Larval feeding activity and use of embryonic resources determine juvenile performance of the common prawn Palaemonserratus.

Phenotypic links are the potential for "carryover" of effects of experience during one life history stage into performance and selection at subsequent stages. They reflect plastic responses to the environment experienced during an early phase on the phenotype of subsequent phases. We are studying these effects by following individuals of the shrimp Palaemon serratus from the embryonic (eggs carried by females) through the larval phase (pelagic) to the juvenile phase (benthic). In experiment 1, we investigated the effects of larval prey concentration (10, 4 and 2 Artemia/mL) and larval incubation temperature (16 and 22 °C) on larval performance (metamorphosis rate, developmental duration and growth) and then on juvenile performance (survival and Specific Growth Rate, SGR, at 18 and 24 °C in 14 days). In experiment 2, we investigated the effects of embryonic incubation temperature (larval biomass and lipid content of newly hatched larvae from embryos incubated at 12 and 18 °C) and larval prey concentration on larval performance and then on juvenile performance. In both experiments, the larvae plastically increased their development time in response to the reduction in temperature and prey concentration, whereas their survival decreased with temperature and prey concentration. The quantity of lipids available at hatching decreased with decreasing embryonic incubation temperature, which reduced the larval performance, particularly with a low concentration of prey. Survival at 14 days post-metamorphosis was significantly reduced when the embryos were incubated at 12 °C compared with those incubated at 18 °C, regardless of the subsequent larval incubation conditions, revealing phenotypic links between overconsumption of embryonic yolk reserves and post-metamorphic fitness. Overall, juveniles had a better SGR at 24 than at 18 °C, and even better when incubated under stressful embryo-larval conditions (temperature and prey concentration). This study highlighted phenotypic links between developmental stages and over developmental periods of several months.

Cadmium Highlights Common and Specific Responses of Two Freshwater Sentinel Species, Dreissena polymorpha and Dreissena rostriformis bugensis.

Zebra mussel (ZM), Dreissena polymorpha, commonly used as a sentinel species in freshwater biomonitoring, is now in competition for habitat with quagga mussel (QM), Dreissena rostriformis bugensis. This raises the question of the quagga mussel's use in environmental survey. To better characterise QM response to stress compared with ZM, both species were exposed to cadmium (100 µg·L-1), a classic pollutant, for 7 days under controlled conditions. The gill proteomes were analysed using two-dimensional electrophoresis coupled with mass spectrometry. For ZM, 81 out of 88 proteoforms of variable abundance were identified using mass spectrometry, and for QM, 105 out of 134. Interestingly, the proteomic response amplitude varied drastically, with 5.6% of proteoforms of variable abundance (DAPs) in ZM versus 9.4% in QM. QM also exhibited greater cadmium accumulation. Only 12 common DAPs were observed. Several short proteoforms were detected, suggesting proteolysis. Functional analysis is consistent with the pleiotropic effects of the toxic metal ion cadmium, with alterations in sulphur and glutathione metabolisms, cellular calcium signalling, cytoskeletal dynamics, energy production, chaperone activation, and membrane events with numerous proteins involved in trafficking and endocytosis/exocytosis processes. Beyond common responses, the sister species display distinct reactions, with cellular response to stress being the main category involved in ZM as opposed to calcium and cytoskeleton alterations in QM. Moreover, QM exhibited greater evidence of proteolysis and cell death. Overall, these results suggest that QM has a weaker stress response capacity than ZM.

Advancing environmental monitoring across the water continuum combining biomarker analysis in multiple sentinel species: A case study in the Seine-Normandie Basin (France).

