Interactive effects of osmotic stress and burrowing activity on protein metabolism and muscle capacity in the soft shell clam Mya arenaria

Effects of hypo-osmotic stress on osmoregulation, antioxidant response, and energy metabolism in sea cucumber Holothuria moebii under desalination environment

With global climate changing, hypo-salinity events are increasing in frequency and duration because of continuous rainfall and freshwater inflow, which causes reduced cytosolic osmolarity and cellular stress responses in aquatic animals. Sea cucumbers are considered stenohaline because they lack osmoregulatory organs and are vulnerable to salinity fluctuations. In this study, we performed multiple biochemical assays, de novo transcriptomics, and widely targeted metabolomics to comprehensively explore the osmoregulatory mechanisms and physiological responses of sea cucumber Holothuria moebii to hypo-osmotic stress, which is a representative specie that is frequently exposed to hypo-saline intertidal zones. Our results found that H. moebii contracted their ambulacral feet and oral tentacles, and the coelomic fluid ion concentrations were reduced to be consistent with the environment. The microvilli of intestines and respiratory trees underwent degeneration, and the cytoplasm exhibited swelling and vacuolation. Moreover, the Na+, K+, and Cl- concentrations and Na+/K+-ATPase activity were significantly reduced under hypo-osmotic stress. The decrease in protein kinase A activity and increase in 5'-AMP level indicated a significant inhibition of the cAMP signaling pathway to regulate ion concentrations. And small intracellular organic molecules (amino acids, nucleotides and their derivatives) also play crucial roles in osmoregulation through oxidative deamination of glutamate, nucleotide catabolism, and nucleic acid synthesis. Moreover, lysosomes and peroxisomes removed oxidative damage, whereas antioxidant metabolites, such as N-acetyl amino acids and glutathione, were increased to resist oxidative stress. With prolonged hypo-osmotic stress, glycerophospholipid metabolism was enhanced to maintain membrane stability. Furthermore, acyl-CoA-binding protein activity was significantly inhibited, and only a small amount of acylcarnitine was significantly accumulated, which indicated a disruption in energy metabolism. PPAR signaling pathway and choline content were up-regulated to promote fatty acid metabolism under hypo-osmotic stress. Overall, our results provide new insights into the osmoregulatory mechanisms and physiological responses of sea cucumbers to hypo-osmotic stress.

NADK-mediated proline synthesis enhances high-salinity tolerance in the razor clam

Euryhaline organisms can accumulate organic osmolytes to maintain osmotic balance between their internal and external environments. Proline is a pivotal organic small molecule and plays an important role in osmoregulation that enables marine shellfish to tolerate high-salinity conditions. During high-salinity challenge, NAD kinase (NADK) is involved in de novo synthesis of NADP(H) in living organisms, which serves as a reducing agent for the biosynthetic reactions. However, the role of shellfish NADK in proline biosynthesis remains elusive. In this study, we show the modulation of NADK on proline synthesis in the razor clam (Sinonovacula constricta) in response to osmotic stress. Under acute hypersaline conditions, gill tissues exhibited a significant increase in the expression of ScNADK. To elucidate the role of ScNADK in proline biosynthesis, we performed dsRNA interference in the expression of ScNADK in gill tissues to assess proline content and the expression levels of key enzyme genes involved in proline biosynthesis. The results indicate that the knock-down of ScNADK led to a significant decrease in proline content (P<0.01), as well as the expression levels of two proline synthetase genes P5CS and P5CR involved in the glutamate pathway. Razor clams preferred to use ornithine as substrate for proline synthesis when the glutamate pathway is blocked. Exogenous administration of proline greatly improved cell viability and mitigated cell apoptosis in gills. In conclusion, our results demonstrate the important role of ScNADK in augmenting proline production under high-salinity stress, by which the razor clam is able to accommodate salinity variations in the ecological niche.

