Biology subjects

Lachs, L.

Publications and source records attributed to Lachs, L..

10 recordsLinked to original sources

bioRxiv2026

Juveniles exhibit lower heatwave tolerance than adults in a reef-building coral

Marine heatwaves cause widespread coral bleaching and mortality, yet thermal responses often differ between adult and juvenile corals, with the latter frequently being more tolerant. Whether the observed enhanced tolerance of juveniles reflects intrinsic physiological differences across life stages or simply reduced environmental stress due to shaded microhabitats occupied by juveniles, or a combination of both factors, remains unclear. To test whether colony size determines thermal responses and if a threshold size exists for tolerance, we exposed 89 colonies of the reef-building coral Acropora aff. digitifera to a 6-week, 32.5 C simulated marine heatwave under controlled light conditions, with cumulative stress reaching 20 C-weeks. Contrary to expectations, ~2 C-weeks less heat stress was required to trigger bleaching and mortality in coral juveniles compared to adults. Mortality divergence across colony sizes was greatest at ~10 C-weeks, when the smallest corals exceeded 80% mortality, whereas the largest colonies showed <10% mortality. A breakpoint in survival odds was apparent at 13.4 cm diameter, corresponding to the size at onset of sexual maturity in the native wild population for this species. These results indicate an ontogenetic shift in physiological heat tolerance, with larger reproductively mature colonies showing enhanced tolerance. The apparent survival advantage of juveniles in the reef frequently reported during bleaching events could be more likely due to micro-environmental buffering than intrinsic physiological tolerance. Our findings suggest that juvenile corals may be more vulnerable to climate change than previously thought, particularly as declining habitat complexity could reduce microhabitat refugia on future reefs.

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bioRxiv2026

The CoralAssist Plug: a novel device with built-in microrefugia for settling, rearing and rapidly outplanting corals

Sexual coral propagation is an emerging technique capable of producing large numbers of corals for coral transplantation and reef rehabilitation. In contrast to asexual coral propagation, sexual propagation increases genotypic diversity and can be used for selective breeding to enhance coral heat tolerance or other desirable traits. However, implementation at meaningful ecological scales is hindered by high mortality during early life stages, high costs associated with nursery rearing facilities, and labour-intensive outplanting methods. To overcome these issues, we developed the CoralAssist Plug (CAP), a ceramic device designed for the rapid and cost-effective outplanting of sexually propagated corals in large numbers that maximises post-outplant survivorship. CAPs combine three important functional features: 1) built-in microrefugia to protect juvenile corals from grazing, 2) a relatively small size, 3 by 1 cm, that is easy to handle and stack efficiently without compromising the survivorship of corals, and 3) a hole in the middle that facilitates handling and attachment. CAPs were settled with Acropora aff. digitifera and outplanted to a reef crest after 1 to 6 months of ex situ nursery rearing. A 3-person dive team was able to outplant ~120 CAPs in one 90-minute shallow dive (just over 2 minutes per CAP per person). With longer nursery durations of 6 months, it was possible to achieve 36 % yield (i.e., the proportion of devices with a surviving coral) 4-years post-outplant. With nursery durations shortened to 1 month, we were able to attain 24 % yield 3-years post-outplant. Microrefugia significantly enhanced post-outplant survivorship leading to an 11 % increase in yield 4 years post outplant compared to devices without microrefugia. Outplanted corals that had reached adult size, were self-attached and were reproductively mature after 4 years. Our results suggest that CAPs can play a meaningful role in reef rehabilitation by efficiently introducing sexually propagated corals into natural populations with clear applications to assisted evolution techniques, such as selective breeding.

