Genetic Connectivity in Corals on the Flower Garden Banks and Surrounding Oil/Gas Platforms, Gulf of Mexico

Abstract

The northern Gulf of Mexico (GOM) currently possesses ∼ 3,600 offshore oil and gas production platforms. These platforms serve as artificial reefs on the continental shelf, where, until their introduction, shallow hard substrata were rare. This newly available substrate has helped to expand scleractinian coral populations in the GOM. Here, we conduct molecular genetic analyses on adult scleractinian corals on the Flower Garden Banks (FGB) coral reefs (∼ 180 km SE of Galveston, TX) and on surrounding oil and gas platforms. We have attempted to determine the degree of genetic affinity among the natural populations and those on the surrounding platforms. The three most abundant hermatypic scleractinian species were sampled: Madracis decactis, Diploria strigosa, and Montastraea cavernosa. Tissue samples were collected from the East (E-) and West (W-) FGB, and seven platforms within a 65 km radius of the FGB, at a depth range of 5-37 m. Genetic variation was assessed using Amplified Fragment Length Polymorphisms (AFLPs). The large number of polymorphic markers generated by AFLPs allowed for the use of standard genetic analysis tools (AMOVA) as well as population allocation techniques (AFLPOP). AMOVA analyses indicated that the E- and W-FGB were genetically homogeneous for populations of Madracis decactis and Diploria strigosa; Montastraea cavernosa populations, however, were significantly different there. In all species, genetic distance (FST) increased significantly with geographic distance between populations. In the brooding species Madracis decactis, this pattern was even stronger when one considered distance between the platforms and the perimeters of the FGB, particularly the nearest FGB, suggesting that the FGB may be a source of larvae for platform populations. AFLPOP analyses showed that the degree of self-allocation to home sites also increased with inter-site distance. Cross allocations between sites dropped significantly and exponentially in all species within only one to several kms of the FGB. Madracis decactis, a brooder with extended larval release periods and near-immediate settlement competence, showed greater affinity to the FGB with distance than Diploria strigosa, a broadcaster. This brooder appears to be more effective at colonizing small, nearby target sites and expanding its geographic range at the meso-scale. The low degree of genetic affinity exhibited by all species on the platforms may be attributed to genetic drift/founder effect or relatively small samples sizes, although total populations were sampled. In general, genetic affinity decreased with inter-site distance. Young coral populations are highly differentiated at the meso-scale during early stages of community succession, implying that much time and repeated colonization of patchy habitats around larger potential larval sources will be required before genetic equilibrium or homogeneity is reached.