Defense Responses to Short-term Hypoxia and Seawater Acidification in the Thick Shell Mussel Mytilus coruscus

Perfluorooctanoate and nano titanium dioxide impair the byssus performance of the mussel Mytilus coruscus

Perfluorooctanoate (PFOA) is widely used as a surfactant and has metabolic, immunologic, developmental, and genetic toxicity on marine organisms. However, the effects of PFOA on individual defense functions in mussels in the presence of titanium dioxide nanoparticles (nano-TiO2) are poorly understood. To investigate the defense strategies and regulatory mechanisms of mussels under combined stressors, the thick-shell mussels Mytilus coruscus were exposed to different PFOA concentrations (0, 2 and 200 μg/L) and nano-TiO2 (0 and 0.1 mg /L, size: 25 nm) for 14 days. The results showed that, compared to the control group, PFOA and nano-TiO2 significantly reduced the number of byssal threads (NBT), byssal threads length (BTL), diameter of proximal threads (DPB), diameter of middle threads (DMB), diameter of distal byssal threads (DDB), adhesive plaque area (BPA), and breaking force of byssal threads (N). Under the influence of PFOA and nano-TiO2, the morphological surface smoothness of the fractured byssal threads surface increased, concurrently inducing an increased surface roughness in the adhesive plaques. Additionally, under the presence of PFOA and nano-TiO2, the foot displayed dispersed tissue organization and damaged villi, accompanied by an increased incidence of cellular apoptosis and an upregulation of the apoptosis gene caspase-8. Expression of the adhesion gene mfp-3 and byssal threads strength genes (preCOL-D, preCOL-NG) was upregulated. An interactive effect on the performance of byssal threads is observed under the combined influence of PFOA and nano-TiO2. Under co-exposure to PFOA and nano-TiO2, the performance of the byssal threads deteriorates, the foot structure is impaired, and the genes mRNA expression of byssal thread secretory proteins have compensated for the adhesion and byssal threads strength by up-regulation. Within marine ecosystems, organic and particulate contaminants exert a pronounced effect on the essential life processes of individual organisms, thereby jeopardizing their ecological niche within community assemblages and perturbing the dynamic equilibrium of the overarching ecosystem. ENVIRONMENTAL IMPLICATION: Perfluorooctanoic acid (PFOA) is prone to accumulate in marine organisms. TiO2 nanoparticles (nano-TiO2) are emerging environmental pollutants frequently found in marine environment. The effects of PFOA and nano-TiO2 on marine mussels are not well understood, and their toxic mechanisms remain largely unknown. We investigated the impacts of PFOA and nano-TiO2 on mussel byssus defense mechanisms. By assessing byssus performance indicators, morphological structures of the byssus, subcellular localization, and changes in byssal secretion-related genes, we revealed the combined effects and mechanisms through which these two types of pollutants may affect the functional capabilities and survival of mussels in the complex marine ecosystem.

Ocean acidification alters anti-predator responses in a competitive dominant intertidal mussel

Widespread intertidal mussels are exposed to a variety of natural and anthropogenic stressors. Even so, our understanding of the combined influence of stressors such as predation risk and ocean acidification (OA) on these species remains limited. This study examined the response of the purple mussel (Perumytilus purpuratus), a species distributed along Pacific southeastern rocky shores, to the effects of predation risk and OA. Using a laboratory 2 × 2 cross design, purple mussels were either devoid or exposed to predator cues from the muricid snail Acanthina monodon, while simultaneously exposing them to current (500 ppm) or projected OA conditions (1500 ppm). The response of purple mussels to these factors was assessed using growth, calcification, clearance, and metabolic rates, in addition to byssus production. After 60 d, the presence of predator cues reduced mussel growth in width and length, and in the latter case, OA enhanced this response making the effects of predator cues more severe. Calcification rates were driven by the interaction between the two stressors, whereas clearance rates increased only in response to OA, likely explaining some of the growth results. Mussel byssus production also increased with pCO₂ but interacted with predation risk: in the absence of predator cues, byssus production increased with OA. These results suggest that projected levels of OA may alter and in some cases prevail over the natural response of purple mussels to predation risk. Considering the role played by this mussel as a dominant competitor and ecosystem engineer in rocky shores, these results have community-wide implications.