The role of the balance between energy production and ammonia detoxification mediated by key amino acids in divergent hypersaline adaptation among crassostrea oysters

Global ocean salinity is changing under rapid climate change and intensified anthropogenic activity. Increased differences in salinity threaten marine biodiversity, organismal survival, and evolution, particularly sessile invertebrates dwelling in highly fluctuating intertidal and estuarine environments. Comparing the responses of closely related species to salinity changes can provide insights into the adaptive mechanisms underlying inter- and intraspecific divergence in salinity tolerance, but are poorly understood in marine bivalves. We collected wild individuals of four Crassostrea species, in addition to two populations of the same species from their native habitats and determined the dynamics of hydrolyzed amino acids (HAAs) and transcriptional responses to hypersaline stress. In response to hypersaline stress, species/populations inhabiting natural high-salinity sea environments showed higher survival and less decline in HAAs than that of congeners inhabiting low-salinity estuaries. Thus, native environmental salinity shapes oyster tolerance. Notably, a strong negative correlation between the decline in HAAs and survival indicated that the HAAs pool could predict tolerance to hypersaline challenge. Four HAAs, including glutamine (Glu), aspartic acid (Asp), alanine (Ala) and glycine (Gly), were identified as key amino acids that contributed substantially to the emergency response to hypersaline stress. High-salinity-adapted oyster species only induced substantial decreases in Glu and Asp, whereas low-salinity-adapted congeners further incresaed Ala and Gly metabolism under hypersaline stress. The dynamics of the content and gene expression responsible for key amino acids pathways revealed the importance of maintaining the balance between energy production and ammonia detoxification in divergent hypersaline responses among oyster species/populations. High constructive or plastic expression of evolutionarily expanded gene copies in high-salinity-adapted species may contribute to their greater hypersaline tolerance. Our findings reveal the adaptive mechanism of key amino acids in salinity adaptation in marine bivalves and provide new avenues for the prediction of adaptive potential and aquaculture with high-salinity tolerant germplasms.

Combined effects of temperature and emersion-immersion cycles on metabolism and bioenergetics of the Pacific oyster Crassostrea (Magallana) gigas

Life on tidal coasts presents physiological major challenges for sessile species. Fluctuations in oxygen and temperature can affect bioenergetics and modulate metabolism and redox balance, but their combined effects are not well understood. We investigated the effects of intermittent hypoxia (12h/12h) in combination with different temperature regimes (normal (15 °C), elevated (30 °C) and fluctuating (15 °C water/30 °C air)) on the Pacific oyster Crassostrea (Magallana) gigas. Fluctuating temperature led to energetic costly metabolic rearrangements and accumulation of proteins in oyster tissues. Elevated temperature led to high (60%) mortality and oxidative damage in survivors. Normal temperature had no major negative effects but caused metabolic shifts. Our study shows high plasticity of oyster metabolism in response to oxygen and temperature fluctuations and indicates that metabolic adjustments to oxygen deficiency are strongly modulated by the ambient temperature. Co-exposure to constant elevated temperature and intermittent hypoxia demonstrates the limits of this adaptive metabolic plasticity.

Metabolism responses in the intestine of Oreochromis mossambicus exposed to salinity, alkalinity and salt-alkalinity stress using LC-MS/MS-based metabolomics

Multiple abiotic stresses are imposed on fish as a result of unprecedented changes in temperature and precipitation patterns in recent decades. It is unclear how teleosts respond to severe ambient salinity, alkalinity, and saline-alkalinity in terms of their metabolic and molecular osmoregulation processes. The metabolic reactions in the intestine of Oreochromis mossambicus under salinity (25 g/L, S_C), alkalinity (4 g/L, A_C), and saline-alkalinity (salinity: 25 g/L & alkalinity: 4 g/L, SA_C) stresses were examined in this research utilizing LC-MS/MS-based metabolomics. The findings demonstrated that the three osmotic-stressed groups' metabolic profiles were considerably different from those of the control group. Osmolytes, energy sources, free amino acids, and several intermediate metabolites were all synthetically adjusted as part of the osmoregulation associated with the salinity, alkalinity, and saline-alkalinity stress. Following osmotic stress, osmoregulation-related pathways, including the mTOR signaling pathway, TCA cycle, glycolysis/gluconeogenesis, etc., were also discovered in the intestine of O. mossambicus. Overall, our findings can assist in better comprehending the molecular regulatory mechanism in euryhaline fish under various osmotic pressures and can offer a preliminary profile of osmotic regulation.

