Physiological-biochemical properties of blue mussel Mytilus edulis adaptation to oil contamination

Pulsative venous return from the branchial vessels to the heart of the bivalve Mytilus galloprovincialis supports the constant-volume mechanism

In bivalves and gastropods, ventricle contraction causes a negative pressure in the auricles and increases venous return from the afferent oblique vein (AOV): the constant-volume (CV) mechanism. The flow in the AOV should be a pulsative flow synchronized with the ventricular contraction. The flow in the heart and adjacent vessels of Mytilus galloprovincialis were measured by magnetic resonance imaging to confirm this hypothesis. Under a regular heartbeat, pulsative flows in the AOV and branchial vessels (BVs) were almost completely synchronized with the flow in the aorta, while filling of the ventricle was in the opposite phase. Flows in the BVs were directed to the posterior direction, and a pair of BVs in the gill axes (the efferent BVs) were connected to the AOV. Based on the images of the whole pathway of the AOV in an oblique slice, we confirmed that haemolymph flow was evoked from the efferent BVs and flow into the ventricle via the auricle was completed in a single heartbeat. Therefore, the walls of the AOV and BVs could resist negative transmural pressure caused by the ventricular contraction. In conclusion, the auricle, the AOV and the BVs, including the gill filaments, act as a suction pump. The pulsative venous return is driven by the negative pressure of the AOV as in the CV mechanism, and the negative pressure in the efferent BVs could draw haemolymph from the sinus via the gill and the afferent BVs. Therefore, Mytilus can start and stop its heartbeat as necessary.

Ioxynil and diethylstilbestrol impair cardiac performance and shell growth in the mussel Mytilus coruscus

The herbicide ioxynil (IOX) and the synthetic estrogen diethylstilbestrol (DES) are environmentally relevant contaminants that act as endocrine disruptors (EDCs) and have recently been shown to be cardiovascular disruptors in vertebrates. Mussels, Mytilus coruscus, were exposed to low doses of IOX (0.37, 0.037 and 0.0037 mg/L) and DES (0.27, 0.027 and 0.0027 mg/L) via the water and the effect monitored by generating whole animal transcriptomes and measuring cardiac performance and shell growth. One day after IOX (0.37 and 0.037 mg/L) and DES (0.27 and 0.027 mg/L) exposure heart rate frequency was decreased in both groups and 0.27 mg/L DES significantly reduced heart rate frequency with increasing time of exposure (P < 0.05) and no acclimatization occurred. The functional effects were coupled to significant differential expression of genes of the serotonergic synapse pathway and cardiac-related genes at 0.027 mg/L DES, which suggests that impaired heart function may be due to interference with neuroendocrine regulation and direct cardiac effect genes. Multiple genes related to detoxifying xenobiotic substances were up regulated and genes related to immune function were down regulated in the DES group (vs. control), indicating that detoxification processes were enhanced, and the immune response was depressed. In contrast, IOX had a minor disrupting effect at a molecular level. Of note was a significant suppression (P < 0.05) by DES of shell growth in juveniles and lower doses (< 0.0027 mg/L) had a more severe effect. The shell growth depression in 0.0027 mg/L DES-treated juveniles was not accompanied by abundant differential gene expression, suggesting that the effect of 0.0027 mg/L DES on shell growth may be direct. The results obtained in the present study reveal for the first time that IOX and DES may act as neuroendocrine disrupters with a broad spectrum of effects on cardiac performance and shell growth, and that DES exposure had a much more pronounced effect than IOX in a marine bivalve.