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bioRxiv2025

Accessible AI Enhances Monitoring of Coral Seeding Devices in Reef Restoration

1. Coral seeding devices (CSDs) - tools designed to deliver sexually propagated corals to target locations - offer a promising means to increase coral abundance on degraded reefs. However, evaluating CSD effectiveness for coral reef restoration across wide areas and over many years is limited by the lack of robust and efficient monitoring approaches. Large-area reef imagery offers an attractive potential solution, but manual detection of CSDs within imagery is slow and limits the scalability of CSD monitoring. 2. We investigated whether machine learning classifiers could accurately detect CSDs in reef orthoimages and tested the performance of classifiers created following minimal manual annotation effort. Using freely available software, we first evaluated classifier performance in a single-site experiment using an orthoimage containing 989 CSDs deployed in Palau. We then also evaluated performance in a multi-site experiment using orthoimages containing a different CSD design deployed across seven sites in the central Great Barrier Reef, Australia. 3. In Experiment 1, classifiers trained on just 30 CSDs annotated within 10 minutes achieved mean recall and precision of 96.7% and 97.1% respectively, reducing manual annotation time by 95.6% whilst still detecting 98.8% of the number of devices found manually. Larger training sets yielded less reliable classifiers and required more manual effort. In Experiment 2, classifiers trained on 30 CSD annotations from one site performed excellently across seven orthoimages from multiple reefs, achieving mean recall and precision of 99.2% and 93.3%. 4. We present evidence that CSD classifiers can be highly effective across both single- and multi-site CSD deployments. In using a free and user-friendly software, we also demonstrate their accessibility to reef restoration practitioners. To facilitate wider uptake of CSD monitoring, we provide a step-by-step protocol for implementing CSD classifiers. By improving access to efficient, direct assessment of intervention outcomes, this method can play a vital role in guiding the enhancement of approaches aiming to restore coral reefs.

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bioRxiv2025

Proximity and current alignment drives fertilisation success in a broadcast-spawning coral

As coral populations decline under climate change and other stressors, surviving corals will become increasingly isolated. While Allee effects that reduce gamete encounters and fertilisation success may lead to reproductive and recruitment failure, the critical population densities and distances between individuals required to maintain viable populations remains uncertain. Here, we investigate the links between colony isolation and fertilisation patterns using an experimentally manipulated patch of the broadcast spawning tabular coral, Acropora hyacinthus. Corals were arranged in a clustered radial design, with colonies arranged at increasing distances radiating downstream from a central spawning aggregation. Fertilisation and paternity assignment analyses were used to examine the influence of parental distance, current alignment, colony size, and genetic relatedness. Fertilisation success declined sharply with increasing distance among corals, and paternity assignments indicated most parents were located within 3 m of each other, underscoring the importance of colony proximity. Additionally, fertilisation success was highest for colonies positioned downstream of the central cluster, and 84% of sequenced progeny were sired by colonies located directly upstream. Simulations of natural reef spatial population distributions projected onto a virtual grid, incorporating empirically derived parentage distances, indicated that populations remained well-mixed at typical adult densities in nearby reefs. However, as densities decreased to 1 colony per 100 m-2 (i.e. 0.01 colonieslllm-2), reproductive isolation and the formation of patchy breeding units became prevalent. Overall, our findings highlight that the spatial arrangement and isolation of corals on reefs, and subsequent Allee effects on fertilisation, can jeopardise the reproductive success of broadcast spawning corals. Conservation efforts are needed to help maintain viable coral populations and successful reproductive aggregations for degraded reef systems.

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bioRxiv2025

A rapidly closing window for coral persistence under global warming

Summary paragraphCoral reefs around the world are threatened by recurrent marine heatwaves causing mass coral bleaching and mortality1. Mitigating future warming impacts requires strategic management2 that adopts a long-term lens. Global analyses of projected heatwaves3,4 are critical for decision-making but how disturbance refugia, coral life-histories and adaptation interact with warming is unknown. Here, we simulate coral eco-evolutionary dynamics across >3,800 individual reefs of Australias iconic Great Barrier Reef under the current suite of climate projections5. We forecast a rapid coral decline before mid-century regardless of emissions scenario. Under the most likely warming6, corals may continue to decline throughout the century. However, corals may recover after mid-century provided that warming is sufficiently slow, allowing thermal adaptation to keep pace with temperature changes - as projected if warming does not exceed 2{degrees}C. Higher emission scenarios would drive most reefs to a near collapse. Future resilient reefs were mostly found in bleaching refugia, which also possessed a greater diversity of thermal phenotypes. Although cool-adapted corals disperse to thermal hotspots, there was no evidence of gene swamping undermining thermal adaptation. While opportunities exist to build management strategies that promote adaptation in thermal refugia and warm spots, curbing the rate of warming by 2050 is crucial for coral persistence.

