Heatwaves alter survival of the Sydney rock oyster, Saccostrea glomerata

Recruitment of a threatened foundation oyster species varies with large and small spatial scales

Understanding how habitat attributes (e.g., patch area and sizes, connectivity) control recruitment and how this is modified by processes operating at larger spatial scales is fundamental to understanding population sustainability and developing successful long-term restoration strategies for marine foundation species-including for globally threatened reef-forming oysters. In two experiments, we assessed the recruitment and energy reserves of oyster recruits onto remnant reefs of the oyster Saccostrea glomerata in estuaries spanning 550 km of coastline in southeastern Australia. In the first experiment, we determined whether recruitment of oysters to settlement plates in three estuaries was correlated with reef attributes within patches (distances to patch edges and surface elevation), whole-patch attributes (shape and size of patches), and landscape attributes (connectivity). We also determined whether environmental factors (e.g., sedimentation and water temperature) explained the differences among recruitment plates. We also tested whether differences in energy reserves of recruits could explain the differences between two of the estuaries (one high- and one low-sedimentation estuary). In the second experiment, across six estuaries (three with nominally high and three with nominally low sedimentation rates), we tested the hypothesis that, at the estuary scale, recruitment and survival were negatively correlated to sedimentation. Overall, total oyster recruitment varied mostly at the scale of estuaries rather than with reef attributes and was negatively correlated with sedimentation. Percentage recruit survival was, however, similar among estuaries, although energy reserves and condition of recruits were lower at a high- compared to a low-sediment estuary. Within each estuary, total oyster recruitment increased with patch area and decreased with increasing tidal height. Our results showed that differences among estuaries have the largest influence on oyster recruitment and recruit health and this may be explained by environmental processes operating at the same scale. While survival was high across all estuaries, growth and reproduction of oysters on remnant reefs may be affected by sublethal effects on the health of recruits in high-sediment estuaries. Thus, restoration programs should consider lethal and sublethal effects of whole-estuary environmental processes when selecting sites and include environmental mitigation actions to maximize recruitment success.

The development of multiplex PCR assays for the rapid identification of multiple Saccostrea species, and their practical applications in restoration and aquaculture

BACKGROUND

The ecology and biology of oysters (Ostreidae) across the tropics is poorly understood. Morphological plasticity and shared characteristics among oysters have resulted in the misidentification of species, creating challenges for understanding basic species-specific biological information that is required for restoration and aquaculture. Genetic barcoding has proven essential for accurate species identification and understanding species geographic ranges. To reduce the costs of molecular species identification we developed multiplex assays using the cytochrome c oxidase subunit I (COI or cox1) barcoding gene for the rapid identification of five species of oysters within the genus Saccostrea that are commonly found in Queensland, Australia: Saccostrea glomerata, Saccostrea lineage B, Saccostrea lineage F, Saccostrea lineage G, and Saccostrea spathulata (lineage J).

RESULTS

Multiplex assays were successful in species-specific amplification of targeted species. The practical application of these primers was tested on wild spat collected from a pilot restoration project in Moreton Bay, Queensland, with identified species (S. glomerata, lineage B and lineage G) validated by Sanger sequencing. DNA sampling by extraction of oyster pallial fluid was also tested on adult oysters collected from the Noosa estuary in Queensland to assess whether oysters were able to be identified non-destructively. DNA concentrations as low as 1 ng/ μL still amplified in most cases, allowing for identification, and mortality at 6 weeks post pallial fluid collection was low (3 out of 104 sampled oysters).

CONCLUSION

These multiplex assays will be essential tools for species identification in future studies, and we successfully demonstrate their practical application in both restoration and aquaculture contexts in Queensland. The multiplex assays developed in this study outline easily replicable methods for the development of additional species-specific primer sets for the rapid identification of other species of Saccostrea found across the Indo-Pacific, which will be instrumental in unravelling the taxonomic ambiguities within this genus in tropical regions.