Nowadays, biomarkers are recognized as valuable tools to complement chemical and ecological assessments in biomonitoring programs. They provide insights into the effects of contaminant exposures on individuals and establish connections between environmental pressure and biological response at higher levels. In the last decade, strong improvements in the design of experimental protocols and the result interpretation facilitated the use of biomarker across wide geographical areas, including aquatic continua. Notably, the statistical establishment of reference values and thresholds enabled the discrimination of contamination effects in environmental conditions, allowed interspecies comparisons, and eliminated the need of a reference site. The aim of this work was to study freshwater-estuarine-coastal water continua by applying biomarker measurements in multi-species caged organisms. During two campaigns, eight sentinel species, encompassing fish, mollusks, and crustaceans, were deployed to cover 25 sites from rivers to the sea. As much as possible, a common methodology was employed for biomarker measurements (DNA damage and phagocytosis efficiency) and data interpretation based on guidelines established using reference values and induction/inhibition thresholds (establishment of three effect levels). The methodology was successfully implemented and allowed us to assess the environmental quality. Employing multiple species per site enhances confidence in observed trends. The results highlight the feasibility of integrating biomarker-based environmental monitoring programs across a continuum scale. Biomarker results align with Water Framework Directive indicators in cases of poor site quality. Additionally, when discrepancies arise between chemical and ecological statuses, biomarker findings offer a comprehensive perspective to elucidate the disparities. Presented as a pilot project, this work contributes to gain insights into current biomonitoring needs, providing new questions and perspectives.

Combined effects of temperature and diet on the performance of larvae produced by young and old Palaemon serratus females.

Seasonal variations in environmental conditions determine the success of decapod larval development, and females transmit more energy in sub-optimal conditions to maximise the fitness of their offspring. The objective of this study was to focus on the combined effects of temperature (14, 18 and 22 °C) and food quality on the performance of larvae produced by 5 young (0+) and 5 old (I+) Palaemon serratus females. We prepared 3 diets based on Artemia, in decreasing order of total fatty acid content: freshly hatched nauplii (N), unenriched metanauplii (M) and metanauplii enriched with a mixture of microalgae (ME). At hatching, the larvae produced by I+ females had a higher biomass but a similar fatty acid concentration to those produced by 0+ females. Larvae survived better and developed relatively faster as temperature increased, and the longer they waited to metamorphose, the greater their weight at metamorphosis. These performances were diet-dependent, with more survival and more growth in less time with diet N than with the other two. Larvae from I+ females performed better than those from 0+ females, especially under the most stressful conditions. The greater biomass of the larvae of I+ females seems to have enabled them to follow a shorter, and therefore faster, development path than those of 0+ females. The larvae's diet also had an impact on post-metamorphic composition: larvae eating a diet richer in fatty acids produced richer juveniles and those eating a poorer diet produced juveniles with slightly more essential fatty acids. This study supports the high plasticity of caridean shrimp larval development and the importance of maternal effects on the fitness of offspring.

N-acetyl-ß-D-glucosaminidase measurement on the freshwater amphipod Gammarus fossarum: development, biological variability and application in an ecotoxicological approach.

Due to its role in the crustacean moulting process, N-acetyl-ß-D-glucosaminidase (NAGase) is interesting to monitor the good proceeding of the moult cycle, as well as relevant in assessing changes in the moulting process caused by stressors. The present study aimed to measure the NAGase activity to monitor the moulting process of the freshwater amphipod Gammarus fossarum. Firstly, an optimised protocol measuring the NAGase activity was made, allowing a robustness and reproducibility of measurements. Then, intrinsic variability of NAGase response was checked under two physiological factors: the gammarid moult cycle and gender. For both genders, a significative increase of activity was observed during premoult, instead of a basal activity detected during postmoult and intermoult. However, the NAGase female profile was preconised to study since it was defined with more precision. Finally, a 16-day exposure of female gammarids to different levels of treated or non-treated wastewater effluents was made. If delays of tissue development appeared on effluent exposed specimens, NAGase activity was similar between the different conditions. This apparent desynchronization between tissue and molecular activities accentuates the diagnostic of moult impairment and raises the interest to use markers at different organisational levels.

Modulation of haemocyte motility by chemical and biological stresses in Mytilus edulis and Dreissena polymorpha.

