Fish predation hinders the success of coral restoration efforts using fragmented massive corals

Shifting reef restoration focus from coral survivorship to biodiversity using Reef Carpets

To enhance the practice of farmed-coral transplantation, we conducted a trial of an approach called "Reef Carpets" (RC), which draws inspiration from the commercial turf-grass sod in land-based lawn gardening. Three 8.4m2 RCs were established on a sandy seabed, containing preselected combinations of branching corals (Acropora cf. variabilis, Pocillopora damicornis, Stylophora pistillata) with nursery recruited dwellers, and were monitored for 17-months. Corals within RCs grew, supported coral recruitment and offered ecological habitats for coral-associated organisms. While the unstable sediment underneath the RCs increased corals' partial mortalities, corals managed to grow and propagate. The extent of fish and gastropods corallivory varied among the coral species and planulation of Stylophora transplants was significantly higher than same-size natal-colonies. The RCs provided conducive environments for fish/invertebrate communities (183 taxa), and each coral species influenced specifically species-diversity and reef-associated communities. Even dead corals played crucial roles as habitats for reef biota, sustaining >80% of the RCs diversity; hence, they should not be considered automatically as indicators of failure. RCs scaled-up reef restoration and generated, in short periods, new reefs in denuded zones with enhanced biodiversity. Yet, RCs employment on soft-beds could be improved by using more structured artificial frameworks, requiring further research efforts.

Stony coral tissue loss disease intervention with amoxicillin leads to a reversal of disease-modulated gene expression pathways

Stony coral tissue loss disease (SCTLD) remains an unprecedented disease outbreak due to its high mortality rate and rapid spread throughout Florida's Coral Reef and wider Caribbean. A collaborative effort is underway to evaluate strategies that mitigate the spread of SCTLD across coral colonies and reefs, including restoration of disease-resistant genotypes, genetic rescue, and disease intervention with therapeutics. We conducted an in-situ experiment in Southeast Florida to assess molecular responses among SCTLD-affected Montastraea cavernosa pre- and post-application of the most widely used intervention method, CoreRx Base 2B with amoxicillin. Through Tag-Seq gene expression profiling of apparently healthy, diseased, and treated corals, we identified modulation of metabolomic and immune gene pathways following antibiotic treatment. In a complementary ex-situ disease challenge experiment, we exposed nursery-cultured M. cavernosa and Orbicella faveolata fragments to SCTLD-affected donor corals to compare transcriptomic profiles among clonal individuals from unexposed controls, those exposed and displaying disease signs, and corals exposed and not displaying disease signs. Suppression of metabolic functional groups and activation of stress gene pathways as a result of SCTLD exposure were apparent in both species. Amoxicillin treatment led to a 'reversal' of the majority of gene pathways implicated in disease response, suggesting potential recovery of corals following antibiotic application. In addition to increasing our understanding of molecular responses to SCTLD, we provide resource managers with transcriptomic evidence that disease intervention with antibiotics appears to be successful and may help to modulate coral immune responses to SCTLD. These results contribute to feasibility assessments of intervention efforts following disease outbreaks and improved predictions of coral reef health across the wider Caribbean.

Dictyota defense: Developing effective chemical protection against intense fish predation for outplanted massive corals

The incorporation of coral species with massive (e.g., boulder, brain) morphologies into reef restoration is critical to sustain biodiversity and increase coral cover on degraded reef ecosystems. However, fragments and colonies of massive corals outplanted in Miami-Dade County, Florida, US, can experience intense predation by fish within the first week of outplanting, resulting in >70% mortality. Here, we tested for the first time the potential benefit of feeding corals powdered Dictyota, a brown reef alga that is chemically defended against grazing, to determine if exposure to Dictyota can confer chemical protection to coral fragments and reduce the impacts of fish predation after outplanting. We found that feeding corals every 2 to 3 days for 2 months with dried and powdered Dictyota prior to outplanting significantly reduced predation levels on Orbicella faveolata and Montastraea cavernosa fragments (with less than 20% of the fragments experiencing predation up to 1-month post-outplanting). We also found that a single exposure to Dictyota at a high concentration 1 to 2 days prior to outplanting significantly reduced predation for six coral species within the first 24 h following outplanting. Thus, feeding corals dry Dictyota ex situ prior to outplanting appears to confer protection from fish predation during the critical first days to weeks after outplanting when predation impacts are commonly high. This simple and cheap method can be easily scaled up for corals kept ex situ prior to outplanting, resulting in an increase in restoration efficiency for massive corals in areas with high fish predation.

