Genetic differentiation in Gulf of California blennioid fishes

Marine organisms inhabit an environment where there are few absolute barriers to movement. In addition, a planktonic larval stage is common to most marine fishes and invertebrates. Consequently, marine organisms are often characterized by little genetic differentiation over large geographic distances, and the factors that might generally promote genetic divergence are not well understood. Here, contributions to population structure from both intrinsic and extrinsic factors were investigated in Gulf of California blennioid fishes. In Appendix A, population structure was estimated from mtDNA control region sequences for three species, Axoclinus nigricaudus, Malacoctenus hubbsi, and Ophioblennius steindachneri, that differ in predicted dispersal patterns (based on ichthyoplankton distribution and abundance patterns relative to rock reefs). FST ranged widely among low to high predicted dispersal species, in the same rank order predicted by larval distribution patterns. In A. nigricaudus (low predicted dispersal), phylogenetic, population genetic, and general linear model analyses of mtDNA (Appendix B) showed that variation was significantly partitioned between two biogeographic regions, and that geographic distance and unsuitable habitat also contributed to mtDNA differentiation. In contrast, allozyme variation in A. nigricaudus showed less partitioning than mtDNA and there was no break between biogeographic regions (Appendix C). In Gulf of California blennies, I find that population structure varies among fishes that have a planktonic larval stage in a manner that can be predicted from larval distribution patterns. The correlation between larval distribution and population structure suggests that some fish larvae, such as A. nigricaudus, actively maintain a position close to their natal reef, preventing substantial gene flow among many populations. In addition, patterns of population subdivision in A. nigricaudus (low dispersal) indicate that when dispersal is restricted, substantial population subdivision due to a combination of factors can occur, despite having a planktonic larval stage. The discordance between mtDNA and allozymes in A. nigricaudus is consistent with non-equilibrium conditions following a population perturbation, selection on allozymes or mtDNA, or some combination of these factors. These results emphasize that multiple genetic markers should be examined when making inferences about the genetic structures of natural populations