Distinct metabolic shifts occur during the transition between normoxia and hypoxia in the hybrid and its maternal abalone

Due to anthropogenic activities that have increased global climate change and nutrient discharges, severe hypoxic events have frequently occurred in coastal waters in recent years. Relying on coastal waters, the aquaculture area has suffered ecological and economic losses caused by hypoxia, especially in summer. In this study, to investigate the stress resistance of the Pacific abalone Haliotis discus hannai (DD) and the hybrid H. discus hannai ♀ × H. fulgens ♂ (DF), a combination of physiological, biochemical, and metabolomic methods were used to compare the metabolic responses of these two abalones to acute hypoxia (~0.5 mg O₂/L, 12 h) and reoxygenation (~6.6 mg O₂/L, 10-20 h). Hemolymph characteristics and aerobic/anaerobic respiratory capacity changed significantly under hypoxia or reoxygenation conditions, and they were regulated in different trends in two abalones. The contents of hepatopancreas glycogen in two abalones reached the trough after 10 h recovery, implying that short-term hypoxia leads to a long-lasting (several hours) imprint on the energy storage of abalone. In response to dissolved oxygen fluctuation, metabolic profiles of two abalones changed in distinct ways both in the hypoxia group or the reoxygenation group. The conversion of carbohydrate metabolism and amino acid metabolism indicated that hypoxia prompts abalone to change the way of energy metabolism, which may also reflect the difference in the energy utilization of DD and DF abalones. In addition, 3 metabolites (L-glutamate, 2-hydroxy-butanoic acid, and 2-methyl-3-hydroxybutyric acid) as potential biomarkers for hypoxia and reoxygenation response in abalone were determined by operating characteristic analysis (ROC). Overall, this study provides information towards understanding the damage caused by frequent hypoxic events and implies the metabolic shifts that occur under hypoxia and reoxygenation conditions in DD and DF abalones.

Negative impacts of realistic doses of spherical and irregular microplastics emerged late during a 42 weeks-long exposure experiment with blue mussels

Microplastics have been found in all compartments of the environment, and numerous life forms are known to take up the anthropogenic particles. Marine filter feeders are particularly susceptible to ingest suspended microplastics, but long-term studies on the potential effects of this uptake are scarce. We exposed juvenile Mytilus spp. to environmentally realistic doses of irregularly shaped polyvinylchloride (PVC) particles (15, 1500, 15,000, 150,000, 1,500,000 particles/individual/week calibrated in the size range 11-60 μm) and regularly shaped polystyrene (PS) beads (15, 1500, 15,000 particles/individual/week, 40 μm) over 42 weeks. During this period, we monitored physiological traits such as clearance rate, byssus production, growth rate, superoxide dismutase (SOD) activity, malondialdehyde (MDA) concentrations, and the condition index (CI). Negative effects of the tested microplastics on mussel performance emerged late in the experiment and were rather weak. Interestingly, even after having received the lowest particle dose of PS, SOD activity in the gill was significantly lower in mussels exposed to microplastics compared to a group of conspecifics that were kept in clean water. However, growth and CI, which are both closely related to the fitness of the mussels, were not found to be impaired at the end of the exposure phase. This is the so far longest laboratory microplastic exposure study on mussels and we worked with particle doses that reflect todays pollution levels. The small effect sizes we observed for the response variables assessed suggest that these specific microplastics pose only a minor threat to blue mussel populations.

Antioxidant responses of triangle sail mussel Hyriopsis cumingii exposed to toxic Microcystis aeruginosa and thermal stress

Harmful algal blooms (HABs) and thermal stress as climate changes become more common in global water ecosystem, especially under eutrophic habitats. Here our study examined the combined impacts of bloom forming cyanobacteria Microcystis aeruginosa and thermal stress on the antioxidant responses of the ecologically important species triangle sail mussel Hyriopsis cumingii. The differential responses of a series of enzymes, e.g. superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) and glutathione-S-transferase (GST), as well as signal metabolites including reactive oxygen species (ROS), malondialdehyde (MDA) and glutathione (GSH) involved in antioxidant defense mechanisms were analyzed during 14 d exposure to toxic cyanobacterium M. aeruginosa and 7 d depuration period. The activities of SOD and GPx as well as the content of ROS and MDA in H. cumingii increased, while CAT activity reduced due to M. aeruginosa exposure. Thermal stress resulted in decrease of CAT, the accumulation of GSH and the enhance of GST and SOD. Meanwhile, the interactive effects among M. aeruginosa, thermal stress and time were also observed on most parameters except for GST activity. The total amount of microcystins (MC) in sail mussels increased with concentrations of exposed M. aeruginosa, independently of the presence or absence of thermal stress. Although around 50% of MC in mussels dropped in the depuration period, most parameters showed alterations because of cyanobacteria exposure and thermal stress. Overall, these findings suggested that toxic cyanobacteria or thermal stress induces oxidative stress and severely affects the enzymes activities and intermediates level associated with antioxidant defense mechanisms in sail mussels respectively. More importantly, the toxic impacts on sail mussels could be intensified by their combination.