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bioRxiv2024

Excluding Large Grazers Dramatically Improves Survival of Outplanted Juvenile Corals

High mortality rates of juvenile corals hinder both the natural recovery of populations and the successful implementation of restoration efforts. Grazing is a significant cause of juvenile coral mortality, and grazer exclusion devices have been shown to increase juvenile coral survivorship. However, most experiments have used cages that typically alter water flow and light conditions and exclude grazers of most sizes, making it difficult to quantify the effects of large grazers alone. Here, we test whether deterring large grazers can increase the survival and growth of six-month-old Acropora digitifera juveniles outplanted to a shallow reef crest, using arrangements of two or four long or short masonry nails that selectively exclude larger grazers (e.g., parrotfish) while minimising abiotic changes. By the end of our study, colonies with deterrents had significantly larger planar area (almost tenfold for the most effective treatment), more branches, greater height, and enhanced survival than those without deterrents. A critical period is the first week after outplanting when colonies with deterrents had significantly less tissue loss from grazing than those without. Less tissue loss in the first week was associated with significantly higher survival over the following 14 months, with an almost threefold improvement for the most effective treatment. For heavily grazed systems, our study highlights the importance of incorporating grazing deterrents into outplant devices to counteract the negative impact of large grazers on outplanted juvenile coral survival and boost restoration success.

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bioRxiv2023

Unexpectedly high coral heat tolerance at thermal refugia

Marine heatwaves and mass bleaching have led to global declines in coral reefs. Corals can adapt, yet, to what extent local variations in thermal stress regimes influence heat tolerance and adaptive potential remains uncertain. Here we identify persistent local-scale thermal refugia and hotspots among the reefs of a remote Pacific archipelago, based on 36 years of satellite-sensed temperatures. Theory suggests that hotspots should promote coral heat tolerance through acclimatisation and directional selection. While historic patterns of mass bleaching and marine heatwaves align with this expectation, we find a contrasting pattern for a single species, Acropora digitifera, exposed to a marine heatwave experiment. Higher heat tolerance at thermal refugia (+0.7 {degrees}C-weeks) and correlations with other traits suggest that non-thermal selective pressures may also influence heat tolerance. We also uncover widespread heat tolerance variability, indicating climate adaptation potential. Compared to the least-tolerant 10% of the A. digitifera population, the most-tolerant 10% could withstand an additional heat stress of 5.2 and 4.1 {degrees}C-weeks for thermal refugia and hotspots, respectively. Despite expectations, local-scale thermal refugia can harbour higher heat tolerance, and mass bleaching patterns do not necessarily predict species responses. This has important implications for designing climate-smart initiatives to tackle global-scale adaptive management problems.

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bioRxiv2022

High-latitude marginal reefs support fewer but bigger corals than their tropical counterparts

AimDespite the awareness that climate change impacts are typically detrimental to tropical coral reefs, the effect of increasing environmental stress and variability on the population size structure of coral species remains poorly understood. This gap in knowledge limits our ability to effectively conserve coral reef ecosystems because size specific dynamics are rarely incorporated. Our aim is to quantify variation in the size structure of coral populations along a tropical-to-subtropical environmental gradient. Location20 coral populations along a latitudinal gradient on the east coast of Australia ([~]23{degrees}S to 30{degrees}S). Time PeriodBetween 2010 and 2018. Major taxa studiedScleractinian corals. MethodsWe apply two methods to quantify the relationship between environmental covariates and coral population size structure along a latitudinal environmental gradient. First, we use linear regression with summary statistics, such as median size as response variables; a method frequently favoured by ecologists. The second method is compositional functional regression, a novel method using entire size-frequency distributions as response variables. We then predict coral population size structure with increasing environmental stress and variability. ResultsCompared to tropical reefs, we find fewer but larger coral colonies in marginal reefs, where environmental conditions are more variable and stressful for corals in the former. Our model predicts that coral populations may become gradually dominated by larger colonies (> 148 cm2) with increasing environmental stress. Main conclusionsWith increasing environmental stress and variability, we can expect shifts in coral population size structure towards more larger colonies. Fewer but bigger corals suggest low survival, slow growth, and poor recruitment. This finding is concerning for the future of coral reefs as it implies populations may have low recovery potential from disturbances. We highlight the importance and usefulness of continuously monitoring changes to population structure over large spatial scales. Data availabilityData is supplied in the supplementary information, or upon request. Once accepted for publication it will be made openly available on Dryad.