Coral micro-fragmentation assays for optimizing active reef restoration efforts

The global decline of coral reefs has driven considerable interest in active coral restoration. Despite their importance and dominance on mature reefs, relatively few coral restoration projects use slower growth forms like massive and encrusting coral species. Micro-fragmentation can increase coral cover by orders of magnitude faster than natural growth, which now allows cultivation of slow growing massive forms and shows promise and flexibility for active reef restoration. However, the major causes of variation in growth and survival of outplanted colonies remain poorly understood. Here, we report simple outplanting assays to aid in active reef restoration of slower growing species and increase the likelihood of restoration success. We used two different micro-fragmentation assays. Pyramid assays were used to examine variation associated with fragment size (ranging from ≈1-9 cm²), nursery residence time (for both in-situ and ex-situ nurseries), and 2D vs. 3D measurements of growth. Block assays were used to examine spatial variation among individual performance at outplanting sites in the field. We found 2D and 3D measurements correlated well, so measured survivorship and growth using top-down planar images for two of the main Hawaiian reef building corals, the plating Montipora capitata and the massive Porites compressa. Pyramid assays housed and outplanted from the in-situ nursery showed no effect of residence time or size on overall survivorship or growth for either species. Results from the ex-situ nursery, however, varied by species, with P. compressa again showing no effect of nursery residence time or size on survivorship or growth. In contrast, nursery culture resulted in improved survivorship of small M. capitata fragments, but net growth showed a weak positive effect of nursery time for medium fragments. Small fragments still suffered higher mortality than either medium or large fragments. Due to their lower mortality, medium fragments (≈3 cm²) appear to be the best compromise between growth and survivorship for outplanting. Likewise, given weak positive gains relative to the investment, our results suggest that it could be more cost-effective to simply outplant medium fragments as soon as possible, without intermediate culture in a nursery. Furthermore, the block assay revealed significant differences in survivorship and growth among sites for individuals of both species, emphasizing the importance of considering spatial variation in coral survival and growth following outplanting. These results highlight the value of using short-term micro-fragmentation assays prior to outplanting to assess size, and location specific performance, optimizing the efficiency of active reef restoration activities and maximizing the probability of success for active coral restoration projects.

Mechanisms and potential immune tradeoffs of accelerated coral growth induced by microfragmentation

Microfragmentation is the act of cutting corals into small pieces (~1 cm²) to accelerate the growth rates of corals relative to growth rates observed when maintaining larger-sized fragments. This rapid tissue and skeletal expansion technique offers great potential for supporting reef restoration, yet the biological processes and tradeoffs involved in microfragmentation-mediated accelerated growth are not well understood. Here we compared growth rates across a range of successively smaller fragment sizes in multiple genets of reef-building corals, Orbicella faveolata and Montastraea cavernosa. Our results confirm prior findings that smaller initial sizes confer accelerated growth after four months of recovery in a raceway. O. faveolata transcript levels associated with growth rate include genes encoding carbonic anhydrase and glutamic acid-rich proteins, which have been previously implicated in coral biomineralization, as well as a number of unannotated transcripts that warrant further characterization. Innate immunity enzyme activity assays and gene expression results suggest a potential tradeoff between growth rate after microfragmentation and immune investment. Microfragmentation-based restoration practices have had great success on Caribbean reefs, despite widespread mortality among wild corals due to infectious diseases. Future studies should continue to examine potential immune tradeoffs throughout the microfragmentation recovery period that may affect growout survival and disease transmission after outplanting.

