Comparative Analysis of Genetic Diversities Among Invasive Populations of Four Gobies in Estuaries of California and Australia

Three Asiatic species of the gobiid genus Tridentiger and one of the genus Acanthogobius have established invasive populations in San Francisco Bay.  Those invasions have been sequential with the earliest two in the 1960’s and the latest in the mid-1990’s.  Two of those species, T. trigoncephalus and A. flavimanus, have also invaded estuaries of southern California, as well as estuaries of eastern Australia.  Except for first records and papers documenting spread, we know little of the population dynamics of the invasions.  Of interest is ascertaining to what relative degrees the invasive populations are supported by in situ growth versus reinforcement through ongoing immigration, for it has been suggested that quick recovery of genetic diversity aids the spread of invasive species.  To investigate, we have applied a founder model approach. Invasive populations founded by few individuals (low effective size) are expected to rapidly lose genetic diversity from inbreeding and genetic drift.  If closed, restoration would require thousands of generations so diversity should still be low under certain assumptions.  Conversely, genetic diversity may be restored quickly through ongoing immigration and be as high as, or higher than, the source population.

Between 2001 and 2010 we acquired population samples of T. trigonocephalus and A. flavimanus (frozen or ethanol-fixed) from Japan (Tokyo Bay), eastern Australia (Port Jackson), San Francisco Bay, and Newport Harbor, and frozen samples of T. bifasciatus and T. barbatus from San Francisco Bay only.  Segments of the mtDNA control region were sequenced for all samples and genetic diversity was determined from the sequences, principally as haplotype diversity (h). A regression of haplotype diversity on years (at the time of our collection) since first record was significant (p = 0.001; R² = 0.94; N = 6) for the invasive populations. Treating the combination as representative of a time-line for any single population, recovery from h = 0 (youngest invasion; T. barbatus) to h = 0.998 (oldest invasion; T. trigonocephlaus) was about 45 yr, or about 20-25 generations.  Haplotype diversity for T. trigonocephalus of San Francisco Bay was significantly higher than that of populations at Tokyo Bay and Port Jackson, which did not differ significantly. The elevated haplotype diversity in San Francisco Bay over the proposed Asiatic source population suggests admixing of T. trigonocephalus in San Francisco Bay from multiple sources. In total, mtDNA evidence suggests that ongoing immigration following initial invasion has restored, or maintained, high genetic diversity for one invasive goby.