. Analysis of molecular variance revealed that only a small proportion (0.24%) of the genetic variance was distributed among regions; accordingly, the corresponding [[PHI].sub.ST] value did not differ significantly from zero. Exact tests of homogeneity of haplotype distributions also were non-significant. Tests for departure from a neutral Wright-Fisher model of genetic polymorphism, however, were significant, and a 'mismatch' distribution of nucleotide-site differences in mtDNA indicated that the departure from neutrality could be due to population expansion. Estimates of the time since expansion ranged from [approximately equal to]270,000 to [approximately equal to]420,000 years before present. The latter is consistent with the hypothesis that red snapper likely colonized the continental shelf in the northern Gulf following a glacial retreat. The observed departure from a neutral Wright-Fisher model also may suggest that insufficient time has lapsed for red snapper in the northern Gulf to attain equilibrium between mutation and genetic drift. However, the temporal signature provided by the 'mismatch' distribution is far older than the last glacial retreat which began [approximately equal to]18,000 years ago. If the departure from neutrality reflects events occurring after the last glacial retreat, tests of present-day population or stock structure may well be compromised. The same may be true for other marine fish species in the northern Gulf.