Growth and survival among Hawaiian corals outplanted from tanks to an ocean nursery are driven by individual genotype and species differences rather than preconditioning to thermal stress

The drastic decline in coral coverage has stimulated an interest in reef restoration, and various iterations of coral nurseries have been used to augment restoration strategies. Here we examine the growth of two species of Hawaiian Montipora that were maintained in mesocosms under either ambient or warmed annual bleaching conditions for two consecutive years prior to outplanting to determine whether preconditioning aided coral restoration efforts. Using coral trees to create a nearby ocean nursery, we examined whether: (1) previous ex situ mesocosm growth would mirror in situ coral tree nursery growth; and (2) thermal ex situ stress-hardening would predict future success during natural warming events in situ for corals moved from tanks to trees. For Montipora capitata, we found that variation in growth was explained primarily by genotype; growth rates in the mesocosms were similar to those in situ, irrespective of preconditioning. Variation in M. flabellata growth, however, was explained by both genotype and culture method such that an individual M. flabellata colony that grew well in the tanks did not necessarily perform as well on the coral trees. For both species, previous exposure to elevated temperatures in the mesocosms provided no benefit to either growth or survival during a warming event in the coral tree nursery compared to those grown in ambient temperatures. Overall, M. capitata performed better in the tree nursery with higher net growth, lower mortality, and was subject to less predation than M. flabellata. Our results show little benefit of the additional cost and time of stress-hardening these corals prior to outplanting because it is unlikely to aid resilience to future warming events. These results also suggest that selecting corals for restoration based on long-term genotype growth performance may be more effective for optimal outcomes but should be weighed against other factors, such as coral morphology, in situ nursery method, location, and other characteristics.

Comparing different farming habitats for mid-water rope nurseries to advance coral restoration efforts in the Maldives

The need for comprehensive and effective coral restoration projects, as part of a broader conservation management strategy, is accelerating in the face of coral reef ecosystem decline. This study aims to expand the currently limited knowledge base for restoration techniques in the Maldives by testing the performance of mid-water rope nurseries in a lagoon and a reef habitat. We examined whether different coral farming habitats impacted fragment survival, health and growth of two coral genera and how the occurrence of mutualistic fauna, predation and disease influenced coral rearing success. Two nurseries were stocked with a total of 448 Pocillopora verrucosa and 96 Acropora spp. fragments, divided into different groups (four Pocillopora groups: lagoon nursery at 5 m; reef nursery at 5, 10 and 15 m; two Acropora groups: lagoon nursery at 5 m and reef nursery at 5 m). Eight fragment replicates from the same donor colony (Pocillopora genets: N = 14, Acropora genets N = 6) were used in each group and monitored for one year. Our results show that fragment survival was high in both farming habitats (>90%), with P. verrucosa surviving significantly better in the lagoon and Acropora spp. surviving and growing significantly faster in the reef nursery. P. verrucosa growth rates were similar between reef and lagoon habitat. Different rearing depths in the reef nursery had no impact on the survival of P. verrucosa but coral growth decreased considerably with depth, reducing fragments' ecological volume augmentation and growth rates by almost half from 5 to 15 m depth. Further, higher fish predation rates on fragments were recorded on the reef, which did not impact overall nursery performance. Mutualistic fauna, which correlated positively with fragment survival, was more frequently observed in the lagoon nursery. The occurrence of disease was noted in both habitats, even though implications for fragment health were more severe in the lagoon. Overall, our study demonstrates that lagoon and reef nurseries are suitable for rearing large numbers of coral fragments for transplantation. Nevertheless, we recommend considering the specific environmental conditions of the farming habitat, in particular water quality and year-round accessibility, in each case and to adjust the coral farming strategy accordingly. We hope that this novel research encourages the increased application of mid-water rope nurseries for 'coral gardening' to advance coral reef recovery and climate resilience in the Maldives.