Gene flow is important for maintaining the genetic diversity required for adaptation to environmental disturbances, though gene flow may be limited by site fidelity in small coastal sharks. Bonnethead sharks (Sphyrna tiburo) - a small coastal hammerhead species - demonstrate site fidelity, as females are philopatric while males migrate to mediate gene flow. Consequently, bonnetheads demonstrate population divergence with distance and Atlantic populations are genetically distinct from those of the Gulf of Mexico. Indeed, Florida forms a vicariant zone between these two bodies of water for many marine species, including some sharks. However, while bonnetheads are expected to have limited dispersal, the extent and rate of bonnethead migration remains uncertain. Thus, we aimed to determine their dispersal capacity by evaluating connectivity between disparate populations from the Gulf of Mexico and Atlantic Ocean. Using 10,733 SNPs derived from 2bRAD sequences, we evaluated genetic connectivity between Tampa Bay on the Gulf Coast of Florida and Biscayne Bay on the Atlantic coast of Florida. While standard analyses of genetic structure revealed little differentiation between Tampa Bay and Biscayne Bay populations, demographic history inference based on the site frequency spectrum favored the model with divergence between 2000 - 6000 years ago, with continuous unidirectional gene flow from Tampa Bay to Biscayne Bay. Combined with prior findings of small-scale genetic divergence in bonnethead mitochondrial (female-transmitted) loci, our findings support the hypothesis that female bonnetheads are highly philopatric while male bonnetheads often migrate over relatively large distances (>300 miles) to find mates. Together, these results provide optimism that under proper management, a small-bodied globally Endangered shark can undergo long migrations to sustain genetic diversity.

Coral populations across the Great Barrier Reef (GBR) could rapidly adapt to the warming climate if they have standing genetic variation for thermal adaptation. Here, we describe a locus likely involved in acclimatization and adaptation of Acropora millepora to cooler temperatures at higher latitudes. This locus shows a strong signal of selection in the A. millepora genome, with a steep latitudinal gradient of derived allele frequency, and harbors a cluster of eight tandemly repeated Δ9-desaturase genes adjacent to a region where a hard sweep likely occurred. In colonies reciprocally transplanted across 4.5 degrees of latitude, the expression of Δ9-desaturase was upregulated at the cooler high-latitude reef. Furthermore, corals from the warmer low-latitude reef with one or two copies of the “cold-adapted” Δ9-desaturase allele expressed the gene more and grew faster than their peers when transplanted to the cooler reef. In other organisms ranging from bacteria to fish, Δ9-desaturase is upregulated under cold conditions to adjust membrane fluidity by introducing double bonds into fatty acid chains of membrane lipids. While all these lines of evidence are suggestive rather than conclusive, they collectively make Δ9-desaturase a strong candidate marker gene for coral thermal acclimatization and adaptation.

Global environmental change is rapidly driving deterioration of many ecosystems- such as coral reefs- though the rate of decline could be offset by genetic adaptation. We aimed to identify which environmental gradients drive local adaptation in two common corals across the Florida Keys Reef Tract, Porites astreoides and Agaricia agaricites. Both species contained three genetically distinct lineages distributed across depths in a remarkably similar way. Additionally, each lineage harbored genetic variation that aligned with other environmental gradients. The most commonly highlighted driver of within-lineage genetic structure was temperature during the coldest winter month, which is warmer at offshore sites especially in the upper Keys. Other repeatedly highlighted environmental drivers were high variation in bottom temperature at nearshore sites along the main island chain, more pronounced ocean stratification west of Key West, and outlying values of several water quality parameters (such as dissolved oxygen, carbon, turbidity, and salinity) at nearshore and Florida Bay locations of the lower and middle Keys. Synthesizing these results, we provide a map of adaptive neighborhoods in the Florida Keys that are likely to harbor differentially adapted coral populations, which can be regarded as different genetic stocks from the perspective of reef conservation and restoration.