Differential responses of selectively bred mussels (Perna canaliculus) to heat stress-survival, immunology, gene expression and microbiome diversity

New Zealand's green-lipped mussel (Perna canaliculus) is an ecologically and economically important species. Marine heatwaves are increasing in frequency around NZ's coastline, and these events are correlated with increased stress and mortality of some aquaculture species. This study aimed to identify general biomarkers of heat stress in P. canaliculus and to assess whether responses differed between genetically distinct selectively bred mussels. We exposed three families of selectively bred mussels (families A, B and C) to three seawater temperature regimes in the laboratory: 1) a "control" treatment (ambient 12°C), 2) a 26°C heat challenge with a subsequent recovery period, and 3) a sustained 26°C heat challenge with no recovery. We investigated whether the survival, immune response (hemocyte concentration and viability, oxidative stress and total antioxidant capacity), hemocyte gene expression and gill microbiome differed between the families during the temperature challenges. In the sustained heat-stress treatment, family A had the highest survival rate (42% compared with 25% and 5% for families C and B, respectively). Gene expression levels significantly shifted during thermal stress and differed between families, with family A more dissimilar than families B and C. Family C had substantially more genes impacted by temperature treatment and timepoint than the other families, while family B had very little genes/pathways that responded to thermal stress. Genes related to heat shock proteins and immune responses (e.g., AIF1, CTSC, TOLL8, CASP9, FNTA, AHCY, CRYAB, PPIF) were upregulated in all families during heat stress. Microbiome species-richness differed between families before and during heat-stress, with family A having a distinctly different microbiome flora than the other families. Microbial diversity changed similarly in all families exposed to prolonged heat-stress, with species of Vibrio and Campylobacter increasing in these mussels. Our study highlights the use of non-lethal sampling of hemocytes as a diagnostic tool to explore the immune response and gene expression of selectively bred mussels, to predict their response to ocean warming. This approach can identify potential thermotolerant candidates for further selective breeding, which may increase the resilience of the mussel aquaculture industry in a warming ocean.

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.

Effect of marine heatwaves on juvenile greater amberjack (Seriola dumerili)

Marine heatwaves (MHWs) have increased in frequency, intensity, and duration in recent years causing significant impacts on marine organisms and fisheries. This study explores the physiological changes of juvenile greater amberjacks (Seriola dumerili) that cope with MHWs. Results showed that physiological parameters were significantly affected by the intensity, duration of MHWs or interaction of two factors (P < 0.05). Repeated MHWs in which water temperatures were increased (24 °C to 28 °C and 32 °C) resulted in changes in enzyme activity levels (catalase (CAT), superoxide dismutase (SOD), and glutathione (GSH)), as well as the level of malondialdehyde (MDA) for antioxidant defense, immune function (acid phosphatase (ACP), alkaline phosphatase (ALP), and lysozyme (LYZ)), and energy metabolism (including triglycerides (TG), glucose (GLU), aspartate aminotransferase (GOT), alanine aminotransferase (ALT), lactate dehydrogenase (LDH), and succinate dehydrogenase (SDH)). The activities of enzymes, including those associated with antioxidant defense, immune function, and energy metabolism, changed significantly in relation to short-term MHWs, indicating a thermal stress response. When S. dumerili were exposed to repeated-MHWs, thermal stress responses increased at 28 °C (T28) and decreased at 32 °C (T32). These results exhibited the inability of S. dumerili to acclimate to severe thermal stress from MHWs. This study examined S. dumerili responses to MHWs and assessed the physiological adaptation of juvenile greater amberjacks to MHWs.

Physiological and biochemical responses of clams to recurrent marine heatwaves

In the past decade, the frequency, intensity and duration of marine heatwaves (MHWs) in the South China Sea have been increasing strikingly, resulting in serious impacts on intertidal bivalves and their ecosystems. The Manila clam, Ruditapes philippinarum, is one of the most ecologically and economically important bivalve species in the South China Sea, yet very little is known about its fate under intensifying MHWs events. Here, we examined how R. philippinarum responded to two consecutive scenarios of MHWs, with each composed of 4 °C and 8 °C rises of seawater temperatures, respectively. Up to 87% of Manila clams survived recurrent MHWs events, and significant increases in standard metabolic rate occurred predominantly under extreme conditions (+8 °C), indicating that the clams could trigger compensatory mechanisms to mitigate MHWs-induced thermal stress. Following acute and repeated exposures to MHWs, Manila clams showed similar responses in enzymes underpinning energy metabolism (NKA, CMA, and T-ATP), antioxidant defence (SOD, CAT, and MDA), and biomineralization (AKP and ACP), most of which exhibited significantly increasing and then decreasing trends with the intensification of MHWs. Of eight genes associated with physiological tolerance and fitness, ATAD3A, PFK, SOD, and C3 were significantly down-regulated in response to recurrent MHWs events, demonstrating the certain resistance to MHWs. These findings provide a better understanding that marine bivalves hold the potential to acclimate simulated MHWs events from the physiological and molecular processes.

