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.
Highlights
Coral reefs are declining worldwide and face a future of increasingly frequent and severe stress
Mesophotic coral reefs, typically defined as reefs at depths greater than ~30 m (Lesser, Slattery, & Leichter, 2009), are unique ecosystems that frequently occur adjacent to shallow reef communities where conspecifics may experience different impacts or stressor severity. This contrast led to the deep reef refugia hypothesis, which suggests that mesophotic reefs may provide refuge habitat and support healthy coral populations to facilitate the recovery of degraded shallow reefs via larval connectivity (Bongaerts, Ridgway, Sampayo, & Hoegh-Guldberg, 2010; Gilmour et al, 2013; Glynn, 1996; Lesser et al, 2009)
Connectivity is an important component of the health and resilience of contemporary reefs, and declining coral communities have prompted a focus on potential links between shallow and mesophotic populations
Summary
Coral reefs are declining worldwide and face a future of increasingly frequent and severe stress. Mesophotic coral reefs, typically defined as reefs at depths greater than ~30 m (Lesser, Slattery, & Leichter, 2009), are unique ecosystems that frequently occur adjacent to shallow reef communities where conspecifics may experience different impacts or stressor severity This contrast led to the deep reef refugia hypothesis, which suggests that mesophotic reefs may provide refuge habitat and support healthy coral populations to facilitate the recovery of degraded shallow reefs via larval connectivity (Bongaerts, Ridgway, Sampayo, & Hoegh-Guldberg, 2010; Gilmour et al, 2013; Glynn, 1996; Lesser et al, 2009). We sampled shallow or mesophotic populations near the endpoints of the Florida Reef Tract to test the hypotheses that (a) there is unresolved population structure within the Florida Reef Tract between mesophotic and shallow coral populations and (b) the remote Pulley Ridge mesophotic ecosystem is part of the same population as other Florida corals
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