Mussels are constantly exposed to various pollutants in the environment, which can impair their immune defences against microbes and thus threaten their survival. In this study, we expand the insight into a key parameter of immune response in two mussel species by exploring the impact of exposure to pollutants or bacteria or simultaneous chemical and biological exposure on haemocyte motility. Basal haemocyte velocity in primary culture was high and increasing over time in Mytilus edulis (mean cell speed of 2.32 µm/min ± 1.57) whereas Dreissena polymorpha showed a constant and rather low cell motility with time (mean cell speed of 0.59 µm/min ± 0.1). In the presence of bacteria, the motility of haemocytes was instantly enhanced and slowed down after 90 min for M. edulis. In contrast, in vitro exposure of haemocytes to chemicals, either Bisphenol A, oestradiol, copper, or caffeine, induced an inhibition of cell motility in both mussel species. Finally, the cellular activation observed during bacterial challenges was inhibited by simultaneous exposure to bacteria and pollutants. Overall, our results indicate that chemical contaminants can alter haemocyte migration in mussels which can weaken their response to pathogens and therefore increase their susceptibility to infectious diseases.

Comparative immune responses of blue mussel and zebra mussel haemocytes to simultaneous chemical and bacterial exposure.

Biomonitoring at the scale of the aquatic continuum and based on biomarkers, requires various representative species and a knowledge of their sensitivity to contaminants. Mussel immunomarkers are established tools for evaluating immunotoxic stress, but little is known about the consequences of an immune activation by local microorganisms on their response to pollution. This study aims to compare the sensitivity of cellular immunomarkers in two mussel species from different environments, the marine mussel Mytilus edulis (blue mussel) and the freshwater mussel Dreissena polymorpha (zebra mussel), to chemical stressors combined with bacterial challenge. Haemocytes were exposed ex vivo to the contaminants (bisphenol A, caffeine, copper chloride, oestradiol, ionomycin) for 4 h. The chemical exposures were coupled with simultaneous bacterial challenges (Vibrio splendidus and Pseudomonas fluorescens) to trigger activation of the immune response. Cellular mortality, phagocytosis efficiency and phagocytosis avidity were then measured by flow cytometry. The two mussel species had different basal levels since D. polymorpha showed higher cell mortality than M. edulis (23.9 ± 11% and 5.5 ± 3% dead cells respectively), and lower phagocytosis efficiency (52.6 ± 12% and 62.2 ± 9%), but similar phagocytosis avidity (17.4 ± 5 and 13.4 ± 4 internalised beads). Both bacterial strains led to an increase in cellular mortality (+8.4% dead cells in D. polymorpha, +4.9% in M. edulis), as well an activation of phagocytosis (+9.2% of efficient cells in D. polymorpha, +6.2% efficient cells and +3 internalised beads per cell in M. edulis). All chemicals triggered an increase in haemocyte mortality and/or phagocytotic modulations, except for bisphenol A. The two species differed in the amplitude of their response. The addition of a bacterial challenge significantly altered cell responses to chemicals with synergetic and antagonistic variations compared to a single exposure, depending on the compound used and the mussel species. This work highlights the species-specific sensitivity of mussel immunomarkers to contaminants, with or without bacterial challenge, and the necessity of considering the presence of in natura non-pathogenic microorganisms for future in situ applications of immunomarkers.

Effects of Chemical Compounds on the Activity of the N-acetyl-ß-D-Glucosaminidase of the Marine Prawn, Palaemon serratus: Screening In Vitro.