Impaired reproduction, energy reserves and dysbiosis: The overlooked consequences of heatwaves in a bivalve mollusc

Extreme events like Marine Heatwaves (MHWs) are becoming more intense, severe, and frequent, threatening benthic communities, specifically bivalves. However, the consequences of non-lethal MHWs on animals are still poorly understood. Here, we exposed the Manila clam Ruditapes philippinarum to non-lethal MHW for 30 days and provided an integrative view of its effects. Our result indicated that albeit non-lethal, MHW reduced clam's energy reserves (by reducing their hepato-somatic index), triggered antioxidant defenses (particularly in males), impaired reproduction (via the production of smaller oocytes in females), triggered dysbiosis in the digestive gland microbiota and altered animals' behaviour (by impacting their burying capacity) and filtration rate. Such effects were seen also at RNA-seq (i.e. many down-regulated genes belonged to reproduction) and metabolome level. Interestingly, negative effects were more pronounced in males than in females. Our results show that MHWs influence animal physiology at multiple levels, likely impacting its fitness and its ecosystem services.

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.

Transgenerational effects of intertidal environment on physiological phenotypes and DNA methylation in Pacific oysters

Climate change and intensifying human activity are posing serious threats to marine organisms. The fluctuating intertidal zone forms a miniature ecosystem of a rapidly changing environment for studying biological adaptation. Transgenerational plasticity (TGP), an evolutionary phenomenon in which parental experience influences offspring phenotypes, provides an avenue for adaptation, but the molecular mechanism was poorly understood in marine molluscs. In this study, wild Pacific oysters (Crassostrea gigas), which were collected from intertidal zones, were used to conduct two-generation breeding in a subtidal area combined with a heat shock experiment in the laboratory to investigate the intertidal environment-induced TGP under temperate subtidal condition and thermally exposed condition, respectively. We showed that TGP could influence the physiological phenotypes related to the status of oxidation and energy in non-stress-exposed subtidal offspring for at least two generations. Genomic DNA methylation exhibited heritable divergence between intertidal and subtidal oysters, and 1655 (or 42.83 %) differentially methylated genes (DMGs) in F₀ were continuously reserved to F₂, which may mediate physiological TGP by participating in biological processes including macromolecule metabolism, cellular responses to stress, and the positive regulation of molecular function, especially fatty acid metabolism. The intertidal experience also influenced the thermal plasticity of physiological phenotypes within and across generations. Totally, 320 (or 14.74 %) specific thermal response DMGs in the intertidal F₀ generation were identified in F₁ and F₂, participating in pathways including carbohydrate, lipid, and energy metabolism, signal transduction, and the organismal immune system, which suggested transgenerational intertidal effect mediated by these genes could positively contribute to stress adaptation and had potential applications for aquaculture. This study demonstrates an epigenetic mechanism for TGP in stress adaptation in marine molluscs, and provides new avenues to improve the stress adaptation for marine resource conservation and aquaculture.

High temperatures enhance the strength of multiple predator effects in a typical crab-clam system

Although marine heatwaves pose urgent threats to marine life, our understanding of how these events influence interactions between key species in marine ecosystems is still inadequate. Herein, we examined the behavioral mechanisms by which heat regulates multiple predator effects in different foraging systems that include Asian paddle crabs (Charybdis japonica) and swimming crabs (Portunus trituberculatus) by quantifying their predation and competition at two temperatures. Our results show that non-independent multiple predator effects occurred in the conspecific treatment of Asian paddle crabs and in the interspecific treatment, whereas independent multiple predator effects occurred in the conspecific treatment of swimming crabs. Asymmetrical behavior responses of these crabs to competition and heat triggered divergences in multiple predator effects. High temperatures increased the strength of multiple predator effects but did not alter their types. The reason is that heat negatively impacts predation by enhancing aggressive interactions, outweighing its direct positive effects on predation.