Changes of Heart Rate and Lipid Composition in Mytilus Edulis and Modiolus Modiolus Caused by Crude Oil Pollution and Low Salinity Effects

Blue mussels, Mytilus edulis, inhabiting tidal zones, are naturally exposed to fluctuating environmental conditions (e.g., fluctuations in temperature and salinities), while horse mussels, Modiolus modiolus, live under relatively invariable shelf water conditions. The present investigation tested the hypothesis: blue mussels, in comparison to horse mussels, have an increased ability to tolerate the stress of pollution combined with low salinity. To assess the response of blue mussels and horse mussels to oil pollution at seawater salinities of 25 psu (normal) and 15 psu (low), we used a combination of heart rate and lipid composition as physiological and biochemical indicators, respectively. A sharp decrease in heart rate as well as important fluctuations in cardiac activity was observed under all oil concentrations. Modifications in the concentrations of the main membrane lipid classes (phosphatidylcholine, phosphatidylethanolamine, and cholesterol) and storage lipids (primarily triacylglycerols) in response to different crude oil concentrations were time- and dose-dependent. Both chosen indicators showed a high sensitivity to crude oil contamination. Furthermore, both bivalve species showed similar responses to oil pollution, suggesting a universal mechanism for biochemical adaptation to crude oil pollution.

Development of Novel Cardiac Indices and Assessment of Factors Affecting Cardiac Activity in a Bivalve Mollusc Chlamys farreri

Cardiac activity has been widely used in marine molluscs as an indicator for their physiological status in response to environmental changes, which is, however, largely less studied in scallops. Here, we monitored cardiac performance of Zhikong scallop Chlamys farreri using an infrared-based method, and evaluated the effects of several biotic (shell height, total weight, and age) and environmental factors (circadian rhythm and temperature) on scallop heart rate (HR), amplitude (HA), and rate-amplitude product (RAP). Results revealed that size has a significant effect on both HR (negative) and HA (positive), but RAP values are similar in different sized scallops. Age also affects scallop cardiac performance, significantly for HR, but not for HA or RAP. Circadian rhythm affects cardiac activity, with significant elevation of HR, HA and RAP during 1:00–8:00 and 17:00–19:00. With seawater temperature elevation, HR peaks at 30.03 ± 0.23°C, HA at 15.08 ± 0.02°C, and RAP at 15.10 ± 0.19 and 30.12 ± 0.28°C. This suggests HR is a good indicator for thermal limit, whereas HA may indicate optimal growth temperature, and RAP could be an index of myocardial oxygen consumption to indicate myocardium stress. Our study provides basic information on the factors that may affect scallop cardiac performance. It also elucidates the feasibility of HA and RAP as cardiac indices in marine molluscs.

Limulus and heart rhythm

Great interest in the comparative physiology of hearts and their functions in Animalia has emerged with classic papers on Limulus polyphemus and mollusks. The recurrent cardiac activity-heart rate-is the most important physiological parameter and when present the kardia (Greek) is vital to the development of entire organs of the organisms in the animal kingdom. Extensive studies devoted to the regulation of cardiac rhythm in invertebrates have revealed that the basics of heart physiology are comparable to mammals. The hearts of invertebrates also beat spontaneously and are supplied with regulatory nerves: either excitatory or inhibitory or both. The distinct nerves and the source of excitation/inhibition at the level of single neurons are described for many invertebrate genera. The vertebrates and a majority of invertebrates have myogenic hearts, whereas the horseshoe crab L. polyphemus and a few other animals have a neurogenic cardiac rhythm. Nevertheless, the myogenic nature of heartbeat is precursor, because the contraction of native and stem-cell-derived cardiomyocytes does occur in the absence of any neural elements. Even in L. polyphemus, the heart rhythm is myogenic at embryonic stages.