Diel-cycling seawater acidification and hypoxia impair the physiological and growth performance of marine mussels

Ocean acidification and hypoxia are concurrent in some coastal waters due to anthropogenic activities in the past decades. In the natural environment, pH and dissolved oxygen (DO) may fluctuate and follow diel-cycling patterns, but such effects on marine animals have not been comprehensively studied compared to their constant effects. In order to study the effects of diel-cycling seawater acidification and hypoxia on the fitness of marine bivalves, the thick shell mussels Mytilus coruscus were exposed to two constant levels of dissolved oxygen (2 mg/L, 8 mg/L) under two pH treatments (7.3, 8.1), as well as single diel fluctuating pH or DO, and the combined diel fluctuating of pH and DO for three weeks. The experimental results showed that constant acidification and hypoxia significantly reduced the extracellular pH (pH_e) and condition index (CI) of mussels, and significantly increased HCO₃^-, pCO₂ and standard metabolic rate (SMR). Diel fluctuating hypoxia and acidification also significantly reduced the pH_e and CI, and significantly increased pCO₂ and SMR, but had no significant effects on HCO₃^-. However, the diel-cycling acidification and hypoxia resulted in a higher CI compared to continuous exposure. In general, continuous and intermittent stress negatively impact the hemolymph and growth performance of mussels. However, mussels possess a little stronger resistance to diel-cycling seawater acidification and hypoxia than sustained stress.

Experimental study of transgenerational effects, pH and predation risk on byssus production in a swiftly spreading invasive fouling Asian mussel, Musculista senhousia (Benson)

Marine biofouling by the highly invasive Asian date mussel, Musculista senhousia (Benson), has caused devastating ecological and economic consequences in most coastal seas. Acute and short-term exposure experiments have demonstrated the susceptibility of mussel byssus - a holdfast structure by which mussels strongly adhere to underwater substrates, to pH. Yet, the influence of long-term exposures, especially across multiple generations, is largely unknown. Here, we evaluated transgenerational effects of pH on byssal threads secreted by M. senhousia, and compared byssus performance in absence versus presence of predators. If no predation occurred, neither pH nor transgenerational exposure significantly affected the number, length and diameter of byssal threads. Under predation risk, mussels, even exposed to low pH, significantly enhanced byssus production. In particular, individuals originating from parents grown under low conditions produced significantly more, longer and stronger byssal threads compared with those spawn from parents exposed to high pH, demonstrating positive transgenerational effects which can confer mussel byssus resilience at low pH. Given the energetically expensive nature of byssus production, these observations can be in line with previously documented plasticity of energy metabolism arose following transgenerational exposure to low pH, which allows mussels to allocate more energy to fulfill the synthesis and secretion of byssal proteins. Our findings demonstrate the remarkable ability of highly invasive fouling mussel species to respond plastically and adapt behaviorally to low pH and hence provide important implications for linking marine biofouling, biological invasion, and coastal acidification.

CO2-driven ocean acidification weakens mussel shell defense capacity and induces global molecular compensatory responses

Oceanic uptake of atmospheric CO₂ is reducing seawater pH and shifting carbonate chemistry within, a process termed as ocean acidification (OA). Marine mussels are a family of ecologically and economically significant bivalves that are widely distributed along coastal areas worldwide. Studies have demonstrated that OA greatly disrupts mussels' physiological functions. However, the underlying molecular responses (e.g., whether there were any molecular compensation mechanisms) and the extent to which OA affects mussel shell defense capacity remain largely unknown. In this study, the thick shell mussels Mytilus coruscus were exposed to the ambient pH (8.1) or one of two lowered pH levels (7.8 and 7.4) for 40 days. The results suggest that future OA will damage shell structure and weaken shell strength and shell closure strength, ultimately reducing mussel shell defense capacity. In addition, future OA will also disrupt haemolymph pH and Ca²⁺ homeostasis, leading to extracellular acidosis and Ca²⁺ deficiency. Mantle transcriptome analyses indicate that mussels will adopt a series of molecular compensatory responses to mitigate these adverse effects; nevertheless, weakened shell defense capacity will increase mussels' susceptibility to predators, parasites and pathogens, and thereby reduce their fitness. Overall, the findings of this study have significant ecological and economic implications, and will enhance our understanding of the future of the mussel aquaculture industry and coastal ecosystems.

