Fast‐growing oysters show reduced capacity to provide a thermal refuge to intertidal biodiversity at high temperatures

Resilience against the impacts of climate change in an ecologically and economically significant native oyster

Climate change is acidifying and warming our oceans, at an unprecedented rate posing a challenge for marine invertebrates vital across the globe for ecological services and food security. Here we show it is possible for resilience to climate change in an ecologically and economically significant oyster without detrimental effects to the energy budget. We exposed 24 pair-mated genetically distinct families of the Sydney rock oyster, Saccostrea glomerata to ocean acidification and warming for 4w and measured their resilience. Resilience was identified as the capacity to defend their acid-base balance without a loss of energy available for Scope for Growth (SFG). Of the 24 families, 13 were better able to defend their acid-base balance while eight had no loss of energy availability with a positive SFG. This study has found oyster families with reslience against climate change without a loss of SFG, is an essential mitigation strategy, in a critical mollusc.

Acclimation in intertidal animals reduces potential pathogen load and increases survival following a heatwave

Intertidal animals can experience intense heat during a heatwave, leading to mortality. The causes of death for intertidal animals following heatwaves have often been attributed to a breakdown in physiological processes. This, however, contrasts with research in other animals where heatwave mortality is attributed to existing or opportunistic diseases. We acclimated intertidal oysters to four treatment levels, including an antibiotic treatment, and then exposed all treatments to a 50°C heatwave for 2 h, replicating what can be experienced on Australian shorelines. We found that both acclimation and antibiotics increased survival and reduced the presence of potential pathogens. Non-acclimated oysters had a significant shift in their microbiome, with increasing abundances of bacteria from the Vibrio genera, including known potential pathogens. Our results demonstrate that bacterial infection plays a pivotal role in post-heatwave mortality. We anticipate these findings to inform the management of aquaculture and intertidal habitats as climate change intensifies.

Spatiotemporal stressors, not secondary structures or small temperature increases, control rapid facilitation of intertidal epifauna

Small animals (epifauna) are ubiquitous in marine systems. Epifauna have high secondary production and provide trophic linkages between primary producers and higher-order consumers, like fish. Despite their importance, little is known about how these animals respond to warming or how their communities vary across spatiotemporal gradients. Here we use mimics of turf seaweed and invasive kelp holdfast to test, in a 5-factorial field experiment, whether intertidal epifauna are facilitated by different habitat structures, temperature conditions, and along cooccurring spatiotemporal gradients. We found that facilitation of epifauna by intertidal turf seaweed peaked in summer, at low elevation, in older habitats and at a less wave-exposed site. However, epifauna were not affected by the presence of a secondary structure like kelp holdfast mimics or small temperature increases from passive solar heating of black and white mimics. There were many significant two-way, but few higher order interactions, showing stronger facilitation under specific environmental conditions, like at low elevation in summer, or low elevation in old habitats. These results highlight that turf-associated epifauna are controlled by vertical elevation, season, hydrodynamics, and habitat age, and appear to be resilient to small temperature increases. Findings are important to better understand linkages between primary producers and higher order consumers and system-wide productivity, and because fast growing turf, facilitated by global warming and eutrophication, are increasingly outcompeting slower growing large perennial canopy forming seaweeds, like kelp and rockweeds.

Assessing the impact of atmospheric heatwaves on intertidal clams

Heatwaves have become more frequent and intense in the last two decades, resulting in detrimental effects on marine bivalves and ecosystems they sustain. Intertidal clams inhabit the most physiologically challenging habitats in coastal areas and live already near their thermal tolerance limits. However, whether and to what extent atmospheric heatwaves affect intertidal bivalves remain poorly understood. Here, we investigated physiological responses of the Manila clam, Ruditapes philippinarum, to heatwaves at air temperature regimes of 40 °C and 50 °C occurring frequently and occasionally at the present day in the Beibu Gulf, South China Sea. With the increasing intensity of heatwaves and following only two days of aerial exposure, Manila clams suffered 100 % mortality at 50 °C, indicating that they succumb to near future heatwaves, although they survived under various scenarios of moderate heatwaves. The latter is couched in energetic terms across levels of biological organization. Specifically, Manila clams acutely exposed to heatwaves enhanced their standard metabolic rate to fuel essential physiological maintenance, such as increasing activities of SOD, CAT, MDA, and AKP, and expression of HSP70. These strategies occur likely at the expense of fitness-related functions, as best exemplified by significant depressions in activities of enzymes (NKA, CMA, and T-ATP) and expression levels of genes (PT, KHK, CA, CAS, TYR, TNF-BP, and OSER). When heatwaves occurred again, Manila clams can respond and acclimate to thermal stress by implementing a suite of more ATP-efficient and less energy-costly compensatory mechanisms at various levels of biological organization. It is consequently becoming imperative to uncover underlying mechanisms responsible for such positive response and rapid acclimation to recurrent heatwaves.

