Although it is generally known that a combination of abiotic and biotic drivers shapes the distribution and abundance of parasites, our understanding of the interplay of these factors remains to be assessed for most marine host species. The present field survey investigated spatial patterns of richness, prevalence and abundance of parasites in Mytilus galloprovincialis along the coast of the northern Adriatic Sea. Herein, the relationships between biotic (host size, density and local parasite richness of mussel population) and abiotic (eutrophication and salinity) drivers and parasite richness of mussel individuals, prevalence and abundance were analysed. Local parasite richness was the most relevant factor driving parasite species richness in mussel individuals. Prevalence was mainly driven by eutrophication levels in three out of four parasite species analysed. Similarly, abundance was driven mainly by eutrophication in two parasite species. Mussel size, density and salinity had only minor contributions to the best fitting models. This study highlights that the influence of abiotic and biotic drivers on parasite infections in mussels can be differentially conveyed, depending on the infection measure applied, i.e. parasite richness, prevalence or abundance. Furthermore, it stresses the importance of eutrophication as a major factor influencing parasite prevalence and abundance in mussels in the Adriatic Sea.

Investigation of parasites and diseases affecting molluscs of ecological and economic interest is critical for the management of native stocks and aquaculture. In recent years, much attention has been devoted to investigating the World Organisation for Animal Health listed infectious diseases, so that communities can be prepared to attend public health emergencies and avoid severe income losses. In this context, the health status of Mytilus galloprovincialis Lamarck, 1819 was analysed in two aquaculture sites (Strunjan Bay and Piran Bay, Slovenia), and in four natural mussel beds (Adriatic Croatia International Marinas Pula and Rovinj and St Catherine, Croatia; Marina Koper, Slovenia) along the coast of the northern Adriatic Sea. The mussels were sampled in February and April 2014, and processed for histological examination of several endosymbionts and pathogens that frequently occur in mytilid mussels. Endosymbionts and pathogens were not detected in farmed mussels. Prokaryotic inclusion bodies, the protozoan Nematopsis, Ancistrocoma-like ciliates, haplosporidian-like plasmodia, turbellarian Urastoma cyprinae, and basophilic inclusion bodies were observed in digestive gland cryosections of wild mussels from the coastal region of the northern Adriatic. Fungal spores of Psilocybe sp., Ulocladium sp. and Alternaria sp. were detected between the digestive tubules based on their morphology. Diagnostic PCR did not reveal infection with Marteilia refringens during the studied period, neither in wild nor farmed mussels. We confirmed the site effects on prevalence of infected mussels. Thus, we can conclude that wild areas are more exposed to endobionts and parasites than aquaculture sites.

This is the first comprehensive research study on the parasites of Mytilus galloprovincialis collected from the Sinop coasts of the Black Sea and their relationships with several environmental and biotic factors. A total of 1740 mussels were collected monthly at three sampling localities representing different ecosystems in the period between August 2012 and July 2013 and examined for parasites. Identified parasites were Nematopsis legeri, Peniculistoma mytili, Urastoma cyprinae, Parvatrema duboisi and Polydora ciliata. Infection prevalence (%), mean intensity and mean abundance values of each parasite species were calculated according to season, sampling localities and length classes of mussel. Nematopsis legeri was the most prevalent species (32.5%), followed by Pe. mytili (6.70%), U. cyprinae (6.30%), Pa. duboisi (4.50%) and Po. ciliata (2.20%). Nematopsis legeri and Parvatrema duboisi had their highest infection prevalence and intensity values in sampling locality III where secondary hosts present to complete their life cycle and larger sized mussels had higher parasite loads. Statistically significant differences were determined in the prevalence of infection and intensity values among seasons, length classes of mussel and sampling localities of each parasite species. The present study provided valuable information on mussel parasites and their relationships with host length, seasons and ecology.

Quantitative real-time polymerase chain reaction (qPCR) is probably the most used method for gene expression quantification because of its high sensitivity and specificity. Nevertheless, this technology can undergo experimental errors and variations. Normalization of the results using a reference gene is therefore necessary to minimize these variations. As the study of immune genes in bivalve mollusks has increased in the last years, the establishment of adequate and stable reference genes for bivalves is strongly required. We analyzed the behavior of four putative reference genes: ribosomal RNA 18S, actin, elongation factor 1 − α and α-tubulin. The suitability of these four genes as internal control for qPCR was evaluated in mussel (Mytilus galloprovincialis) and clam (Ruditapes philippinarum) hemocytes after bacterial challenge. Four independent approaches (BestKeeper, GeNorm, NormFinder and DeltaCt ) were used to assess the suitable genes for stable expression. For these particular circumstances, the most stable gene in hemocytes was elongation factor 1 − α for mussels and α-tubulin for clams.