An integrated approach to assess the impacts of zinc pyrithione at different levels of biological organization in marine mussels

The mechanisms of sublethal toxicity of the antifouling biocide, zinc pyrithione (ZnPT), have not been well-studied. This investigation demonstrates that 14-d sublethal exposure to ZnPT (0.2 or 2 μM, alongside inorganic Zn and sea water controls) is genotoxic to mussel haemocytes but suggests that this is not caused by oxidative DNA damage as no significant induction of oxidised purines was detected by Fpg-modified comet assay. More ecologically relevant endpoints, including decreased clearance rate (CR), cessation of attachment and decreased tolerance of stress on stress (SoS), also showed significant response to ZnPT exposure. Our integrated approach was underpinned by molecular analyses (qRT-PCR of stress-related genes, 2D gel electrophoresis of proteins) that indicated ZnPT causes a decrease in phosphoenolpyruvate carboxykinase (PEPCK) expression in mussel digestive glands, and that metallothionein genes are upregulated; PEPCK downregulation suggests that altered energy metabolism may also be related to the effects of ZnPT. Significant relationships were found between % tail DNA (comet assay) and all higher level responses (CR, attachment, SoS) in addition to PEPCK expression. Principal component analyses suggested that expression of selected genes described more variability within groups whereas % tail DNA reflected different ZnPT concentrations.

Temperature increases, hypoxia, and changes in food availability affect immunological biomarkers in the marine mussel Mytilus galloprovincialis

Temperature increases, hypoxia, and changes in food availability are predicted to occur in the future. There is growing concern for the health status of wild and farmed organisms, since environmental stressors alter organism functions, and elicit coordinated physiological responses for homeostasis. Mussels are good bioindicators of environmental conditions. Their ability to maintain unaltered immunosurveillance under adverse environmental conditions may enhance their survival capability. Few studies are currently concerned with the relationships and feedback among multiple stressors. Here, food concentration, temperature, and oxygenation treatments were evaluated for their effects on immune enzymatic parameters of Mytilus galloprovincialis detected in the digestive gland and the lysosomal viability by neutral red uptake. Mussels were exposed to three temperatures (12, 20, and 28 °C) under normoxic (8 mg O₂l^-1) and anoxic conditions and specimens were fed with six food concentrations, ranging 0.2-5 g chlorophyll l^-1. Temperature increases affected esterase and alkaline phophatase enzyme functionality, and addition of food buffered detrimental effects generated by harsh conditions, such as those provided by low oxygen concentrations. Kinetics of the phenoloxidase was negatively correlated with increasing temperature. In this case, food had a buffering effect that counteracted the limiting temperature only under normoxic conditions. In addition, the stability of the lysosomal membrane was altered under conditions of thermal stress and food change, under normoxic and anoxic conditions. Overall, environmental stress factors affected immune biomarkers of Mediterranean mussels, and the level of food acted as a buffer, increasing the thermal resistance of the specimens.

A portable infrared photoplethysmograph: heartbeat of Mytilus galloprovincialis analyzed by MRI and application to Bathymodiolus septemdierum

Infrared photoplethysmogram (IR-PPG) and magnetic resonance image (MRI) of the Mytilus galloprovincialis heart were obtained simultaneously. Heart rate was varied by changing temperature, aerial exposure and hypoxia. Higher heart rates (35-20 beat min⁻¹) were usually observed at 20°C under the aerobic condition, and typical IR-PPG represented a single peak (peak v). The upward and downward slopes of the peak v corresponded to the filling and contracting of the ventricle, respectively. A double-peak IR-PPG was observed in a wide range of heart rates (5 to 35 beats min⁻¹) under various conditions. The initial peak v corresponded to the filling of the ventricle, and the origin of the second peak (v’) varied with the heart rate. A flat IR-PPG with a noise-level represented cardiac arrest. Although large movement of the shells and the foot caused slow waves or a baseline drift of the IR-PPG, the heart rate can be calculated from the v-v interval. Based on these results, we assembled a portable IR-PPG recording system, and measured the heartbeats of Bathymodiolus septemdierum (Mytilidae) for 24 h on a research vessel just after sampling from the deep sea, showing that IR-PPG is a noninvasive, economical, robust method that can be used in field experiments.

Summary: Infrared photoplethysmogram of Mytilus heart was analyzed by magnetic resonance imaging. Portable photoplethysmographs provide a noninvasive, economical and robust method to monitor the heartbeat of mussels in field experiments.