N-acetyl-ß-D-glucosaminidase (NAGase) is important for crustaceans because the enzyme activity is necessary for the molting process. The present study aimed to assess the sensitivity of Palaemon serratus NAGase activity to a set of compounds of diverse chemical families in the context of in vitro exposures. Compounds representing different chemical families were selected according to their abundance, impact in the environment, and relevance as disruptors of the molting process. In a first step, four solvents (dimethylsulfoxide [DMSO], methanol, acetone, and ethanol) were tested to determine their suitability to dissolve hydrophobic compounds without affecting NAGase activity. Exclusively, ethanol had no effect on enzyme activity and on the integrity of the proteins present in the enzyme extract. The 18 other compounds were tested and four of these compounds, pentoxifylline, fenoxycarb, dithiocarbamate, and RH5849, showed a specific alteration on the activity of NAGase, without affecting the protein content. However, cadmium, zinc, and glyphosate showed a nonspecific alteration, affecting both the enzyme activity and the proteins, whereas ibuprofen exclusively altered the protein content. Finally, 10 of the 22 tested compounds (including DMSO, acetone, and methanol) showed a direct alteration of NAGase activity. Environ Toxicol Chem 2023;42:846-858. © 2023 SETAC.

Temperature-mediated developmental plasticity in winter and summer larvae of Palaemon serratus.

In a seasonal environment, variation in larval phenotype and developmental plasticity allow crustacean larvae to maximise survival by lengthening or shortening their development. The aim of this study is to investigate the effects of temperature, laying season and their interaction on larval developmental pathways (larval instars and larval stages). We monitored the different larval stages and calculated the number of larval instars reached during the development of winter and summer larvae of Palaemon serratus incubated at 12, 16 or 20 °C. We observed a great variability in the larval development (6-13 larval instars and 6 to 11 larval stages). A higher temperature decreases the development time and the number of larval instars. At a given temperature, the development time of winter and summer larvae was not different. Two larval stages were considered supernumerary (zoea 4 and 6), as they were more frequent at low temperatures. At higher temperatures, some larvae started to develop pleopods as early as the third instar, larval stage which had never been described (named here zoea 3'). This phenomenon was more common in winter larvae than in summer larvae. These results provide new insights into the expression of developmental plasticity in decapod larvae.

Blue mussel (Mytilus edulis)-A bioindicator of marine water contamination by protozoa: Laboratory and in situ approaches.

AIMS: The protozoan parasites Cryptosporidium spp., Giardia duodenalis and Toxoplasma gondii are identified as public health priorities and are present in a wide variety of environments including the marine ecosystem. The objective of this study was to demonstrate that the marine bivalve blue mussel (Mytilus edulis) can be used as a tool to monitor the contamination of marine waters by the three protozoa over time. METHODS AND RESULTS: In order to achieve a proof of concept, mussels were exposed to three concentrations of G. duodenalis cysts and Cryptosporidium parvum/T. gondii oocysts for 21 days, followed by 21 days of depuration in clear water. Then, natural contamination by these protozoa was sought for in wild marine blue mussels along the northwest coast of France to validate their relevance as bioindicators in the field. Our results highlighted that: (a) blue mussels bioaccumulated the parasites for 21 days, according to the conditions of exposure, and parasites could still be detected during the depuration period (until 21 days); (b) the percentage of protozoa-positive M. edulis varied under the degree of protozoan contamination in water; (c) mussel samples from eight out of nine in situ sites were positive for at least one of the protozoa. CONCLUSIONS: The blue mussel M. edulis can bioaccumulate protozoan parasites over long time periods, according to the degree of contamination of waters they are inhabiting, and can highlight recent but also past contaminations (at least 21 days). SIGNIFICANCE AND IMPACT OF THE STUDY: Mytilus edulis is a relevant bioaccumulators of protozoan (oo)cysts in laboratory and field conditions, hence its potential use for monitoring parasite contamination in marine waters.

Centrosome, the Newly Identified Passenger through Tunneling Nanotubes, Increases Binucleation and Proliferation Marker in Receiving Cells.