Extreme heatwave drives topography-dependent patterns of mortality in a bed-forming intertidal barnacle, with implications for associated community structure

Heatwave frequency and intensity will increase as climate change progresses. Intertidal sessile invertebrates, which often form thermally benign microhabitats for associated species, are vulnerable to thermal stress because they have minimal ability to behaviourally thermoregulate. Understanding what factors influence the mortality of biogenic species and how heatwaves might impact their ability to provide habitat is critical. Here, we characterize the community associated with the thatched barnacle, Semibalanus cariosus (Pallass, 1788), in British Columbia (BC), Canada. Then, we investigate what site-level and plot-level environmental factors explained variations in barnacle mortality resulting from an unprecedented regional heatwave in BC, Canada. Furthermore, we used a manipulative shading experiment deployed prior to the heatwave to examine the effect of thermal stress on barnacle survival and recruitment and the barnacle-associated community. We identified 50 taxa inhabiting S. cariosus beds, with variations in community composition between sites. Site-scale variables and algal canopy cover did not predict S. cariosus mortality, but patch-scale variation in substratum orientation did, with more direct solar irradiance corresponding with higher barnacle mortality. The shading experiment demonstrated that S. cariosus survival, barnacle recruitment, and invertebrate community diversity were higher under shades where substratum temperatures were lower. Associated community composition also differed between shaded and non-shaded plots, suggesting S. cariosus was not able to fully buffer acute thermal stress for its associated community. While habitat provisioning by intertidal foundation species is an important source of biodiversity, these species alone may not be enough to prevent substantial community shifts following extreme heatwaves. As heatwaves become more frequent and severe, they may further reduce diversity via the loss of biogenic habitat, and spatial variation in these impacts may be substantial.

Transcriptomic responses reveal impaired physiological performance of the pearl oyster following repeated exposure to marine heatwaves

Marine heatwaves are predicted to become more intense and frequent in the future, possibly threatening the survival of marine organisms and devastating their communities. While recent evidence reveals the adaptability of marine organisms to heatwaves, substantially overlooked is whether they can also adjust to repeated heatwave exposure, which can occur in nature. By analysing transcriptome, we examined the fitness and recoverability of the pearl oyster (Pinctada maxima) after two consecutive heatwaves (24 °C to 32 °C for 3 days; recovery at 24 °C for 4 days). In the first heatwave, 331 differentially expressed genes (DEGs) were found, such as AGE-RAGE, MAPK, JAK-STAT, FoxO and mTOR. Despite the recovery after the first heatwave, 2511 DEGs related to energy metabolism, body defence, cell proliferation and biomineralization were found, where 1655 of them were downregulated, suggesting a strong negative response to the second heatwave. Our findings imply that some marine organisms can indeed tolerate heatwaves by boosting energy metabolism to support molecular defence, cell proliferation and biomineralization, but this capacity can be overwhelmed by repeated exposure to heatwaves. Since recurrence of heatwaves within a short period of time is predicted to be more prevalent in the future, the functioning of marine ecosystems would be disrupted if marine organisms fail to accommodate repeated extreme thermal stress.

Post-responses of intertidal bivalves to recurrent heatwaves

Heatwaves are becoming hotter, longer and more frequent, threatening the survival of intertidal bivalves and devastating their ecosystems. Yet, substantially overlooked are heatwave-induced post-responses, which are important to assess cascading consequences. Here, we investigated responses of intertidal bivalves, Ruditapes philippinarum, to recurrent heatwaves. Physiological and gene expression analyses demonstrated that the mantle tissue of R. philippinarum did not sensitively respond to heatwaves, but revealed post-responses under recovery scenarios. Of 20 genes related to essential physiology and fitness, 18 were down-regulated during the 1st recovery period, but following repeated exposure, 13 genes were up-regulated, in line with significantly increased activities of energy-metabolizing enzymes, and antioxidant and nonspecific enzymes. The down-regulation of genes involved in biomineralization, nevertheless, was observed under recovery scenarios, implying the trade-off between essential physiological and fitness-related functions. These findings pave the way for understanding the physiological plasticity of marine bivalves in response to intensifying heatwaves.

