Population genetic structure of the broadcast spawning coral, Montastraea cavernosa, demonstrates refugia potential of upper mesophotic populations in the Florida Keys

In the Florida Keys, coral cover on shallow reef systems (0-30 m) has declined over the past several decades, punctuated by severe losses during coral disease outbreaks and bleaching events. However, certain areas within the Florida Keys, especially the Dry Tortugas and many upper mesophotic habitats (30-60 m), have maintained relatively healthy coral communities, even in the face of recent severe and widespread coral disease outbreaks. Relatively little is known about the genetic connectivity of corals among these sites or the potential for mesophotic sites to act as refugia by contributing to metapopulation recovery and persistence. Using a paired shallow and upper mesophotic sampling design, we assessed the genetic connectivity of a dominant, broadcast spawning coral species, Montastraea cavernosa, across the Northern and Southern Dry Tortugas, Lower Florida Keys, and Upper Florida Keys. A genetic dataset based on a suite of > 9000 single-nucleotide polymorphism loci indicated that the level of vertical genetic connectivity between paired shallow and upper mesophotic populations varied significantly based on location. Shallow and upper mesophotic M. cavernosa populations in the Northern Dry Tortugas and the Upper Keys were genetically similar. In contrast, populations were significantly differentiated across depth in the Lower Keys and Southern Dry Tortugas. While upper mesophotic populations in the Lower Keys and Southern Dry Tortugas were distinct from their shallow counterparts, there was evidence of relatively high levels of genetic connectivity to both the shallow and upper mesophotic populations downstream in the Upper Keys. These results suggest that while vertical connectivity between paired shallow and mesophotic populations can vary, certain upper mesophotic populations may fill an important role in maintaining coral metapopulations throughout the Florida Keys and should be considered in future management strategies.

Automated summary

The study reveals that certain areas within the Florida Keys, especially upper mesophotic habitats, maintain relatively healthy coral communities and may play an important role in maintaining coral metapopulations. The level of genetic connectivity between shallow and upper mesophotic populations varies significantly based on location, with distinct differences across depth in some areas.