Type 1 tunneling nanotubes (TNTs-1) are long, cytoplasmic protrusions containing actin, microtubules and intermediate filaments that provide a bi-directional road for the transport of various components between distant cells. TNT-1 formation is accompanied by dramatic cytoskeletal reorganization offering mechanical support for intercellular communication. Although the centrosome is the major microtubule nucleating center and also a signaling hub, the relationship between the centrosome and TNTs-1 is still unexplored. We provide here the first evidence of centrosome localization and orientation towards the TNTs-1 protrusion site, which is implicated in TNT-1 formation. We also envision a model whereby synchronized reorientation of the Golgi apparatus along with the centrosome towards TNTs-1 ensures effective polarized trafficking through TNTs-1. Furthermore, using immunohistochemistry and live imaging, we observed for the first time the movement of an extra centrosome within TNTs-1. In this regard, we hypothesize a novel role for TNTs-1 as a critical pathway serving to displace extra centrosomes and potentially to either protect malignant cells against aberrant centrosome amplification or contribute to altering cells in the tumor environment. Indeed, we have observed the increase in binucleation and proliferation markers in receiving cells. The fact that the centrosome can be both as the base and the user of TNTs-1 offers new perspectives and new opportunities to follow in order to improve our knowledge of the pathophysiological mechanisms under TNT control.

N-acetyl-ß-d-glucosaminidase activity in Palaemon serratus - Methodological optimisation and intrinsic variability.

Chitinolytic enzymes fulfil a key role in the moulting process of crustaceans, in degrading the endocuticle during apolysis. Measuring the enzyme activity is an interesting manner to monitor the moult process at sub-individual level, complementary to the classical observation of the integument morphogenesis, ecdysis success, or moult cycle duration. The present study aimed to optimise the methodology of using N-acetyl-ß-D-glucosaminidase (NAGase) activity to monitor moulting in the marine prawn Palaemon serratus, and to compare NAGase activity levels along the moult cycle of both male and female specimens. First, to optimise protocols for five different organs, different reaction medium compositions were tested, considering the type buffer, concentration of the substrate, and the load in enzymatic extract. Second, levels of NAGase activity were closely monitored during eight moulting stages in male prawns. Variations in NAGase activity were observed during the moult cycle, with an increase in activity in the late premoult phase of approximately 2.4-fold the level of the intermoult phase. This response profile was observed for each tested organ. The levels of NAGase activity of male and female specimens were compared during three stages of the premoult phase. The patterns observed for both sexes were similar for all the tested organs.

Comparison of viability and phagocytic responses of hemocytes withdrawn from the bivalves Mytilus edulis and Dreissena polymorpha, and exposed to human parasitic protozoa.

Bivalve molluscs are now considered indicator species of aquatic contamination by human parasitic protozoa. Nonetheless, the possible effects of these protozoa on the immune system of their paratenic hosts are poorly documented. The aim of this study was to evaluate the effects of two protozoa on hemocyte viability and phagocytosis from two mussels, the zebra mussel (freshwater habitat) and the blue mussel (seawater habitat). For these purposes, viability and phagocytic markers have been analysed on hemocytes from mussels without biological stress (control hemocytes), and on hemocytes exposed to a biological stress (Toxoplasma gondii and Cryptosporidium parvum oocysts). We report, for the first known time, the interactions between protozoa and hemocytes of mussels from different aquatic environments. Zebra mussel hemocytes showed a decrease in phagocytosis of fluorescent microbeads after exposure to both protozoa, while blue mussel hemocytes reacted only to T. gondii oocysts. These decreases in the ingestion of microbeads can be caused by competition between beads and oocysts and can be influenced by the size of the oocysts. New characterisations of their immune capacities, including aggregation, remain to be developed to understand the specificities of both mussels.

First evidence of cytotoxic effects of human protozoan parasites on zebra mussel (Dreissena polymorpha) haemocytes.

The interaction between human protozoan parasites and the immune cells of bivalves, that can accumulate them, is poorly described. The purpose of this study is to consider the mechanisms of action of some of these protozoa on zebra mussel haemocytes, by evaluating their cytotoxic potential. Haemocytes were exposed to Toxoplasma gondii, Giardia duodenalis or Cryptosporidium parvum (oo)cysts. The results showed a cytotoxic potency of the two largest protozoa on haemocytes and suggested the formation of haemocyte aggregates. Thus, this study reveals the first signs of a haemocyte:protozoan interaction.