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.

Genome-wide identification and analysis of HECT E3 ubiquitin ligase gene family in Ruditapes philippinarum and their involvement in the response to heat stress and Vibrio anguillarum infection

E3 ubiquitin ligase (E3s) plays an important role in ubiquitin proteasome pathway, proteins containing homologous E6-AP carboxyl terminus (HECT) domains. However, the role of HECT E3 ubiquitin ligase in mollusk was rarely explored. In this study, we performed a genome-wide analysis of the HECT domain-containing gene in Ruditapes philippinarum to identify and predict the structural and functional characterization of HECT genes in response to abiotic and biotic stress. A total of sixteen members of HECT gene family were identified and analyzed for the gene structure, phylogenetic relation, three-dimensional structure, protein interaction network, and expression patterns. Experimental results demonstrated that Rph.HUWE1, Rph.HECTD1, Rph.Ubr5 were significantly up-regulated in response to heat stress and bacterial challenge. Taken together, our data provide insights into the potential function of HECT E3 ligase in heat stress and Vibrio anguillarum infection.

Water quality and the health of remnant leaf oyster (Isognomon ephippium) populations in four Australian estuaries

Leaf oysters (Isognomon ephippium) are large intertidal bivalves that form shellfish reefs. They have a patchy and restricted distribution in estuaries in northern New South Wales, Australia, where the water quality is impacted by a range of anthropogenic stressors from coastal agriculture, urbanisation, industry and recreational activities, along with natural stochastic events such as flooding. Little, however, is currently known about the tolerance of leaf oysters to poor water quality. This study investigated the condition of leaf oyster populations in four estuaries, by assessing the density, size-frequency and condition index. These biological parameters were modelled against habitat water quality parameters including nutrient loads, organic and inorganic carbon, algal biomass, sedimentation and total alkalinity. The water quality assessments were replicated during dry conditions and after rain in two estuaries. Leaf oyster density, size, and body condition significantly differed among the four estuaries. Density ranged from as few as 0.04 oysters per m² in the heavily impacted Richmond River, up to 267 oysters per m² in the Tweed River. Overall water quality was also significantly different among the four estuaries, and significant declines in water quality were observed after rain. Distance based linear models revealed significant correlations between biological indicators of leaf oyster reef health and a range of water quality parameters. Leaf oysters with higher condition occur in locations with high dissolved oxygen and salinity levels, whereas low pH and high nutrient run-off were associated with low density and poorer oyster condition. Nevertheless, dense populations were found to persist in areas with pH below 7.9, in high turbidity (over 27 NTU) and areas dominated by fine silt. Their ability to form dense reefs on estuarine soft muddy bottom habitats with low water flow, suggests that leaf oysters could be used to complement rock oysters in future intertidal shellfish reef restoration programs.

Staying in place and moving in space: Contrasting larval thermal sensitivity explains distributional changes of sympatric sea urchin species to habitat warming

For marine ectotherms, larval success, planktonic larval duration and dispersal trajectories are strongly influenced by temperature, and therefore, ocean warming and heatwaves have profound impacts on these sensitive stages. Warming, through increased poleward flow in regions with western boundary currents, such as the East Australia Current (EAC), provides opportunities for range extension as propagules track preferred conditions. Two sea urchin species, Centrostephanus rodgersii and Heliocidaris tuberculata, sympatric in the EAC warming hotspot, exhibit contrasting responses to warming. Over half a century, C. rodgersii has undergone marked poleward range extension, but the range of H. tuberculata has not changed. We constructed thermal performance curves (TPC) to determine if contrasting developmental thermal tolerance can explain this difference. The temperatures tested encompassed present-day distribution and forecast ocean warming/heatwave conditions. The broad and narrow thermal optimum (Topt) ranges for C. rodgersii and H. tuberculata larvae (7.2 and 4.7°C range, respectively) matched their realized (adult distribution) thermal niches. The cool and warm temperatures for 50% development to the feeding larva approximated temperatures at adult poleward range limits. Larval cool tolerances with respect to mean local temperature differed, 6.0 and 3.8°C respectively. Larval warm tolerances were similar for both species as are the adult warm range edges. The larvae of both species would be sensitive to heatwaves. Centrostephanus rodgersii has stayed in place and shifted in space, likely due to its broad cold-warm larval thermal tolerance and large thermal safety margins. Phenotypic plasticity of the planktonic stage of C. rodgersii facilitated its range extension. In contrast, larval cold intolerance of H. tuberculata explains its restricted range and will delay poleward extension as the region warms. In a warming ocean, we show that intrinsic thermal biology traits of the pelagic stage provide an integrative tool to explain species-specific variation in range shift patterns.

