Surviving after settlement through the first year of life is a recognised bottleneck in up-scaling reef coral restoration. Incorporating spatial refugia in settlement devices has the potential to alleviate some hazards experienced by young recruits, such as predation and accidental grazing, and can increase the likelihood of survival to size-escape thresholds. Yet optimising the design of microrefugia is challenging due to the complexity of physical and biological processes that occur at fine spatial scales around a recruit. Here, we investigated the effects of microhabitat features on the survival of Acropora tenuis spat in a year-long experimental field deployment of two types of artificial settlement devices—grooved-tiles and lattice-grids—onto three replicate racks on a shallow, central mid-shelf reef of the Great Barrier Reef. Spat survival across device types averaged between 2 and 39% and about half of all devices had at least one surviving coral after a year. While the larvae settled across all micro-habitats available on the devices, there was strong post-settlement selection for corals on the lower edges, lower surfaces, and in the grooves, with 100% mortality of recruits on upper surfaces, nearly all within the first 6 months of deployment. The device type that conferred the highest average survival (39%) was a tile with wide grooves (4 mm) cut all the way through, which significantly improved survival success over flat and comparatively featureless control tiles (13%). We hypothesise that the wide grooves provided protection from accidental grazing while also minimising sediment accumulation and allowing higher levels of light and water flow to reach the recruits than featureless control devices. We conclude that incorporating design features into deployment devices such as wide slits has the potential to substantially increase post-deployment survival success of restored corals.
Natural bleaching events provide an opportunity to examine how local‐scale environmental variation influences bleaching severity and recovery. During the 2020 marine heat wave, we documented widespread and severe coral bleaching affecting 75%–98% of coral cover throughout the Keppel Islands in the southern inshore Great Barrier Reef. Acropora, Pocillopora, and Porites were the most severely affected genera, while Montipora was comparatively less susceptible. Site‐specific heat‐exposure metrics were not correlated with Acropora bleaching severity, but recovery was faster at sites that experienced lower heat exposure. Despite severe bleaching and exposure to accumulated heat that often results in coral mortality (degree heating weeks ~4–8), cover remained stable. Approximately 94% of fate‐tracked Acropora millepora colonies survived, perhaps due to reduced irradiance stress from high turbidity, heterotrophic feeding, and large tidal flows that can increase mass transfer. Severe bleaching followed by rapid recovery and the continuing dominance of Acropora populations in the Keppel Islands is indicative of high resilience. These coral communities have survived a 0.8°C increase in average temperatures over the last 150 years. However, recovery following the 2020 bleaching was driven by the easing of thermal stress, which may challenge their recovery potential under further warming.
Coral seeding is a promising intervention being tested and applied in global reef-restoration efforts. However, high mortality of newly settled corals, particularly in degraded environments, is one of several hurdles to overcome to up-scale this technique. Incorporating design features that mitigate drivers of mortality could increase coral-seeding effectiveness. Here we tested five ceramic seeding-device designs that incorporated various features intended to act as coral microrefugia. Devices seeded with Acropora tenuis spat were deployed to three sites on a shallow mid-shelf reef on the central Great Barrier Reef (GBR), and survival was assessed after 219 days. Average yield (device-level survival) was moderate across the device designs (22%) and all devices performed comparably. However, there was a nearly threefold increase in survival at one site compared with the other, similar sites less than 100 m away. Assessment of the benthic community around each device suggested that the sites hosted similar communities, although multivariate analyses identified a weak but significant positive relationship between survival and the first principal component (only 17% of variance), characterized by higher cover of sponges and epilithic algal communities. These results suggest that within-reef scale variations (10s of meters) in environmental conditions, or variations in community constituents not investigated here (e.g. grazing fishes, mobile invertebrates), may impact the success of coral seeding. Further investigations into the drivers of coral survival at fine spatial scales, particularly abiotic conditions, are required to guide the placement of coral-seeding devices and to predict their effectiveness.
Embedding and immobilisation of living cells and microorganisms is used in a variety of research and commercial applications. Here we report the successful extended immobilisation of coral larvae in a low-gelling temperature agarose. Embryos and larvae of five broadcast-spawning Scleractinian species were immobilised in agarose gel and tested in a series of exploratory survival and settlement assays. The optimal developmental stage for immobilisation was after ciliation at approximately 24 hours post-fertilisation, after which, survival of immobilised larvae of all species was nearly 100%. In long-term assays, 50% of Montipora digitata larvae survived immobilised for 89 days. Furthermore, immobilised larvae of multiple species, that were released from the agarose, generally remained capable of settlement. These results demonstrate that the immobilisation of the early life-history stages of corals is possible for a variety of applications in basic and applied science.
Many species of colonial corals have sterile zones—areas where polyps are sexually immature. While inconspicuous in many species, the sterile zones observed in Acropora glauca in 2019 were striking, likely due to the tabular growth morphology, highly pigmented eggs, and high degree of intracolony spawning synchrony in this species.
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