Seawater acidification and temperature modulate anti-predator defenses in two co-existing Mytilus species

The effects of short-term (7 days) experimental ocean acidification (-0.4 pH units) and warming (+5 °C) on anti-predator defenses of two sympatric Mytilus species from China, M. coruscus and M. edulis, in the presence and absence of predator cues were investigated. Results suggested species-specific independent negative effects of acidification and warming on the number and weight of byssal threads, the force of thread attachment, and total thread plaque area. Similar negative effects were observed for clustering behaviour, with acidification and warming independently increasing the number of solitary individuals and decreasing the percentage of mussels in clusters. Acidification effects on byssus were strongly exacerbated when predators were present. Ultimately, this study suggests that short-term exposure to experimental warming and acidification can negatively impact anti-predator defense strategies in mussels with potential ramifications for predator-prey interactions and ecological functioning in systems where mussel beds play a key ecological role.

Nano-ZnO impairs anti-predation capacity of marine mussels under seawater acidification

Artificial nanoparticles and ocean acidification (OA) caused by the rapid increase of CO₂ absorbed by the ocean are both ecologically hazardous to marine organisms. The combined effects of the two environmental stressors on the anti-predation ability of marine mussels were studied. Mytilus coruscus was exposed to three different gradient concentrations of nano-ZnO (0, 2.5, 10 mg/L) in combination of two pH levels (7.7 and 8.1). The crab Charybdis japonica was used as its predator. During the experiment, anti-predator indexes, including number of byssus threads (NBT), shell-closing strength (SCS), diameter of byssus thread (BTD), length of byssus thread (BTL), cumulative length of byssus thread (CBTL) and cumulative volume of byssus thread (CBTV) were studied. The results showed that predator induced the anti-predation responses in M. coruscus, and NBT, SCS, BTL, CBTL and CBTV were significantly increased. Under the conditions of pH 7.7 and 10 mg/L nano-ZnO, NBT, SCS, BTD, BTL, CBTL, and CBTV were significantly reduced. What's more, significant interactions among pH, nano-ZnO and predator were observed in CBTL and CBTV. Therefore, the joint treatment of nano-ZnO and low pH reduces the adhesion strength of byssus thread and may increase the probability of mussels being preyed.

Only as strong as the weakest link: structural analysis of the combined effects of elevated temperature and pCO2 on mussel attachment

Predicting how combinations of stressors will affect failure risk is a key challenge for the field of ecomechanics and, more generally, ecophysiology. Environmental conditions often influence the manufacture and durability of biomaterials, inducing structural failure that potentially compromises organismal reproduction, growth, and survival. Species known for tight linkages between structural integrity and survival include bivalve mussels, which produce numerous byssal threads to attach to hard substrate. Among the current environmental threats to marine organisms are ocean warming and acidification. Elevated pCO₂ exposure is known to weaken byssal threads by compromising the strength of the adhesive plaque. This study uses structural analysis to evaluate how an additional stressor, elevated temperature, influences byssal thread quality and production. Mussels (Mytilus trossulus) were placed in controlled temperature and pCO₂ treatments, and then, newly produced threads were counted and pulled to failure to determine byssus strength. The effects of elevated temperature on mussel attachment were dramatic; mussels produced 60% weaker and 65% fewer threads at 25°C in comparison to 10°C. These effects combine to weaken overall attachment by 64-88% at 25°C. The magnitude of the effect of pCO₂ on thread strength was substantially lower than that of temperature and, contrary to our expectations, positive at high pCO₂ exposure. Failure mode analysis localized the effect of temperature to the proximal region of the thread, whereas pCO₂ affected only the adhesive plaques. The two stressors therefore act independently, and because their respective target regions are interconnected (resisting tension in series), their combined effects on thread strength are exactly equal to the effect of the strongest stressor. Altogether, these results show that mussels, and the coastal communities they support, may be more vulnerable to the negative effects of ocean warming than ocean acidification.

