Coral-algal symbioses in mesophotic Montastraea cavernosa in the Gulf of Mexic

Mesophotic reefs represent biodiverse ecosystems that may act as a refuge for depth-generalist coral species threatened in shallow habitats. Despite the importance of coral-algal symbioses, few studies focus on mesophotic zooxanthellae assemblages and their influence on connectivity. This study compared zooxanthellae in Montastraea cavernosa at shallow and mesophotic depths at Flower Garden Banks National Marine Sanctuary and McGrail Bank. Mesophotic corals contained more zooxanthellae and more chlorophyll a and c2 per unit area coral. Increased zooxanthellae within mesophotic corals may represent an adaptive strategy to optimize light capture in low-light environments. Genetic profiles for zooxanthellae assemblages from shallow and mesophotic corals showed similar diversity across banks and between depths. The dominant sequence making up assemblages was identified as Symbiodinium type C1. Similar assemblage diversity suggests that zooxanthellae assemblages will not limit connectivity potential between shallow and mesophotic corals at these reefs.