Temperature-buffering by oyster habitat provides temporal stability for rocky shore communities

Intertidal rocky shores are considered among the most thermally stressful marine ecosystems, where many species live close to their upper thermal limit and depend on access to cool microclimates to persist through heat events. In such environments, the provision of cool microclimates by habitat-forming species enables persistence of associated species during high temperature events. We assessed whether, by maintaining cool microclimates through heat events, habitat formed by rock oysters (Saccostrea cucullata) provides temporal stability to associated invertebrate communities over periods of extreme temperatures. On three tropical rocky shores of Hong Kong, which experiences a monsoonal climate, we compared changes in microclimates and invertebrate communities associated with oyster and bare rock habitats between the cool and hot seasons. Oyster habitats were, across both seasons, consistently characterised by lower maximum temperatures and greater thermal stability than bare rock habitats. Invertebrate communities in the bare rock habitat were less diverse and abundant in the hot than the cool season, but communities in the cooler habitats provided by oysters did not display temporal change. These results demonstrate that microclimates formed by oysters provide temporal stability to associated communities across periods of temperature change and are key determinants of species distributions in thermally stressful environments. The conservation and restoration of oyster habitats may, therefore, build resilience in associated ecological communities subject to ongoing environmental change.

Impacts of marine heatwaves on pearl oysters are alleviated following repeated exposure

Marine heatwaves (MHWs) have occurred with increasing duration, frequency and intensity in the past decade in the South China Sea, posing serious threats to marine ecosystems and fisheries. However, the impact of MHWs on marine bivalves - one of the most ecologically and economically important fauna in coastal ecosystems - remains largely unknown. Here, we investigated physiological responses of the pearl oyster, Pinctada maxima inhabiting a newly identified climate change hotspot (Beibu Gulf, South China Sea) to short-lasting and repeatedly-occurring MHWs scenarios. Following 3-day exposure to short-lasting MHWs scenarios with water temperature rapidly arising from 24 °C to 28 °C, 32 °C and 36 °C, respectively, mortality rates of pearl oysters increased, and especially they suffered 100% mortality at 36 °C. Activities of enzymes including acid phosphatase (ACP), alkaline phosphatase (AKP), glutathione (GSH) and level of malondialdehyde (MDA) increased significantly with increasing intensity and duration of MHWs, indicating thermal stress responses. When exposed to repeatedly-occurring MHWs scenarios, mortality rates of pearl oysters increased slightly, and thermal stress responses were alleviated, as exemplified by significant decreases in ACP, AKP, GSH and MDA activities compared with those during short-lasting MHWs scenarios, demonstrating the potential of P. maxima to acclimate rapidly to MHWs. These findings advance our understanding of how marine bivalves respond to MHWs scenarios varying in duration, frequency, and intensity.

Spatially Variable Effects of Artificially-Created Physical Complexity on Subtidal Benthos

In response to the environmental damage caused by urbanization, Nature-based Solutions (NbS) are being implemented to enhance biodiversity and ecosystem processes with mutual benefits for society and nature. Although the field of NbS is flourishing, experiments in different geographic locations and environmental contexts have produced variable results, with knowledge particularly lacking for the subtidal zone. This study tested the effects of physical complexity on colonizing communities in subtidal habitats in two urban locations: (1) Plymouth, United Kingdom (northeast Atlantic) and (2) Tel Aviv, Israel (eastern Mediterranean) for 15- and 12-months, respectively. At each location, physical complexity was manipulated using experimental tiles that were either flat or had 2.5 or 5.0 cm ridges. In Plymouth, biological complexity was also manipulated through seeding tiles with habitat-forming mussels. The effects of the manipulations on taxon and functional richness, and community composition were assessed at both locations, and in Plymouth the survival and size of seeded mussels and abundance and size of recruited mussels were also assessed. Effects of physical complexity differed between locations. Physical complexity did not influence richness or community composition in Plymouth, while in Tel Aviv, there were effects of complexity on community composition. In Plymouth, effects of biological complexity were found with mussel seeding reducing taxon richness, supporting larger recruited mussels, and influencing community composition. Our results suggest that outcomes of NbS experiments are context-dependent and highlight the risk of extrapolating the findings outside of the context in which they were tested.

