Fine‐scale structure among mesophotic populations of the great star coral Montastraea cavernosa revealed by SNP genotyping

Abstract

Mesophotic reefs (30‐150 m) have been proposed as potential refugia that facilitate the recovery of degraded shallow reefs following acute disturbances such as coral bleaching and disease. However, because of the technical difficulty of collecting samples, the connectivity of adjacent mesophotic reefs is relatively unknown compared with shallower counterparts. We used genotyping by sequencing to assess fine‐scale genetic structure of Montastraea cavernosa at two sites at Pulley Ridge, a mesophotic coral reef ecosystem in the Gulf of Mexico, and downstream sites along the Florida Reef Tract. We found differentiation between reefs at Pulley Ridge (~68 m) and corals at downstream upper mesophotic depths in the Dry Tortugas (28–36 m) and shallow reefs in the northern Florida Keys (Key Biscayne, ~5 m). The spatial endpoints of our study were distinct, with the Dry Tortugas as a genetic intermediate. Most striking were differences in population structure among northern and southern sites at Pulley Ridge that were separated by just 12km. Unique patterns of clonality and outlier loci allele frequency support these sites as different populations and suggest that the long‐distance horizontal connectivity typical of shallow‐water corals may not be typical for mesophotic systems in Florida and the Gulf of Mexico. We hypothesize that this may be due to the spawning of buoyant gametes, which commits propagules to the surface, resulting in greater dispersal and lower connectivity than typically found between nearby shallow sites. Differences in population structure over small spatial scales suggest that demographic constraints and/or environmental disturbances may be more variable in space and time on mesophotic reefs compared with their shallow‐water counterparts.

Soft coral reproductive phenology along a depth gradient: Can “going deeper” provide a viable refuge?

“…It would seem that coral planulae may be less successful in migrating from deeper habitats to shallow ones and establishing there, than the other way around (Prada & Hellberg, 2021; Rippe et al, 2021; Serrano et al, 2014; Shlesinger & Loya, 2021). Indeed, the current findings are consistent with several studies suggesting that population connectivity along depth may be relevant only for some depth‐generalist species but not for all, or not in all biogeographic regions (Bongaerts et al, 2017; Drury et al, 2020; Liberman et al, 2018; Rippe et al, 2021; Scucchia et al, 2021; Serrano et al, 2014; Shlesinger et al, 2018; Shlesinger & Loya, 2021). Taken together with our findings of reduced reproductive activity at mesophotic depth, it seems less likely that migrating into the deeper MCEs can provide corals with a long‐term viable refuge that will enable them to successfully replenish and sustain shallow‐water populations.…”

Depth-dependent parental effects create invisible barriers to coral dispersal

“…Whether our findings generally hold for corals as well as for other marine organisms, awaits similar assessments of broadcast-spawning species to be carried out. Yet, despite the natural variability between species and sites, genetic segregation of both the coral host and its symbionts across depth was found to date in both brooding and spawning corals 10,[42][43][44][45][46][47][48][49][50]83,84 . Regardless, our results demonstrate that although not fully hindered, corals' early life-history stages may have differential success and fitness in different conditions, depending on their natal habitats.…”

“…At the community level, coral reefs have long been recognized as displaying a distinct depth zonation, dominated by different species [91][92][93][94][95][96][97] . At the population level, recent studies have also found evidence of ecological adaptive divergence and genetic segregation across depth in several species and localities 10,[42][43][44][45][46][47][48][49][50]83,84 . Here, we have shown that local adaptations and parental effects may considerably influence brooding coral abundance across depth through selective larval settlement and phenotype-environment mismatches.…”

“…Phenotype-environment mismatches (or "immigrant inviability" 38 ) may therefore constitute a considerable barrier to population connectivity in the sea 8,[39][40][41] . Indeed, recent studies on coral population genetic structures have found evidence of segregation across depth 10,[42][43][44][45][46][47][48][49][50] (although this may differ between sites among species [46][47][48][49] ). These studies motivated us to seek an explanation for the seemingly paradoxical findings of genetic differentiation in populations separated vertically by only tens of meters, alongside the findings of large-scale horizontal genetic connectivity, sometimes across hundreds of kilometers 10,[42][43][44][45][46][47][48][49][50] .…”

Depth-Dependent Genetic Structuring of a Depth-Generalist Coral and Its Symbiodiniaceae Algal Communities at Campeche Bank, Mexico

“…Applying molecular techniques to the depth-stratified collection of corals can characterize potential ecological differences between shallow and mesophotic populations of depth-generalist coral species and begin to evaluate the refugia potential of mesophotic populations (Bongaerts et al, 2015;Studivan and Voss, 2018b;Eckert et al, 2019;Drury et al, 2020;Eckert et al, 2020;Sturm et al, 2021). Restriction-site associated DNA sequencing (RADseq) methods have become an increasingly popular tool in the field of coral population genetics, as they allow for relatively inexpensive but high-resolution genotyping (on the order of thousands of single nucleotide polymorphisms, SNPs; Puritz et al, 2014).…”