Highly polluted life history and acute heat stress, a hazardous mix for blue mussels.

Intertidal sessile organisms constitute through their life history unintended stress recorders. This study focuses on the impact of pollution on Mytilus edulis ability to cope with an additional stress. For this purpose, two acclimation stages to different temperatures were conducted before an acute stress exposure in mussels collected from a heavily polluted site. Gill proteomes were analyzed by 2DE and regulated proteins identified. Massive mortality was observed for organisms acclimated to colder temperatures. Despite this major difference, both groups shared a common response with a strong representation of proteoforms corresponding to "folding, sorting and degradation" processes. Nevertheless, surviving mussels exhibit a marked increase in protein degradation consistent with the observed decrease of cell defense proteins. Mussels acclimated to warmer temperature response is essentially characterized by an improved heat shock response. These results show the differential ability of mussels to face both pollution and acute heat stress, particularly for low-acclimated organisms.

Use of sperm DNA integrity as a marker for exposure to contamination in Palaemon serratus (Pennant 1777): Intrinsic variability, baseline level and in situ deployment.

In a previous study, the Comet assay was optimized for Palaemon serratus prawns in order to propose a biomarker for sperm quality in this species. However, better knowledge of its basal level and its natural variability, related to intrinsic biotic and environmental abiotic factors, is required before any relevant use of this biomarker in the field. To fulfill this goal, the present study proceeded in three steps: (i) the temporal variability of DNA integrity was followed monthly in a reference population over a 2-year period, (ii) the correlation between the main intrinsic biotic (i.e. size, weight and molting stage) and abiotic factors (i.e. water temperature) were recorded in the field, and the basal DNA integrity was assessed in order to scrutinize any confounding influence of factors unrelated to toxic response, (iii) the baseline level was used to discriminate biomarker response among different stations displaying contrasting contamination levels. The results of the two-year monitoring in the reference population revealed no correlation between the levels of spermatozoa DNA damage and temperature, body size, weight or molting stage. Only a slight variability between monthly samplings was detected. On the basis of these field-collected data, we defined a reference distribution (i.e. 52.6 ±â€¯5.6 A.U) with a threshold value (i.e. 61.7 A.U). Finally, this threshold value proved its relevance to discriminate among stations with contrasting pollution levels around the Seine Bay. Indeed, the results suggest significant DNA damage in populations nearest the Seine estuary, a major source of contaminants in the Bay, and a lower effect in populations further away from the estuary. The overall conclusion was that the Comet assay on P. serratus spermatozoa could be a useful tool for the monitoring of the toxicological print within sperm and main globally the contamination exposure of crustaceans in marine waters.

Multixenobiotic resistance in Mytilus edulis: Molecular and functional characterization of an ABCG2- type transporter in hemocytes and gills.

Among the cellular protection arsenal, ABC transporters play an important role in xenobiotic efflux in marine organisms. Two pumps belonging to B and C subfamily has been identified in Mytilus edulis. In this study, we investigated the presence of the third major subtype ABCG2/BCRP protein in mussel tissues. Transcript was expressed in hemocytes and with higher level in gills. Molecular characterization revealed that mussel ABCG2 transporter shares the sequence and organizational structure with mammalian and molluscan orthologs. Overall identity of the predicted amino acid sequence with corresponding homologs from other organisms was between 49% and 98%. Moreover, protein efflux activity was demonstrated using a combination of fluorescent allocrites and specific inhibitors. The accumulation of bodipy prazosin and pheophorbide A was heterogeneous in gills and hemocytes. Most of the used blockers enhanced probe accumulation at different levels, most significantly for bodipy prazosin. Moreover, Mrp classical blocker MK571 showed a polyspecificity. In conclusion, our data demonstrate that several ABC transporters contribute to MXR phenotype in the blue mussel including ABCG2 that forms an active pump in hemocytes and gills. Efforts are needed to distinguish between the different members and to explore their single function and specificity towards allocrites and chemosensitizers.