Effects of different oxygen regimes on ecological performance and bioenergetics of a coastal marine bioturbator, the soft shell clam Mya arenaria

Benthic species are exposed to oxygen fluctuations that can affect their performance and survival. Physiological effects and ecological consequences of fluctuating oxygen are not well understood in marine bioturbators such as the soft-shell clam Mya arenaria. We explored the effects of different oxygen regimes (21 days of exposure to constant hypoxia (~4.1 kPa P_O2), cyclic hypoxia (~2.1-~10.4 kPa P_O2) or normoxia (~21 kPa P_O2)) on energy metabolism, oxidative stress and ecological behaviors (bioirrigation and bioturbation) of M. arenaria. Constant hypoxia and post-hypoxic recovery in cyclic hypoxia led to oxidative injury of proteins and lipids, respectively. Clams acclimated to constant hypoxia maintained aerobic capacity similar to the normoxic clams. In contrast, clams acclimated to cyclic hypoxia suppressed aerobic metabolism and activated anaerobiosis during hypoxia, and strongly upregulated aerobic metabolism during recovery. Constant hypoxia led to decreased lipid content, whereas in cyclic hypoxia proteins and glycogen accumulated during recovery and were broken down during the hypoxic phase. Digging of clams was impaired by constant and cyclic hypoxia, and bioirrigation was also suppressed under constant hypoxia. Overall, cyclic hypoxia appears less stressful for M. arenaria due to the metabolic flexibility that ensures recovery during reoxygenation and mitigates the negative effects of hypoxia, whereas constant hypoxia leads to depletion of energy reserves and impairs ecological functions of M. arenaria potentially leading to negative ecological consequences in benthic ecosystems.

Salinity variation modulates cellular stress response to ZnO nanoparticles in a sentinel marine bivalve, the blue mussel Mytilussp

Zinc oxide nanoparticles are released into marine environments from industrial, medical and consumer uses sparking concerns about their potential ecotoxicological effects. Ecological hazard assessment of nZnO in marine ecosystems is hindered by the lack of understanding of the potential interactive effects of nZnO toxicity with other common abiotic stressors, such as salinity fluctuations, in marine organisms. To close this gap in our knowledge, we carried out a comprehensive biomarker-based assessment of the combined effects of salinity and nZnO in a sentinel marine bivalve, the blue mussels Mytilus edulis. The mussels were exposed for 21 days to clean seawater (control), an environmentally relevant concentration (100 μg Zn l^-1) of nZnO or dissolved Zn (to identify the toxic effects attributable to Zn²⁺ toxicity) under the normal (15), low (5) and fluctuating (5-15) salinity regimes. The selected molecular and biochemical markers focused on the oxidative stress, apoptosis, detoxification system and inflammation in the gills and the digestive gland of the mussels. Biomarker analysis showed different effects of nZnO and dissolved Zn on biomarkers of oxidative stress, xenobiotic detoxification and apoptosis but similar effects of both pollutants on the levels of metallothioneins and inflammatory markers. Exposure to nZnO led to elevated levels of lipid peroxidation, upregulation of p53 and p38 stress kinases and apoptosis-related genes, most notably in the gills. Exposure to dissolved Zn led to accumulation of protein carbonyls and activated redox-sensitive detoxification enzymes (NADPH-P450 reductase and glutathione-S-transferase) in the mussels. The ambient salinity had significant effects the cellular adverse effects of nZnO in the mussels. The nZnO-induced cellular stress was detectable under the normal (15) and fluctuating (5-15) salinity conditions in the studied brackish water population of the mussels. At low salinity (5), nZnO toxicity signal was almost completely dampened. These findings indicate that chronic osmotic stress close to the tolerance limits of M. edulis prevails over the effects of the environmentally relevant nZnO and dissolved Zn concentrations in combined exposures. These stressor interactions might ameliorate the cellular toxicity of nZnO in the mussels but limit applicability of cellular stress biomarkers for detecting the toxic effects of nanopollutants in low salinity habitats.

Effect of hypo- and hypersaline stress conditions on physiological, metabolic, and immune responses in the oyster Crassostrea corteziensis (Bivalvia: Ostreidae)