Contemporary Oyster Reef Restoration: Responding to a Changing World

Globally, there is growing interest in restoring previously widespread oyster reefs to reinstate key ecosystem services such as shoreline protection, fisheries productivity and water filtration. Yet, since peak expiration of oysters in the 1800s, significant and ongoing environmental change has occurred. Estuaries and coasts are undergoing some of the highest rates of urbanization, warming and ocean acidification on the planet, necessitating novel approaches to restoration. Here, we review key design considerations for oyster reef restoration projects that maximize the probability that they will meet biological and socio-economic goals not only under present-day conditions, but into the future. This includes selection of sites, and where required, substrates and oyster species and genotypes for seeding, not only on the basis of their present and future suitability in supporting oyster survival, growth and reproduction, but also based on their match to specific goals of ecosystem service delivery. Based on this review, we provide a road map of design considerations to maximize the success of future restoration projects.

Making seawalls multifunctional: The positive effects of seeded bivalves and habitat structure on species diversity and filtration rates

The marine environment is being increasingly modified by the construction of artificial structures, the impacts of which may be mitigated through eco-engineering. To date, eco-engineering has predominantly aimed to increase biodiversity, but enhancing other ecological functions is arguably of equal importance for artificial structures. Here, we manipulated complexity through habitat structure (flat, and 2.5 cm, 5 cm deep vertical and 5 cm deep horizontal crevices) and seeding with the native oyster (Saccostrea glomerata, unseeded and seeded) on concrete tiles (0.25 m × 0.25 m) affixed to seawalls to investigate whether complexity (both orientation and depth of crevices) influences particle removal rates by suspension feeders and colonisation by different functional groups, and whether there are any ecological trade-offs between these functions. After 12 months, complex seeded tiles generally supported a greater abundance of suspension feeding taxa and had higher particle removal rates than flat tiles or unseeded tiles. The richness and diversity of taxa also increased with complexity. The effect of seeding was, however, generally weaker on tiles with complex habitat structure. However, the orientation of habitat complexity and the depth of the crevices did not influence particle removal rates or colonising taxa. Colonisation by non-native taxa was low compared to total taxa richness. We did not detect negative ecological trade-offs between increased particle removal rates and diversity and abundance of key functional groups. Our results suggest that the addition of complexity to marine artificial structures could potentially be used to enhance both biodiversity and particle removal rates. Consequently, complexity should be incorporated into future eco-engineering projects to provide a range of ecological functions in urbanised estuaries.

Heatwaves alter survival of the Sydney rock oyster, Saccostrea glomerata

Heatwaves are an increasing threat to organisms across the globe. Marine and atmospheric heatwaves are predicted to impact sessile intertidal marine organisms, especially when exposed at low tide and unable to seek refuge. The study aimed to determine whether a simulated atmospheric heatwave will alter the survival of selectively bred families of Sydney rock oysters (Saccostrea glomerata), and whether survival is dependent on morphological and physiological traits. The survival of S. glomerata families to a simulated atmospheric heatwave varied from 25 to 60% and was not correlated with morphology or physiology. Survival may depend on the presence of genotypes that translate into molecular defenses such as heat-shock proteins and inhibitor of apoptosis proteins that provide oysters with resilience. Understanding the responses among families of oysters to heatwaves is critical if we are to restore the ecological services of oyster reefs and sustain oyster aquaculture.

