Genetic structure among 50 species of the northeastern Pacific rocky intertidal community

Abstract

Comparing many species' population genetic patterns across the same seascape can identify species with different levels of structure, and suggest hypotheses about the processes that cause such variation for species in the same ecosystem. This comparative approach helps focus on geographic barriers and selective or demographic processes that define genetic connectivity on an ecosystem scale, the understanding of which is particularly important for large-scale management efforts. Moreover, a multispecies dataset has great statistical advantages over single-species studies, lending explanatory power in an effort to uncover the mechanisms driving population structure. Here, we analyze a 50-species dataset of Pacific nearshore invertebrates with the aim of discovering the most influential structuring factors along the Pacific coast of North America. We collected cytochrome c oxidase I (COI) mtDNA data from populations of 34 species of marine invertebrates sampled coarsely at four coastal locations in California, Oregon, and Alaska, and added published data from 16 additional species. All nine species with non-pelagic development have strong genetic structure. For the 41 species with pelagic development, 13 show significant genetic differentiation, nine of which show striking FST levels of 0.1-0.6. Finer scale geographic investigations show unexpected regional patterns of genetic change near Cape Mendocino in northern California for five of the six species tested. The region between Oregon and Alaska is a second focus of intraspecific genetic change, showing differentiation in half the species tested. Across regions, strong genetic subdivision occurs more often than expected in mid-to-high intertidal species, a result that may reflect reduced gene flow due to natural selection along coastal environmental gradients. Finally, the results highlight the importance of making primary research accessible to policymakers, as unexpected barriers to marine dispersal break the coast into separate demographic zones that may require their own management plans.

Figure 1. Map of primary collection locations, geographic regions, and major ecological features of the Pacific coast of North America.

The background color represents sea surface temperature for a day in July, 2008. Bar graphs on the left-hand side of the figure show the number of species with strong, moderate and mild genetic structure in southern, central and northern comparisons. Structure categories are mild: Φ_ST = 0.02 – 0.05, moderate: 0.05 – 0.10, strong:>0.1.

Figure 2. Genetic differentiation in the vicinity of Cape Mendocino, CA, for six invertebrate species.

Squares represent sampling localities for each species (different habitat requirements precluded identical sampling sites). Squares with different shading are significantly different in Arlequin analyses based on COI haplotypes for each species. Sample sizes and exact collection locations are shown in Table 1. Inset is the correlation between geographic and genetic distance for the same six species, for collections made in the Cape Mendocino region. Asterisk denotes significance at the p = 0.05 level.

Figure 3. Overall genetic subdivision (Φ_ST) and pelagic larval duration (PLD).

Brooding species (nonpelagic larvae) in black filled circles, nonbrooding species (pelagic larvae) in orange filled circles. Statistical results for the nonparametric Spearman Rank Correlation are shown.

Figure 4. Overall genetic subdivision (Φ_ST) and habitat depth for nonbrooding species for four habitat categories.

Asterisk denotes significant the difference between the high intertidal depth category and both the mid-intertidal and low-subtidal categories (p<0.05). Box widths are 25^th and 75^th quartiles, respectively; bars are means, and whiskers are the extreme data points. Φ_ST values are significantly or marginally significantly apportioned among depth categories (ANOVA R² = 0.24, p = 0.02). High and Mid-Intertidal species had significantly and marginally greater Φ_ST values than Low to Subtidal species, respectively (Wilcoxon, p = 0.03 and p = 0.07). Depth categories were sampled approximately equally in each geographic region (chi-square, all region pairs p>0.2).