Salinity in the oceans is changing due to climate change and global warming. Intense rainfalls and freshwater runoff decrease salinity along the coastal areas. In contrast, intense drought seasons and river damming have certainly increased salinity in lagoons and estuaries. Few studies have focused on aspects of the biology and culture of oyster Crassostrea corteziensis, but until now, physiological and immunological responses in this species have not been assessed under acute hypo- and hypersaline stress conditions. Oysters obtained from a local farm were acclimated for three weeks in laboratory conditions. To avoid closure of oyster valves during salinity induced-stress conditions, a notch was done on each organism shell not only to facilitate oyster tissue exposure to rearing water but also for sampling hemolymph. Oysters (N = 180) were abruptly exposed to three salinity treatments: (HO) hypo-, (C) control, and (HP) hypersaline stress conditions (10, 35, and 50 PSU, respectively). Four oysters per treatment were sampled at 1, 2, 3, 6, 12, 24, and 48 h after exposure. Hemolymph osmolality, water content and total protein concentration in tissues, metabolic and immune responses were assessed for each organism. Oyster survival was not different among treatments and was maintained above 96% at the end of the experimental trial. Hemolymph osmolality reached the value of rearing water at 6 and 48 h of exposure to HP and HO stress conditions, where oysters exposed to salinity increase showed less resilience than those to decrease. Higher glucose levels in plasma and lower ones of hemocyanin were assessed in the oysters exposed to HP compared to HO conditions, suggesting more stressful conditions or susceptibility of oysters during salinity increase. Total hemocyte (THC), hyalinocyte (HC), and granulocyte (GC) counts decreased in oysters exposed to HP condition, while total and differential hemocyte counts were similar among oysters exposed to HO and control conditions. Despite hemocyte phagocytosis was not different among treatments, viability decreased in those exposed to HP condition. Contrastingly, superoxide anion (SOA) production (oxidative capacity) increased in oysters exposed to both induced salinity-stress conditions, which suggest susceptibility increase in oysters, particularly during salinity increase. The results show that HP condition is particularly stressful for C. corteziensis. In turn, this condition could increase both their vulnerability to other environmental stressors, such as temperature and/or acidification or susceptibility to opportunistic pathogenic microorganisms that cause the most common oyster diseases.

Tissue- and substrate-dependent mitochondrial responses to acute hypoxia-reoxygenation stress in a marine bivalve Crassostrea gigas (Thunberg, 1793)

Hypoxia is a major stressor for aquatic organisms, yet intertidal organisms like the oyster Crassostrea gigas are adapted to frequent oxygen fluctuations by metabolically adjusting to shifts in oxygen and substrate availability during hypoxia-reoxygenation (H/R). We investigated the effects of acute H/R stress (15 min at ∼0% O2, and 10 min reoxygenation) on isolated mitochondria from the gill and the digestive gland of C. gigas respiring on different substrates (pyruvate, glutamate, succinate, palmitate and their mixtures). Gill mitochondria showed better capacity for amino acid and fatty acid oxidation compared to the mitochondria from the digestive gland. Mitochondrial responses to H/R stress strongly depended on the substrate and the activity state of mitochondria. In mitochondria oxidizing NADH-linked substrates exposure to H/R stress suppressed oxygen consumption and ROS generation in the resting state, whereas in the ADP-stimulated state, ROS production increased despite little change in respiration. As a result, electron leak (measured as H2O2 to O2 ratio) increased after H/R stress in the ADP-stimulated mitochondria with NADH-linked substrates. In contrast, H/R exposure stimulated succinate-driven respiration without an increase in electron leak. Reverse electron transport (RET) did not significantly contribute to succinate-driven ROS production in oyster mitochondria except for a slight increase in the OXPHOS state during post-hypoxic recovery. A decrease in NADH-driven respiration and ROS production, enhanced capacity for succinate oxidation and resistance to RET might assist in post-hypoxic recovery of oysters mitigating oxidative stress and supporting rapid ATP re-synthesis during oxygen fluctuations such as commonly observed in estuaries and intertidal zones.

Time-course changes in the regulation of ions and amino acids in the hard clam Meretrix lusoria upon lower salinity challenge

In this study, we examined ion and amino acid regulation in the gill and mantle of the hard clam Meretrix lusoria. We found that the osmolality and Na⁺ and Cl^- concentrations of hard clam hemolymph were significantly reduced after transferring clams from the salinity of their natural habitat [20‰ saltwater (SW)] to a lower salinity environment (10‰ SW). Specific activities of Na⁺ , K⁺ -ATPase (NKA), which provides the driving force for the secondary ion transport associated with cell osmoregulation in gills and mantles, were unaffected during the acclimation to lower salinity. In contrast, there was a significant decline in the contents of free amino acids (FAAs) in the gills and mantles of hard clams during lower salinity acclimation. Taurine was established to be the dominant FAA, the content of which is considerably higher than that of other FAAs in the hard clam. Following acclimation to the lower salinity environment, mRNA expression of the taurine transporter (TAUT), which plays a pivotal role in regulating intracellular taurine contents, was significantly upregulated in the gill and downregulated in the mantle of hard clams at different time points. However, the relative abundance of TAUT protein in the gill and mantle was significantly increased after transfer from 20‰ SW to 10‰ SW, which may reflect feedback regulation in response to reduced taurine contents in the gill and mantle of hard clams. Collectively, the findings of this study provide important insights on the dynamic processes of ion and amino acid regulation in the peripheral tissues of bivalves.