Seawater acidification aggravated cadmium toxicity in the oyster Crassostrea gigas: Metal bioaccumulation, subcellular distribution and multiple physiological responses

Mounting evidence has demonstrated the combined effects of ocean acidification (OA) and other environmental stressors on marine organisms. Although metal pollution is widely distributed in coasts and estuaries, the combined effects of OA and metal pollution have received little attention until recent years. In this study, the accumulation and subcellular distribution of cadmium (Cd) and the physiological responses of the oyster Crassostrea gigas were investigated after 31 days of exposure to OA and Cd, either alone or in combination. Increased Cd accumulation was found both in gills (about 57% increase at pH 7.8, 22% increase at pH 7.6) and digestive glands (about 38% increase at pH 7.8, 22% increase at pH 7.6) of C. gigas under elevated pCO₂ exposure. Although a similar total Cd accumulation pattern was seen in oyster gills and digestive glands, a higher partition of Cd in the BIM (biologically inactive metal) fractions of gills (about 60%) was found in Cd-exposed treatments compared to the digestive glands (about 45%), which might correspond to the generally lower toxicity in gills. Moreover, synergetic effects of Cd and OA on the oxidative stresses, histopathological damage, and apoptosis of exposed oysters were observed in this study, which might be explained by significant interactions of these two factors on increased generation of ROS. These findings demonstrated that OA could aggravate the toxicity of metals in marine organisms, with significant implications for coastal benthic ecosystems regarding the widespread metal contamination and the concurrent increase of acidified seawater.

Impact of zinc oxide nanoparticles and ocean acidification on antioxidant responses of Mytilus coruscus

Increased production of engineered nanoparticles (NPs) has raised extensive concerns about the potential toxic effects on marine organisms. Extensive evidences documented the impact of ocean acidification (OA) on the physiology and fitness of bivalves. In the present study, we investigated the biochemical responses of the mussel Mytilus coruscus exposed to both nano-ZnO and low pH relevant for ocean acidification conditions for 14 d followed by a 7-d recovery period. Most biochemical indexes (superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), acid phosphatase (ACP) and alkaline phosphatase (ALP)) measured in gills and hemocytes were increased when the mussels were subject to low pH or high concentration of nano-ZnO, suggesting oxidative stress responses. No significant interactions between the two stressors were observed for most measured parameters. After a 1 week recovery period, low pH and nano-ZnO had less marked impact for SOD, GPx, ACP and ALP in hemocytes as compared to the end of the 14 d exposure. However, no recovery was observed in gills. Overall, our results suggest that both low pH and nano-ZnO induce an anti-oxidative response in Mytilus coruscus with gills being more sensitive than hemocytes.

Short-Term Exposure of Mytilus coruscus to Decreased pH and Salinity Change Impacts Immune Parameters of Their Haemocytes

With the release of large amounts of CO₂, ocean acidification is intensifying and affecting aquatic organisms. In addition, salinity also plays an important role for marine organisms and fluctuates greatly in estuarine and coastal ecosystem, where ocean acidification frequently occurs. In present study, flow cytometry was used to investigate immune parameters of haemocytes in the thick shell mussel Mytilus coruscus exposed to different salinities (15, 25, and 35‰) and two pH levels (7.3 and 8.1). A 7-day in vivo and a 5-h in vitro experiments were performed. In both experiments, low pH had significant effects on all tested immune parameters. When exposed to decreased pH, total haemocyte count (THC), phagocytosis (Pha), esterase (Est), and lysosomal content (Lyso) were significantly decreased, whereas haemocyte mortality (HM) and reactive oxygen species (ROS) were increased. High salinity had no significant effects on the immune parameters of haemocytes as compared with low salinity. However, an interaction between pH and salinity was observed in both experiments for most tested haemocyte parameters. This study showed that high salinity, low salinity and low pH have negative and interactive effects on haemocytes of mussels. As a consequence, it can be expected that the combined effect of low pH and changed salinity will have more severe effects on mussel health than predicted by single exposure.

Ocean acidification hampers sperm-egg collisions, gamete fusion, and generation of Ca2+ oscillations of a broadcast spawning bivalve, Tegillarca granosa

Although the effect of ocean acidification on fertilization success of marine organisms is increasingly well documented, the underlying mechanisms are not completely understood. The fertilization success of broadcast spawning invertebrates depends on successful sperm-egg collisions, gamete fusion, and standard generation of Ca²⁺ oscillations. Therefore, the realistic effects of future ocean pCO₂ levels on these specific aspects of fertilization of Tegillarca granosa were investigated in the present study through sperm velocity trials, fertilization kinetics model analysis, and intracellular Ca²⁺ assays, respectively. Results obtained indicated that ocean acidification significantly reduced the fertilization success of T. granosa, which could be accountable by (i) decreased sperm velocity hence reducing the probability for sperm-egg collisions; (ii) lowered probability of gamete fusion for each gamete collision event; and (iii) disrupted intracellular Ca²⁺ oscillations.