Is altered behavior linked to cellular energy regulation in a freshwater mussel (Elliptio complanata) exposed to triclosan?

Environmental stress may alter the bioenergetic balance of organisms by resulting in greater energy investment into detoxification processes, which diverts energy from other biological functions. Here, we examine responses to triclosan (TCS) exposure in a freshwater mussel across multiple biological levels: behavioral (e.g., burrowing and movement activity), organismal (e.g., metabolic rate and heart rate), and subcellular (e.g., gene expression and protein abundance/activity). At the subcellular level, we employed both energetic (i.e., AMP-activated protein kinase (AMPK)) and traditional (i.e., heat shock protein (HSP70), superoxide dismutase (SOD), glutathione-S-transferase (GST)) biomarkers. We found a significant reduction in burrowing and movement behaviors, a 1.8-fold increase in total-AMPK protein abundance, and a 2.8-fold increase in AMPK activity after 21d. GST activity increased after 4d, but not after 21d. Our findings suggest that TCS exposure results in an energetic tradeoff between detoxification at the cellular level and whole-animal activity.

Genotoxic potential and heart rate disorders in the Mediterranean mussel Mytilus galloprovincialis exposed to Superdispersant-25 and dispersed diesel oil

The effects of ex situ exposure of Mytilus galloprovincialis to Superdispersant-25 (S-25), diesel oil and dispersed diesel oil mixtures were studied by the impact on level of DNA damage in haemocytes (comet assay) and the cardiac activity patterns of mussels. Specimens were exposed for 72 h in a static system to diesel oil (100 μL/L and 1 mL/L), S-25 (5 and 50 μL/L), and dispersed diesel oil mixtures M1 (diesel oil 100 μL/L + S-25 5 μL/L) and M2 (diesel oil 1 mL/L + S-25 50 μL/L). For positive control 40 μM CdCl2 was used. The comet assay results indicated genotoxic potential of S-25 while the effects of diesel oil alone were not observed. The highest response was detected for M1 while the effects of M2 were not detected. The heart rate disorders were recorded for the diesel oil (1 mL/L), S-25 (50 μL/L) and both dispersed diesel oil mixtures.

Effect of seawater desalination and oil pollution on the lipid composition of blue mussels Mytilus edulis L. from the White Sea

A study on the effect oil pollution under normal and reduced salinity had on blue mussels Mytilus edulis L. from the White Sea in an aquarium-based experiment and in the natural habitat revealed a change in gill total lipids as a compensatory response. The cholesterol concentration and the cholesterol/phospholipids ratio in gills were found to reflect the impact of the environmental factors (oil pollution and desalination), and evidence adaptive changes in the cell membrane structure. An elevated content of storage lipids (chiefly triacylglycerols) in the mussels in the aquarium experiment indicates, first of all, the uptake and accumulation of oil products in gill cells under both normal and reduced seawater salinity, while high triacylglycerols level in gill littoral mussels from 'control' biotope in the Gulf of Kandalaksha is primarily associated with the mussel׳s pre-spawning period.

Biochemical markers of pollutant responses in macrozoobenthos from the White Sea: intracellular proteolysis

Coastal environments of Kandalaksha Gulf in the White Sea (Russia) despite nature conservation efforts are heavily influenced by human activities. Biological effects of complex environmental pollution, including organic substances, heavy metals, and oil hydrocarbons, were assessed in widely distributed marine invertebrates, Gammarus duebeni (Crustacea, Amphipoda) and Mytilus edulis (Mollusca, Bivalvia), collected from a series of anthropogenically-impacted areas and distanced reference sites in Kandalaksha Gulf. The parameters of intracellular protein degradation pathways such as cytosol calpain system and lysosomal cathepsins B (CatB) and cathepsin D (CatD) were studied. The response reactions observed in invertebrates vary in specificity and ranged from adaptive to destructive depending on the total contaminant level and the nature of predominant pollutant. The ecological relevance of studied parameters as biomarkers was confirmed by their ability to indicate both expose to pollutants and adverse effects at the organism level.

