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Deep reefs are not universal refuges: Reseeding potential varies among coral species.

ABSTRACT: Deep coral reefs (that is, mesophotic coral ecosystems) can act as refuges against major disturbances affecting shallow reefs. It has been proposed that, through the provision of coral propagules, such deep refuges may aid in shallow reef recovery; however, this "reseeding" hypothesis remains largely untested. We conducted a genome-wide assessment of two scleractinian coral species with contrasting reproductive modes, to assess the potential for connectivity between mesophotic (40 m) and shallow (12 m) depths on an isolated reef system in the Western Atlantic (Bermuda). To overcome the pervasive issue of endosymbiont contamination associated with de novo sequencing of corals, we used a novel subtraction reference approach. We have demonstrated that strong depth-associated selection has led to genome-wide divergence in the brooding species Agaricia fragilis (with divergence by depth exceeding divergence by location). Despite introgression from shallow into deep populations, a lack of first-generation migrants indicates that effective connectivity over ecological time scales is extremely limited for this species and thus precludes reseeding of shallow reefs from deep refuges. In contrast, no genetic structuring between depths (or locations) was observed for the broadcasting species Stephanocoenia intersepta, indicating substantial potential for vertical connectivity. Our findings demonstrate that vertical connectivity within the same reef system can differ greatly between species and that the reseeding potential of deep reefs in Bermuda may apply to only a small number of scleractinian species. Overall, we argue that the "deep reef refuge hypothesis" holds for individual coral species during episodic disturbances but should not be assumed as a broader ecosystem-wide phenomenon.

SUBMITTER: Bongaerts P

PROVIDER: S-EPMC5310828 | biostudies-other | 2017 Feb

REPOSITORIES: biostudies-other

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Deep reefs are not universal refuges: Reseeding potential varies among coral species.

Bongaerts Pim P Riginos Cynthia C Brunner Ramona R Englebert Norbert N Smith Struan R SR Hoegh-Guldberg Ove O

Science advances 20170215 2

Deep coral reefs (that is, mesophotic coral ecosystems) can act as refuges against major disturbances affecting shallow reefs. It has been proposed that, through the provision of coral propagules, such deep refuges may aid in shallow reef recovery; however, this "reseeding" hypothesis remains largely untested. We conducted a genome-wide assessment of two scleractinian coral species with contrasting reproductive modes, to assess the potential for connectivity between mesophotic (40 m) and shallow ...[more]

PMID: 28246645

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Project description:Loss of larger consumers from stressed ecosystems can lead to trophic release of mid-level consumers that then impact foundation species, suppressing ecosystem function and resilience. For example, in coral reef ecosystems, outbreaks of coral predators like crown-of-thorns sea stars have been associated with fishing pressure and can dramatically impact the composition and persistence of corals. However, the ecological impacts, and consequences for management, of smaller, less obvious corallivores remain inadequately understood. We investigated whether reef state (coral vs. seaweed domination) influenced densities and size frequencies of the corallivorous gastropod Coralliophila violacea on its common host, the coral Porites cylindrica, within three pairs of small Marine Protected Areas (MPAs) and adjacent fished areas in Fiji. C. violacea densities were 5-35 times greater, and their size frequencies more broadly distributed, within seaweed-dominated fished areas than in adjacent MPAs dominated by corals. Tethering snails (4-9 mm in shell height) in place on their coral hosts indicated that suppression of snails in MPAs was due to predation, apparently by fishes. When tethered on the benthos (where they rarely occur), rather than on their host, mortality of larger snails (15.0-25.0 mm in shell height) was high in all areas, primarily due to hermit crabs killing them and occupying their shells. Because C. violacea is a sessile gastropod that feeds affixed to the base of corals and produces minimal visible damage, it has been considered a "prudent feeder" that minimally impacts its host coral. We assessed this over a 24-d feeding period in the field. Feeding by individual C. violacea reduced P. cylindrica growth by ~18-43% depending on snail size. Our findings highlight the considerable, but underappreciated, negative impacts of this common corallivore on degraded reefs. As reefs degrade and corals are lost, remaining corals (often species of Porites) may gain the full attention of elevated densities of coral consumers. This will further damage the remaining foundation species, suppressing the resilience of corals and enhancing the resilience of degraded, seaweed-dominated reefs.

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Deep reefs are not universal refuges: Reseeding potential varies among coral species.

Publications

Deep reefs are not universal refuges: Reseeding potential varies among coral species.

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