Salinity-dependent effects of ZnO nanoparticles on bioenergetics and intermediate metabolite homeostasis in a euryhaline marine bivalve, Mytilus edulis

Engineered nanoparticles including ZnO nanoparticles (nZnO) are important emerging pollutants in aquatic ecosystems creating potential risks to coastal ecosystems and associated biota. The toxicity of nanoparticles and its interaction with the important environmental stressors (such as salinity variation) are not well understood in coastal organisms and require further investigation. Here, we examined the interactive effects of 100 μg l^-1 nZnO or dissolved Zn (as a positive control for Zn²⁺ release) and salinity (normal 15, low 5, and fluctuating 5-15) on bioenergetics and intermediate metabolite homeostasis of a keystone marine bivalve, the blue mussel Mytilus edulis from the Baltic Sea. nZnO exposures did not lead to strong disturbances in energy or intermediate metabolite homeostasis regardless of the salinity regime. Dissolved Zn exposures suppressed the mitochondrial ATP synthesis capacity and coupling as well as anaerobic metabolism and modified the free amino acid profiles in the mussels indicating that dissolved Zn is metabolically more damaging than nZnO. The environmental salinity regime strongly affected metabolic homeostasis and altered physiological and biochemical responses to nZnO or dissolved Zn in the mussels. Exposure to low (5) or fluctuating (5-15) salinity affected the physiological condition, energy metabolism and homeostasis, as well as amino acid metabolism in M. edulis. Generally, fluctuating salinity (5-15) appeared bioenergetically less stressful than constantly hypoosmotic stress (salinity 5) in M. edulis indicating that even short (24 h) periods of recovery might be sufficient to restore the metabolic homeostasis in this euryhaline species. Notably, the biological effects of nZnO and dissolved Zn became progressively less detectable as the salinity stress increased. These findings demonstrate that habitat salinity must be considered in the biomarker-based assessment of the toxic effects of nanopollutants on coastal organisms.

Modulation of physiological oxidative stress and antioxidant status by abiotic factors especially salinity in aquatic organisms

Exposure to a variety of environmental factors such as temperature, pH, oxygen and salinity may influence the oxidative status in aquatic organisms. The present review article focuses on the modulation of oxidative stress with reference to the generation of reactive oxygen species (ROS) in aquatic animals from different phyla. The focus of the review article is to explore the plausible mechanisms of physiological changes occurring in aquatic animals due to altered salinity in terms of oxidative stress. Apart from the seasonal variations in salinity, global warming and anthropogenic activities have also been found to influence oxidative health status of aquatic organisms. These effects are discussed with an objective to develop precautionary measures to protect the diversity of aquatic species with sustainable conservation. Comparative analyses among different aquatic species suggest that salinity alone or in combination with other abiotic factors are intricately associated with modulation in oxidative stress in a species-specific manner in aquatic animals. Osmoregulation under salinity stress in relation to energy demand and supply are also discussed. The literature survey of >50 years (1960-2020) indicates that oxidative stress status and comparative analysis of redox modulation have evolved from the analysis of various biotic and/or abiotic factors to the study of cellular signalling pathways in these aquatic organisms.

Bioenergetics in environmental adaptation and stress tolerance of aquatic ectotherms: linking physiology and ecology in a multi-stressor landscape

Energy metabolism (encompassing energy assimilation, conversion and utilization) plays a central role in all life processes and serves as a link between the organismal physiology, behavior and ecology. Metabolic rates define the physiological and life-history performance of an organism, have direct implications for Darwinian fitness, and affect ecologically relevant traits such as the trophic relationships, productivity and ecosystem engineering functions. Natural environmental variability and anthropogenic changes expose aquatic ectotherms to multiple stressors that can strongly affect their energy metabolism and thereby modify the energy fluxes within an organism and in the ecosystem. This Review focuses on the role of bioenergetic disturbances and metabolic adjustments in responses to multiple stressors (especially the general cellular stress response), provides examples of the effects of multiple stressors on energy intake, assimilation, conversion and expenditure, and discusses the conceptual and quantitative approaches to identify and mechanistically explain the energy trade-offs in multiple stressor scenarios, and link the cellular and organismal bioenergetics with fitness, productivity and/or ecological functions of aquatic ectotherms.