Genetic variation of mussels Mytilus galloprovincialis in central–eastern Mediterranean Sea is investigated in this study. A total of 550 individuals sampled from two cultured and 11 wild populations from Italy, Croatia, Greece and Turkey were genotyped at 10 microsatellite loci. Significant deviations from Hardy–Weinberg expectations were observed in more than 75% of the tests performed. All populations showed extensive heterozygote deficits, which remained at significant levels even after correction for null alleles, providing evidence that null alleles were only partly responsible for deviations from Hardy–Weinberg equilibrium in these molluscs. Moreover, null alleles seem to have limited influence on the population genetic differentiation. Similar levels of multi-locus heterozygosity and allelic richness were observed in all populations, cultured and wild, implying the sustainability of the exploited populations. Lack of isolation by distance and markedly low genetic differentiation between the nine Greek sampling sites (shoreline >1000 km) was revealed by Mantel tests, FST values, exact tests and analyses of molecular variance, indicating that mussels from these regions are either at or close to panmixia. Similarly, patterns of genetic homogeneity were also found between the two Italian samples, whereas the observed genetic differentiation of the populations from Turkey and Croatia probably reflects the specific topographic and oceanographic conditions of these regions.

The presence of the non-indigenous species, the black-pygmy mussel Limnoperna securis, was surveyed for the first time in the Rias of Pontevedra and Arousa, areas adjacent to the first location (Ria of Vigo) of this potential invader in Atlantic waters. Molecular identification of the mussels was conducted by polymerase chain reaction amplification and sequencing of nuclear and mitochondrial genes. This paper describes for the first time the species in the Ria of Pontevedra, confirming that this invader was absent in an intense shellfish farming area of the Ria of Arousa (Galicia, north-western Spain). Field sampling revealed that relatively high concentrations of this mytilid bivalve have colonized some localities of the inner part of the Ria of Pontevedra located in brackish waters. A comparison between population densities, size and substrate preference of L. securis in the Rias of Pontevedra and Vigo was conducted. Dispersion capacity of L. securis is discussed based on molecular detection of larval stages in the stomach contents of the copepod Centropages typicus coupled with physical oceanography of the southern Galician Rias. The invasive role of L. securis is also discussed in the context of the bissus secretion and attachment strength, ecological tolerance of the species, and the recent finding of settlements of this species on numerous colonies of the economically important blue mussel Mytilus galloprovincialis.

The impact of an alien species is correlated with its abundance and potential to colonisenew environments. Consequently, the crucial aspects that give a mytilid species invasivepotential include its strength and capacity for adhesion to a wide variety of substratesand its ecological tolerance to environmental conditions. The alien black pygmy musselXenostrobus securis settles together with the indigenous and raftcultured mussel Mytilus galloprovincialis on hard substrates of the innercoastline of the Ría de Vigo (Atlantic Coast NW Spain). Key parameters for the ecologicalsuccess of these species could include their byssus structure and ecological plasticity.In this study, we assessed their comparative ability to attach to different substratesunder different environmental conditions, using both in situ and laboratory approaches.Byssus threads secreted by X. securis were found to be extremely thin andtheir thickness did not vary with mussel size. This thinness of byssus filaments inX. securis was compensated by the secretion of a huge number (up tothousand of byssus filaments). Although no increase in attachment strength was found withincreasing mussel size in this species, multiple byssus thread secretion might be a keyfactor in the successful invasion of inner areas of the estuary. Furthermore, X.securis exposes a much lower shell area to lift and drag forces in musselconglomerates. By contrast, the indigenous species M. galloprovincialissecretes thicker byssus threads, which provide stronger in situ attachmentdisplaying a linear increase in strength relative to the size of mussels. The exposure ofboth mytilid species to different abiotic factors in the laboratory confirmed the weakerbyssus secretion and limited variation in attachment strength in the alien species. On onehand, X. securis seemed to be better adapted to life on soft bottoms andnot capable of much plasticity when attached to different substrates. However, X.securis might be also well adapted to variable environmental conditions, e.g.,salinity fluctuations, with no need to modify its byssus properties to ensure secureattachment. Attachment strength of M. galloprovincialis was higher thanX. securis in all abiotic conditions tested. Results are discussed inrelation to the ecological tolerance and impact of these mytilid species.

