Does depth divide? Variable genetic connectivity patterns among shallow and mesophotic Montastraea cavernosa coral populations across the Gulf of Mexico and western Caribbean.

Despite general declines in coral reef ecosystems in the tropical western Atlantic, some reefs, including mesophotic reefs (30-150 m), are hypothesized to function as coral refugia due to their relative isolation from anthropogenic stressors. Understanding the connectivity dynamics among these putative refugia and more degraded reefs is critical to develop effective management strategies that promote coral metapopulation persistence and recovery. This study presents a geographically broad assessment of shallow (<30 m) and mesophotic (>30 m) connectivity dynamics of the depth-generalist coral species Montastraea cavernosa. Over 750 coral genets were collected across the Northwest and Southern Gulf of Mexico, Florida, Cuba, and Belize, and ~5000 SNP loci were generated to quantify high-resolution genetic structure and connectivity among these populations. Generally, shallow and mesophotic populations demonstrated higher connectivity to distant populations within the same depth zone than to adjacent populations across depth zones. However, exceptions to this pattern include the Northwest Gulf of Mexico and the Florida Keys which exhibited relatively high vertical genetic connectivity. Furthermore, estimates of recent gene flow emphasize that mesophotic M. cavernosa populations are not significant sources for their local shallow counterparts, except for the Northwest Gulf of Mexico populations. Location-based differences in vertical connectivity are likely a result of diverse oceanographic and environmental conditions that may drive variation in gene flow and depth-dependent selection. These results highlight the need to evaluate connectivity dynamics and refugia potential of mesophotic coral species on a population-by-population basis and to identify stepping-stone populations that warrant incorporation in future international management approaches.

Population genetic structure of the great star coral, Montastraea cavernosa, across the Cuban archipelago with comparisons between microsatellite and SNP markers.

Coral reef habitats surrounding Cuba include relatively healthy, well-developed shallow and mesophotic (30-150 m) scleractinian communities at the cross-currents of the Tropical Western Atlantic (TWA). However, Cuba's coral communities are not immune to the declines observed throughout the TWA, and there is limited information available regarding genetic connectivity, diversity, and structure among these populations. This represents an immense gap in our understanding of coral ecology and population dynamics at both local and regional scales. To address this gap, we evaluated the population genetic structure of the coral Montastraea cavernosa across eight reef sites surrounding Cuba. Colonies were genotyped using nine microsatellite markers and > 9,000 single nucleotide polymorphism (SNP) markers generated using the 2bRAD approach to assess fine-scale genetic structure across these sites. Both the microsatellite and SNP analyses identified patterns of genetic differentiation among sample populations. While the microsatellite analyses did not identify significant genetic structure across the seven shallow M. cavernosa sampling sites, the SNP analyses revealed significant pairwise population differentiation, suggesting that differentiation is greater between eastern and western sites. This study provides insight into methodological differences between microsatellite and SNP markers including potential trade-offs between marker-specific biases, sample size, sequencing costs, and the ability to resolve subtle patterns of population genetic structure. Furthermore, this study suggests that locations in western Cuba may play important roles in this species' regional metapopulation dynamics and therefore may merit incorporation into developing international management efforts in addition to the local management the sites receive.

Depth Influences Symbiodiniaceae Associations Among Montastraea cavernosa Corals on the Belize Barrier Reef.

In Belize, shallow populations (10 and 16 m) of the coral species Montastraea cavernosa from the back reef and reef crest are genetically differentiated from deeper populations on the fore reef and reef wall (25 and 35 m). Like many species of scleractinian corals, M. cavernosa has an obligate symbiosis with dinoflagellate microalgae from the family Symbiodiniaceae. Here, we describe the Symbiodiniaceae taxa found within previously sampled and genotyped M. cavernosa populations along a depth gradient on the Belize Barrier Reef by implementing high-throughput sequencing of the ITS2 region of Symbiodiniaceae ribosomal DNA and the SymPortal analysis framework. While Symbiodiniaceae ITS2 type profiles across all sampling depths were almost entirely (99.99%) from the genus Cladocopium (formerly Symbiodinium Clade C), shallow (10 and 16 m) populations had a greater diversity of ITS2 type profiles in comparison to deeper (25 and 35 m) populations. Permutational multivariate analysis of variance (PERMANOVA) confirmed significant differences in ITS2 type profiles between shallow and deep sample populations. Overall Symbiodiniaceae communities changed significantly with depth, following patterns similar to the coral host's population genetic structure. Though physiological differences among species in the cosmopolitan genus Cladocopium are not well-described, our results suggest that although some members of Cladocopium are depth-generalists, shallow M. cavernosa populations in Belize may harbor shallow-specialized Symbiodiniaceae not found in deeper populations.

Populations of the coral species Montastraea cavernosa on the Belize Barrier Reef lack vertical connectivity.

Larval connectivity among and within coral reefs is important for sustaining coral metapopulations, enhancing ecosystem resilience through species and genetic diversity, and maintaining reef ecosystems' structure and functions. This study characterized genetic structure and assessed horizontal and vertical connectivity among populations of the ubiquitous gonochoric broadcast spawning coral Montastraea cavernosa in Belize. Using nine polymorphic microsatellite loci, we genotyped M. cavernosa colonies from four depth zones at four study sites within Belizean marine management zones. Study sites were selected within South Water Caye Marine Reserve (3 sites) and Glover's Reef Marine Reserve (1 site). Strong contemporary genetic differentiation was observed between relatively shallow M. cavernosa populations (10 m, 16 m) and relatively deep (25 m, 35 m) populations, coinciding with a transition from reef crest to reef slope. These results were consistent across both marine reserves. Vertical and horizontal migration models suggest that all populations were historically panmictic, with little unidirectional migration. The relative local isolation of shallow and mesophotic M. cavernosa populations in Belize, coupled with the importance of Belize's upper mesophotic populations as potential larval sources for other areas in the Tropical Western Atlantic, reinforces the need for management strategies that conserve coral populations across all depth zones.