Integrated analysis of transcriptomic and metabolomic data to evaluate responses to hypersalinity stress in the gill of the razor clam (Sinonovacula constricta)

Salinity is an important ecological factor that affects physiological metabolism, survival, and distribution of marine organisms. Despite changes in the osmolarity and composition of the cytosol during salinity shifts, marine mollusks are able to maintain their metabolic function. The razor clam (Sinonovacula constricta) survives the wide range of salinity in the intertidal zone via changes in behavior and physiology. To explore the stress responses and mechanisms of salinity tolerance in razor clams, we collected transcriptomic and metabolomic data from a control group (salinity 20‰, S20) and a salinity-stress group (salinity 35‰, S35). The transcriptome data showed that genes related to the immune system, cytoskeleton remodeling, and signal transduction pathways dominated in the S35 group to counteract hypersalinity stress in the gill. The metabolomic analysis showed that 142 metabolites were significantly different between the S35 and S20 groups and that amino acid and carbohydrate metabolism were affected by hypersalinity stress. Levels of amino acids and energy substances, such as l-proline, isoleucine, and fructose, were higher in the gill of the S35 group. The combination of transcriptomic and metabolomic data indicated that metabolism of amino acids, carbohydrates, and lipids was enhanced in the gill during adaptation to high salinity. These results clarified the complex physiological processes involved in the response to hyperosmotic stress and maintenance of metabolism in the gill of razor clams. These findings provide a reference for further study of the biological responses of euryhaline shellfish to hyperosmotic stress and a molecular basis for the search for populations with high salinity tolerance.

Effects of prolonged food limitation on energy metabolism and burrowing activity of an infaunal marine bivalve, Mya arenaria

Benthic organisms are subject to prolonged seasonal food limitation in the temperate shallow coastal waters that can cause energetic stress and affect their performance. Sediment-dwelling marine bivalves cope with prolonged food limitation by adjusting different physiological processes that might cause trade-offs between maintenance and other fitness-related functions. We investigated the effects of prolonged (42 days) food deprivation on bioenergetics, burrowing performance and amino acid profiles in a common marine bivalve, Mya arenaria collected in winter and spring. Food limitation of >15 days decreased respiration of the clams by 80%. Total tissue energy content was higher in spring-collected clams (reflecting higher lipid content) than in their winter counterparts. Prolonged food deprivation decreased the tissue energy content of clams, especially in winter. The levels of free amino acids transiently increased during the early phase of food deprivation possibly reflecting suppression of the protein synthesis or enhanced protein degradation. The levels of amino acids considered essential for bivalves were more tightly conserved than those of non-essential amino acids during starvation. The burrowing capacity of clams was negatively affected by food deprivation so that the time required for a burial cycle increased by 35-50% after 22-42 days of starvation. During the early phase of starvation, clams preferentially used lipids as fuel for burrowing, whereas carbohydrates were used at the later phase. These findings suggest that although M. arenaria can withstand prolonged food deprivation by lowering their basal maintenance costs and switching their fuel usage, their ecological functions (e.g. bioturbation and the energy transferable to the next trophic level) could be negatively impacted by starvation.

Effects of seawater salinity and pH on cellular metabolism and enzyme activities in biomineralizing tissues of marine bivalves

Molluscan shell formation is a complex energy demanding process sensitive to the shifts in seawater CaCO₃ saturation due to changes in salinity and pH. We studied the effects of salinity and pH on energy demand and enzyme activities of biomineralizing cells of the Pacific oyster (Crassostrea gigas) and the hard-shell clam (Mercenaria mercenaria). Adult animals were exposed for 14 days to high (30), intermediate (18), or low (10) salinity at either high (8.0-8.2) or low (7.8) pH. Basal metabolic cost as well as the energy cost of the biomineralization-related cellular processes were determined in isolated mantle edge cells and hemocytes. The total metabolic rates were similar in the hemocytes of the two studied species, but considerably higher in the mantle cells of C. gigas compared with those of M. mercenaria. Cellular respiration was unaffected by salinity in the clams' cells, while in oysters' cells the highest respiration rate was observed at intermediate salinity (18). In both studied species, low pH suppressed cellular respiration. Low pH led to an upregulation of Na⁺/K⁺ ATPase activity in biomineralizing cells of oysters and clams. Activities of Ca²⁺ ATPase and H⁺ ATPase, as well as the cellular energy costs of Ca²⁺ and H⁺ transport in the biomineralizing cells were insensitive to the variation in salinity and pH in the two studied species. Variability in cellular response to low salinity and pH indicates that the disturbance of shell formation under these conditions has different underlying mechanisms in the two studied species.