Changes of blue mussels Mytilus edulis L. lipid composition under cadmium and copper toxic effect

The lipid and fatty acid composition of the blue mussels Mytilus edulis L. gills and digestive glands was evaluated after 24 and 72 h of cadmium (Cd) and copper (Cu) exposure. Mussels were exposed to different cadmium (10, 100, and 500 μg/L) and copper (5, 50, and 250 μg/L) concentrations. Similar stress response of predominant membrane phospholipids level as well as polyenoic and non-methylene interrupted (NMI) fatty acids content was observed in mussel gills under both cadmium and copper effects. Increased NMI fatty acids level after 24 h, the metal ions treatment suggests that these acids contribute to the protective response to the membrane oxidative stress caused by accumulation of the metals. The content of cholesterol, some minor membrane phospholipids, and storage lipids (triacylglycerols, TAG) in the mussels' organs alter significantly under the cadmium and copper effect. A two-step response at the digestive glands TAG level depends on the duration of the cadmium and copper treatments (24 and 72 h) on the mussels. The results demonstrate that Cd and Cu impact has adverse effects on gills and digestive glands lipid and fatty acids composition. The type of observed effects varies with the nature and concentration of the metal ions and depends on the role of the metals in the mussels' life activity.

Effect of copper and cadmium ions on heart function and calpain activity in blue mussel Mytilus edulis

The heart rate and calpain activity of blue mussels Mytilus edulis from the sublittoral zone, exposed to different levels of water-borne copper and cadmium, was investigated in a long-term experiment. The content of cadmium and copper in the blue mussel was determined using flame and graphite Atomic absorption spectroscopy. The observed concentrations ranged from 2.5 to 89.1 μg/g dry weight for cadmium and from 6.1 to 51.0 μg/g dry weight for copper in the control and highest concentration, respectively. Initially, increase in cardiac activity in response to copper and Cadmium exposure was observed under all pollutant concentrations (5-250 and 10-500 μg/L, respectively). The calpain-like activity in gills and hepatopancreas of the mussels treated with metals changed in dose- and time-dependent manner: from a sharp rise at the 250 μg/L concentration of copper on the first day to a significant decrease under the effect of Cadmium in the concentration of 500 μg/L on the third day of the experiment. These results suggest that: (i) heart rate oscillation may reflect active adaptation of blue mussels to contamination and (ii) animals have different sensitivity to copper and Cadmium according to the role of the metals in the mussels' life activity.

The neuronal control of cardiac functions in Molluscs

In this manuscript, I review the current and relevant classical studies on properties of the Mollusca heart and their central nervous system including ganglia, neurons, and nerves involved in cardiomodulation. Similar to mammalian brain hemispheres, these invertebrates possess symmetrical pairs of ganglia albeit visceral (only one) ganglion and the parietal ganglia (the right ganglion is bigger than the left one). Furthermore, there are two major regulatory drives into the compartments (pericard, auricle, and ventricle) and cardiomyocytes of the heart. These are the excitatory and inhibitory signals that originate from a few designated neurons and their putative neurotransmitters. Many of these neurons are well-identified, their specific locations within the corresponding ganglion are mapped, and some are termed as either heart excitatory (HE) or inhibitory (HI) cells. The remaining neurons are classified as cardio-regulatory, and their direct and indirect actions on the heart's function have been documented. The cardiovascular anatomy of frequently used experimental animals, Achatina, Aplysia, Helix, and Lymnaea is relatively simple. However, as in humans, it possesses all major components including even trabeculae and atrio-ventricular valves. Since the myocardial cells are enzymatically dispersible, multiple voltage dependent cationic currents in isolated cardiomyocytes are described. The latter include at least the A-type K(+), delayed rectifier K(+), TTX-sensitive Na(+), and L-type Ca(2+) channels.