Infection dynamics of a V. splendidus strain pathogenic to Mytilus edulis: In vivo and in vitro interactions with hemocytes.

The pathogenic strain V. splendidus 10/068 1T1 has previously been reported for its virulence to the blue mussel and for its capacity to alter immune responses. In this study, we expanded the knowledge on hemocyte-pathogen interactions by using in vitro and in vivo assays. V. splendidus 10/068 1T1 severely inhibited cell adhesion and acidic vacuole formation unlike the innocuous phylogenetically related V. splendidus 12/056 M24T1 which had no effect on these cell functions. Furthermore, the virulent bacteria decreased hemocyte viability (59% of viability after 24 h). Infection dynamics were explored by using a model based on water tank cohabitation with septic mussels infected by GFP-tagged V. splendidus 10/068 1T1. Experimental infections were successfully produced (16.6% and 45% mortalities in 3 days and 6 days). The amount of GFP Vibrio in seawater decreased during the experiment suggesting its horizontal transfer from diseased animals to healthy ones. At the same time periods, bacteria were detected in hemocytes and in various organs and caused necrosis especially in gills. Total hemocyte count and viability were affected. Taken together, our results indicate that the pathogen V. splendidus 10/068 1T1 colonizes its host both by bypassing external defense barriers and impairing hemocyte defense activities.

Consequences of acclimation on the resistance to acute thermal stress: Proteomic focus on mussels from pristine site.

Climate change constitutes an additional threat for intertidal species that already have to cope with a challenging environment. The present study focuses on the blue mussel Mytilus edulis and aims at investigating the importance of thermal acclimation in heat stress response. Microcosm exposures were performed with mussels submitted to an identical acute thermal stress following two thermal summer acclimations standing for present or future temperature conditions. Gill proteomes were analyzed by 2DE and 96 differentially expressed proteoforms were identified. Our results show that cell integrity appears to be maintained by the rise in molecular protective systems (i.e. Heat Shock Proteins), and by the reallocation of energy production via a switch to anaerobic metabolism and the setting up of alternative energy pathways. Finally, our results indicate that the response of mussels to acute thermal stress is conditioned by the acclimation temperature with an improved response in organisms acclimated to higher temperatures.

First evidence for a Vibrio strain pathogenic to Mytilus edulis altering hemocyte immune capacities.

Bacterial isolates were obtained from mortality events affecting Mytilus edulis and reported by professionals in 2010-2013 or from mussel microflora. Experimental infections allowed the selection of two isolates affiliated to Vibrio splendidus/Vibrio hemicentroti type strains: a virulent 10/068 1T1 (76.6% and 90% mortalities in 24 h and 96 h) and an innocuous 12/056 M24T1 (0% and 23.3% in 24 h and 96 h). These two strains were GFP-tagged and validated for their growth characteristics and virulence as genuine models for exposure. Then, host cellular immune responses to the microbial invaders were assessed. In the presence of the virulent strain, hemocyte motility was instantaneously enhanced but markedly slowed down after 2 h exposure. By contrast, hemocyte velocity increased in the presence of the innocuous 12/056 M24T1. At the same time interval, 10/068 1T1 invaded hemocytes and was more rapidly internalized than the innocuous strain. Extracellular products (ECPs) prepared from 10/068 1T1 cultures significantly inhibited phagocytic activity while 12/056 M24T1 ECPs had no effect. Furthermore, the pathogenic strain and its ECPs inhibited oxidative burst unlike 12/056 M24T1 strain/ECPs which enhanced ROS production. Taken together, our results suggest that the mussel pathogen 10/068 1T1 may escape immune response by altering hemocytes functions.