Interacting effects of habitat structure and seeding with oysters on the intertidal biodiversity of seawalls

The construction of artificial structures, such as seawalls, is increasing globally, resulting in loss of habitat complexity and native species biodiversity. There is increasing interest in mitigating this biodiversity loss by adding topographic habitat to these structures, and/or seeding them with habitat-forming species. Settlement tile experiments, comparing colonisation of species to more and less complex habitats, have been used to inform eco-engineering interventions prior to their large-scale implementation. Most studies have focused on applying one type of intervention (either adding habitat structure or seeding with native organisms), so it is unclear whether there are greater benefits to biodiversity when multiple interventions are combined. Using a fully orthogonal experiment, we assessed the independent and interactive effects of habitat structure (flat vs. crevice/ridges) and seeding with native oysters (unseeded vs. seeded) on the biodiversity of four different functional groups (sessile and mobile taxa, cryptobenthic and pelagic fishes). Concrete tiles (flat unseeded, flat seeded, complex unseeded and complex seeded) were deployed at two sites in Sydney Harbour and monitored over 12 months, for the survival and colonisation of oysters and the species density and abundances of the four functional groups. The survival of seeded oysters was greater on the complex than flat tiles, at one of the two sites, due to the protective role of crevices. Despite this, after 12 months, the species density of sessile invertebrates and the percentage cover of seeded and colonising oysters did not differ between complex and seeded tiles each of which supported more of these variables than the flat unseeded tiles. In contrast, the species density of mobile invertebrates and cryptobenthic fishes and the MaxN of pelagic fishes, at 1 month, were only positively influenced by seeding with oysters, which provided food as well as habitat. Within the complex seeded and unseeded tiles, there was a greater species density of sessile taxa, survival and percentage cover of oysters in the crevices, which were more humid and darker at month 12, had lower high temperature extremes at months 1 and 12, than on the ridges or flat tiles. Our results suggest that eco-engineering projects which seek to maximise the biodiversity of multiple functional groups on seawalls, should apply a variety of different microhabitats and habitat-forming species, to alter the environmental conditions available to organisms.

Can prior exposure to stress enhance resilience to ocean warming in two oyster species?

Securing economically and ecologically significant molluscs, as our oceans warm due to climate change, is a global priority. South eastern Australia receives warm water in a strengthening East Australia Current and so resident species are vulnerable to elevated temperature and marine heat waves. This study tested whether prior exposure to elevated temperature can enhance resilience of oysters to ocean warming. Two Australian species, the flat oyster, Ostrea angasi, and the Sydney rock oyster, Saccostrea glomerata, were obtained as adults and "heat shocked" by exposure to a dose of warm water in the laboratory. Oysters were then transferred to elevated seawater temperature conditions where the thermal outfall from power generation was used as a proxy to investigate the impacts of ocean warming. Shell growth, condition index, lipid content and survival of flat oysters and condition of Sydney rock oysters were all significantly reduced by elevated seawater temperature in the field. Flat oysters grew faster than Sydney rock oysters at ambient temperature, but their growth and survival was more sensitive to elevated temperature. "Stress inoculation" by heat shock did little to ameliorate the negative effects of increased temperature, although the survival of heat-shocked flat oysters was greater than non-heat shocked oysters. Further investigations are required to determine if early exposure to heat stress can enhance resilience of oysters to ocean warming.

Limitations of cross- and multigenerational plasticity for marine invertebrates faced with global climate change

Although cross generation (CGP) and multigenerational (MGP) plasticity have been identified as mechanisms of acclimation to global change, the weight of evidence indicates that parental conditioning over generations is not a panacea to rescue stress sensitivity in offspring. For many species, there were no benefits of parental conditioning. Even when improved performance was observed, this waned over time within a generation or across generations and fitness declined. CGP and MGP studies identified resilient species with stress tolerant genotypes in wild populations and selected family lines. Several bivalves possess favourable stress tolerance and phenotypically plastic traits potentially associated with genetic adaptation to life in habitats where they routinely experience temperature and/or acidification stress. These traits will be important to help 'climate proof' shellfish ventures. Species that are naturally stress tolerant and those that naturally experience a broad range of environmental conditions are good candidates to provide insights into the physiological and molecular mechanisms involved in CGP and MGP. It is challenging to conduct ecologically relevant global change experiments over the long times commensurate with the pace of changing climate. As a result, many studies present stressors in a shock-type exposure at rates much faster than projected scenarios. With more gradual stressor introduction over longer experimental durations and in context with conditions species are currently acclimatized and/or adapted to, the outcomes for sensitive species might differ. We highlight the importance to understand primordial germ cell development and the timing of gametogenesis with respect to stressor exposure. Although multigenerational exposure to global change stressors currently appears limited as a universal tool to rescue species in the face of changing climate, natural proxies of future conditions (upwelling zones, CO₂ vents, naturally warm habitats) show that phenotypic adjustment and/or beneficial genetic selection is possible for some species, indicating complex plasticity-adaptation interactions.