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.

Marine heatwaves have minimal influence on the quality of adult Sydney rock oyster flesh

Marine heatwaves (MHWs) are impacting marine biodiversity, including fisheries and aquaculture. However, it is largely unknown which species will be able to endure MHWs and at what price. Here, we applied elevated temperature (2 °C above ambient) and two different heatwave scenarios to adults of the economically important Sydney rock oyster (SRO, Saccostrea glomerata), and evaluated the impact on nutritional properties, gene expression profiles and immune health indicators. We found that elevated temperature (23 °C) and a variable heatwave (VHW) during winter caused some significant differences in the micronutrient and trace elements levels in oyster flesh. There was an increase of lead under VHW and a decrease in chromium, barium and aluminium under elevated temperature. Conversely, gene expression profiles and other physiological parameters, including flesh protein, fatty acid profiles and hemocyte numbers, were not affected by MHWs. These results indicate that adult SRO are reasonably resilient, and should continue to provide high-quality seafood, under near-future ocean warming and moderate heatwave scenarios.

Repeated exposure to simulated marine heatwaves enhances the thermal tolerance in pearl oysters

In an era of unprecedented climate change, marine heatwaves (MHWs) are projected to increase in frequency, intensity, and duration, severely affecting marine organisms and fisheries and causing profound ecological and socioeconomic impacts. However, very little is known about effects of MHWs on ecologically and economically important bivalve species. Here, we investigate how pearl oysters, Pinctada maxima (Jameson), respond to MHWs under various simulated scenarios. Acute 3-day exposure to MHWs, mimicked by increasing the ambient seawater temperature from 24°C to 28°C, 32°C, and 36°C, respectively, induced significant changes (initially sustained increase and late decrease) in activities of antioxidant enzymes (GSH-Px, SOD, CAT, MDA, and T-AOC) and biomineralizaiton-related enzymes (AKP and ACP). Likewise, energy-metabolizing enzymes (NKA, CMA, and T-ATP) showed remarkable acute responses, with significant increases occurring at the beginning and end of thermal exposure. Following repeated exposure to MHWs, without exception, acute responses of assayed enzymes to MHWs were significantly alleviated, implying that pearl oysters have the ability to implement more efficient and less costly energy-utilizing strategies to compensate for thermal stress induced physiological interferences. These findings indicate that marine bivalves can respond plastically and acclimate rapidly to MHWs and pave the way for predicting the fate of this important taxonomic groups in rapidly changing oceans.

Biotic habitats as refugia under ocean acidification

Habitat-forming organisms have an important role in ameliorating stressful conditions and may be of particular relevance under a changing climate. Increasing CO₂ emissions are driving a range of environmental changes, and one of the key concerns is the rapid acceleration of ocean acidification and associated reduction in pH. Such changes in seawater chemistry are anticipated to have direct negative effects on calcifying organisms, which could, in turn, have negative ecological, economic and human health impacts. However, these calcifying organisms do not exist in isolation, but rather are part of complex ecosystems. Here, we use a qualitative narrative synthesis framework to explore (i) how habitat-forming organisms can act to restrict environmental stress, both now and in the future; (ii) the ways their capacity to do so is modified by local context; and (iii) their potential to buffer the effects of future change through physiological processes and how this can be influenced by management adopted. Specifically, we highlight examples that consider the ability of macroalgae and seagrasses to alter water carbonate chemistry, influence resident organisms under current conditions and their capacity to do so under future conditions, while also recognizing the potential role of other habitats such as adjacent mangroves and saltmarshes. Importantly, we note that the outcome of interactions between these functional groups will be context dependent, influenced by the local abiotic and biotic characteristics. This dependence provides local managers with opportunities to create conditions that enhance the likelihood of successful amelioration. Where individuals and populations are managed effectively, habitat formers could provide local refugia for resident organisms of ecological and economic importance under an acidifying ocean.

