Cryptic diversity of shallow and mesophotic Stephanocoenia intersepta corals across Florida Keys National Marine Sanctuary.

Population genetic analyses can provide useful data on species' regional connectivity and diversity which can inform conservation and restoration efforts. In this study, we quantified the genetic connectivity and diversity of Stephanocoenia intersepta corals from shallow (<30 m) to mesophotic (30-45 m) depths across Florida Keys National Marine Sanctuary. We generated single nucleotide polymorphism (SNP) markers to identify genetic structuring of shallow and mesophotic S. intersepta corals. We uncovered four distinct, cryptic genetic lineages with varying levels of depth-specificity. Shallow-specific lineages exhibited lower heterozygosity and higher inbreeding relative to depth-generalist lineages found across both shallow and mesophotic reefs. Estimation of recent genetic migration rates demonstrated that mesophotic sites are more prolific sources than shallow sites, particularly in the Lower Keys and Upper Keys. Additionally, we compared endosymbiotic Symbiodiniaceae among sampled S. intersepta using the ITS2 region and SYMPORTAL analysis framework, identifying symbionts from the genera Symbiodinium, Breviolum, and Cladocopium. Symbiodiniaceae varied significantly across depth and location and exhibited significant, but weak correlation with host lineage and genotype. Together, these data demonstrate that despite population genetic structuring across depth, some mesophotic populations may provide refuge for shallow populations moving forward and remain important contributors to the overall genetic diversity of this species throughout the region. This study highlights the importance of including mesophotic as well as shallow corals in population genetic assessments and informs future science-based management, conservation, and restoration efforts within Florida Keys National Marine Sanctuary.