A Decision Framework for Coastal Infrastructure to Optimize Biotic Resistance and Resilience in a Changing Climate

Coastal ecosystems are under growing pressure from human activities such as pollution and climate change. Although the rapidly growing numbers of humans living in coastal areas is a large part of the problem, there is great opportunity to improve the resistance and resilience of biotic communities via creative changes to the engineering design of built infrastructure. Here, we apply ecological theories to create a framework for adaptive building in marine systems that can be applied by managers worldwide. We explain how climate effects could be mitigated across different spatial scales with both physical and biological interventions. This requires an approach based on ecological theory that incorporates our understanding of how systems withstand (resistance) or recover (resilience) from impacts and takes into account future local and global environmental conditions. By translating ecological theory into practical application, we propose a framework for the choice and design of coastal infrastructure that can underpin effective, forward-looking conservation strategies.

The mechanisms by which oysters facilitate invertebrates vary across environmental gradients

The effective use of ecosystem engineers to conserve biodiversity requires an understanding of the types of resources an engineer modifies, and how these modifications vary with biotic and abiotic context. In the intertidal zone, oysters engineer ecological communities by reducing temperature and desiccation stress, enhancing the availability of hard substrate for attachment, and by ameliorating biological interactions such as competition and predation. Using a field experiment manipulating shading, predator access and availability of shell substrate at four sites distributed over 900 km of east Australian coastline, we investigated how the relative importance of these mechanisms of facilitation vary spatially. At all sites, and irrespective of environmental conditions, the provision of hard substrate by oysters enhanced the abundance and richness of invertebrates, in particular epibionts (barnacles and oyster spat) and grazing gastropods. Mobile arthropods utilised the habitat provided by disarticulated dead oysters more than live oyster habitat, whereas the abundance of polychaetes and bivalves were much greater in live oysters, suggesting the oyster filter-feeding activity is important for these groups. In warmer estuaries, shading by oysters had a larger effect on biodiversity, whereas in cooler estuaries, the provision of a predation refuge by oysters played a more important role. Such knowledge of how ecosystem engineering effects vary across environmental gradients can help inform management strategies targeting ecosystem resilience via the amelioration of specific environmental stressors, or conservation of specific community assemblages.

Restoring oysters to urban estuaries: Redefining habitat quality for eastern oyster performance near New York City

Restoring and conserving coastal resilience faces increasing challenges under current climate change predictions. Oyster restoration, in particular, faces threats from alterations in precipitation, warming water temperatures, and urbanization of coastlines that dramatically change salinity patterns, foster the proliferation and spread disease, and disrupt habitat connectivity, respectively. New York City (NYC) coastal waters, once home to a booming oyster fishery for eastern oysters (Crassostrea virginica), are now nearly devoid of live oyster reefs. Oyster restoration in urban estuaries is motivated by the synergistic ecosystem benefits this native keystone species can deliver. Recent surveys have documented substantial remnant populations of adult oysters in the upper low salinity zone of the Hudson/Raritan Estuary (HRE) near Tarrytown, NY. This study assessed fitness-related performance across the HRE salinity gradient to evaluate habitat suitability on an estuarine scale. Oysters were hatchery-produced from wild, moderate-salinity broodstock, then outplanted for measurement of growth, survival, reproduction and disease prevalence over two years. Survival was generally higher in the lower salinity river sites and in the higher salinity Jamaica Bay sites relative to mesohaline NYC harbor sites. Growth rate was highest in Jamaica Bay and had high variation among other sites. Surprisingly, the highest proportion of individuals with sex-differentiated gametes and the highest average gonad maturation index was found at a low salinity site. Consistent with the advanced gametogenesis measured in experimental animals at low salinity, annual wild recruitment was documented near the low salinity remnant population in each of five monitored years. These results suggest that the remnant HRE oyster population is a robust, self-sustaining population that can be leveraged to support restoration of subpopulations in other parts of the estuary, but further research is required to determine if the mesohaline and near-ocean reaches of the HRE can support the full oyster life cycle.