Microbiome response differs among selected lines of Sydney rock oysters to ocean warming and acidification

Oyster microbiomes are integral to healthy function and can be altered by climate change conditions. Genetic variation among oysters is known to influence the response of oysters to climate change and may ameliorate any adverse effects on oyster microbiome; however, this remains unstudied. Nine full-sibling selected breeding lines of the Sydney rock oyster (Saccostrea glomerata) were exposed to predicted warming (ambient = 24°C, elevated = 28°C) and ocean acidification (ambient pCO2 = 400, elevated pCO2 = 1000 µatm) for 4 weeks. The haemolymph bacterial microbiome was characterized using 16S rRNA (V3-V4) gene sequencing and varied among oyster lines in the control (ambient pCO2, 24°C) treatment. Microbiomes were also altered by climate change dependent on oyster lines. Bacterial α-diversity increased in response to elevated pCO2 in two selected lines, while bacterial β-diversity was significantly altered by combinations of elevated pCO2 and temperature in four selected lines. Climate change treatments caused shifts in the abundance of multiple amplicon sequence variants driving change in the microbiome of some selected lines. We show that oyster genetic background may influence the Sydney rock oyster haemolymph microbiome under climate change and that future assisted evolution breeding programs to enhance resilience should consider the oyster microbiome.

Energetic lipid responses of larval oysters to ocean acidification

Climate change will increase energetic demands on marine invertebrate larvae and make planktonic food more unpredictable. This study determined the impact of ocean acidification on larval energetics of the oysters Saccostrea glomerata and Crassostrea gigas. Larvae of both oysters were reared until the 9-day-old, umbonate stage under orthogonal combinations of ambient and elevated p CO ₂ (340 and 856 μatm) and food was limited. Elevated p CO ₂ reduced the survival, size and larval energetics, larvae of C. gigas being more resilient than S. glomerata. When larvae were fed, elevated p CO ₂ reduced lipid levels across all lipid classes. When larvae were unfed elevated p CO ₂ resulted in increased lipid levels and mortality. Ocean acidification and food will interact to limit larval energetics. Larvae of S. glomerata will be more impacted than C. gigas and this is of concern given their aquacultural status and ecological function.

Climate change alters the haemolymph microbiome of oysters

The wellbeing of marine organisms is connected to their microbiome. Oysters are a vital food source and provide ecological services, yet little is known about how climate change such as ocean acidification and warming will affect their microbiome. We exposed the Sydney rock oyster, Saccostrea glomerata, to orthogonal combinations of temperature (24, 28 °C) and pCO₂ (400 and 1000 μatm) for eight weeks and used amplicon sequencing of the 16S rRNA (V3-V4) gene to characterise the bacterial community in haemolymph. Overall, elevated pCO₂ and temperature interacted to alter the microbiome of oysters, with a clear partitioning of treatments in CAP ordinations. Elevated pCO₂ was the strongest driver of species diversity and richness and elevated temperature also increased species richness. Climate change, both ocean acidification and warming, will alter the microbiome of S. glomerata which may increase the susceptibility of oysters to disease.

Effects of marine heatwave conditions across the metamorphic transition to the juvenile sea urchin (Heliocidaris erythrogramma)

For short development species, like the sea urchin Heliocidaris erythrogramma, the entire planktonic duration can be impacted by marine heatwaves (MHW). Developmental thermal tolerance of this species through metamorphosis was investigated over a broad range (7.6-28.0 °C), including temperatures across its distribution and MHW conditions. In controls (19.5-21.0 °C), 80% of individuals developed to metamorphosis at day 5, doubling to 10 days at 14.0 °C. The thermal range (14.4-21.2 °C) of metamorphosis on day 7 reflected the realised thermal niche with 25.9 °C the upper temperature for success (T₄₀). By day 10, juvenile tolerance narrowed to the local range (16.2-19.0 °C), similar to levels tolerated by adults, indicating negative carryover effects across the metamorphic transition. Without phenotypic adjustment or adaptation, regional warming will be detrimental, although populations may be sustained by thermotolerant offspring. Our results show the importance of the metamorphic transition in understanding the cumulative sensitivity of species to MHW.