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bioRxiv2021

Fine-tuning heat stress algorithms to optimise global predictions of mass coral bleaching

Increasingly severe marine heatwaves under climate change threaten the persistence of many marine ecosystems. Mass coral bleaching events, caused by periods of anomalously warm sea surface temperatures (SST), have led to catastrophic levels of coral mortality globally. Remotely monitoring and forecasting such biotic responses to heat stress is key for effective marine ecosystem management. The Degree Heating Week (DHW) metric, designed to monitor coral bleaching risk, reflects the duration and intensity of heat stress events, and is computed by accumulating SST anomalies (HotSpot) relative to a stress threshold over a 12-week moving window. Despite significant improvements in the underlying SST datasets, corresponding revisions of the HotSpot threshold and accumulation window are still lacking. Here, we fine-tune the operational DHW algorithm to optimise coral bleaching predictions using the 5km satellite-based SSTs (CoralTemp v3.1) and a global coral bleaching dataset (37,871 observations, National Oceanic and Atmospheric Administration). After developing 234 test DHW algorithms with different combinations of HotSpot threshold and accumulation window, we compared their bleaching-prediction ability using spatiotemporal Bayesian hierarchical models and sensitivity-specificity analyses. Peak DHW performance was reached using HotSpot thresholds less than or equal to Maximum Monthly Mean SST and accumulation windows of 4 - 8 weeks. This new configuration correctly predicted up to an additional 310 bleaching observations compared to the operational DHW algorithm, an improved hit rate of 7.9 %. Given the detrimental impacts of marine heatwaves across ecosystems, heat stress algorithms could also be fine-tuned for other biological systems, improving scientific accuracy, and enabling ecosystem governance.

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bioRxiv2021

A framework for selectively breeding corals for assisted evolution

Coral cover on tropical reefs has declined during the last three decades due to the combined effects of climate change, destructive fishing, pollution, and land use change. Drastic reductions in greenhouse gas emissions combined with effective coastal management and conservation strategies are essential to slow this decline. Innovative approaches, such as selective breeding for adaptive traits combined with large-scale sexual propagation, are being developed with the aim of pre-adapting reefs to increased ocean warming. However, there are still major gaps in our understanding of the technical and methodological constraints to producing corals for such restoration interventions. Here we propose a framework for selectively breeding corals and rearing them from eggs to 2.5-year old colonies using the coral Acropora digitifera as a model species. We present methods for choosing colonies for selective crossing, enhancing early survivorship in ex situ and in situ nurseries, and outplanting and monitoring colonies on natal reefs. We used a short-term (7-day) temperature stress assay to select parental colonies based on heat tolerance of excised branches. From six parental colonies, we produced 12 distinct crosses, and compared survivorship and growth of colonies transferred to in situ nurseries or outplanted to the reef at different ages. We demonstrate that selectively breeding and rearing coral colonies is technically feasible at small scales and could be upscaled as part of restorative assisted evolution initiatives. Nonetheless, there are still challenges to overcome before selective breeding can be implemented as a viable conservation tool, especially at the post-settlement and outplanting phases. Although interdisciplinary approaches will be needed to overcome many of the challenges identified in this study, selective breeding has the potential to be a viable tool within reef managers toolbox to support the persistence of selected reefs in the face of climate change.

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