Over a 52-month period beginning in May 2004 and concluding in August/September 2008, and coinciding with the period over which tributyltin was banned as a ship anti-foulant globally, a population of the dogwhelk, Nucella lapillus on the south-eastern coast of England, was studied for changes in population size and structure, reproduction and feeding behaviour. During the study period, the size of the N. lapillus population grew from ~25 individuals to >500, i.e. a 20-fold increase, and recovery from imposex was total. Significant changes in feeding behaviour were also reported. For example, peaks in N. lapillus predation were recorded over the winter months, but as population size increased, this temporal seasonality was masked due to the growing numbers of juveniles feeding on smaller and smaller individuals of Mytilus galloprovincialis (as opposed to barnacles) as the study progressed. Similarly, with freedom from imposex, the numbers of failed drilling attempts declined and numbers of prey with more than one drill hole increased as the incidence of kleptoparasitism increased. Predated M. galloprovincialis were also increasingly attacked in the antero- and postero-dorsal quadrants of their shells as the study progressed. Contrary to expectations, only a slightly positive relationship between predator and prey sizes was recorded overall however, suggesting that beyond a shell height of ~13 mm, when the transition from barnacle to mussel feeding occurs, M. galloprovincialis individuals of virtually any size are preyed upon by N. lapillus.

Byssogenesis rate and attachment strength of the mussel Mytilus galloprovincialis were investigated in the laboratory considering different body sizes and feeding conditions. Byssal thread secretion was significantly higher in juveniles as compared to larger mussels of approximately 87 mm shell length. Asymptotic number of threads attached was obtained from approximately 72 hours onwards within a range of 42–46 and 27–31 for juveniles and larger experimental mussels, respectively (P < 0.05 ANOVA). Absorption efficiency values of control fed individuals dropped significantly from 0.78 in juveniles to 0.70 in larger mussels (P < 0.001 ANOVA) which pointed out energetic constraints of mussels with regard to their size and its probable effect on byssus secretion rates. Attachment force was 2.8 times higher in larger mussels than in juveniles (2.21 versus 0.78 N for both sizes, respectively; P < 0.001 ANOVA) which in turn followed the same order of magnitude than differences in the thread's thickness values of both experimental mussel sizes. Tenacity followed a reverse pattern with juveniles presenting two-fold higher values than larger mussels (P < 0.001 ANOVA) based on a lower increment of attachment force (x2.8) as compared to shell area (x6) for the comparison larger versus juvenile mussel size, which in turn might suggest that larger specimens secreted weaker threads. When animals were maintained unfed for a week, a significant drop in both byssus secretion and attachment force were observed in juveniles but was not the case for larger mussels most likely as a consequence of a relatively short period of maintenance under food availability stress for the latter individuals that showed significantly higher initial condition and/or energetic store values. Accordingly, the use of energetic reserves in juveniles kept unfed for a week together with a significant drop in byssus secretion and attachment force might suggest a link, i.e. transfer of energy between soft tissues and byssus under stress. Quantitative values of byssal threads, based on the significance of the byssus versus attachment force relationship, together with its morphometric value, i.e. thread's thickness, represented a primary mechanism to explain variability in attachment strength of M. galloprovincialis of the different body sizes studied here.

The blue mussel, Mytilus edulis, and the Mediterranean mussel, Mytilus galloprovincialis, occur widely over much of northern Europe, and wherever they are sympatric they hybridize. The hybrid zone is large, ranging from western France to the north of Scotland, and is spatially complex, containing a mixture of pure, hybrid and introgressed individuals. Results from an Irish study in 1981, using partially diagnostic allozyme markers, indicated that mussels on the Irish Sea coast were solely M. edulis, but on Atlantic wave-exposed shores, and to a much lesser extent on wave-protected shores, mussels comprised an interbreeding mixture of M. edulis and M. galloprovincialis. In this study mussels were analysed from 20 locations on Irish coasts, using the Me15/16 nuclear DNA marker. The results showed a high frequency of M. galloprovincialis (0.378 ± 0.198) and hybrid (0.429 ± 0.175) genotypes, and correspondingly low frequencies of the M. edulis genotype (0.194 ± 0.107) at both exposed and sheltered locations on Atlantic coasts, indicating no apparent advantage for the M. edulis genotype at wave-protected sites. Mytilus galloprovincialis was virtually absent from the Irish Sea. Mussels in this area may be a self-recruiting population of M. edulis due to thermal front development at the northern and southern entrances to the Irish Sea in late spring, thereby preventing an influx of spring-spawned Mytilus larvae. The apparent change in the genetic composition of mussels on Atlantic coasts since the early 1980s could be related to climate change, or to aquaculture practice in Ireland whereby mussels from exposed shores are used to seed ropes in wave-protected bays and estuaries.

Since the first description of Marteilia refringens (Paramyxea) in flat oysters Ostrea edulis in 1968 in the Aber Wrach, Brittany (France), the life-cycle of this parasite has remained unknown. However, recent studies, conducted in the ‘claire’ system, have proposed the planktonic copepod Acartia grani as a potential intermediate host for the parasite. Nevertheless, experimental transmission of the parasite through the copepod has failed. Recent studies in this field have reported the presence of the parasite in zooplankton from the bays of the Delta de l'Ebre, a more complex and natural estuarine environment than that of the claire. As a result, 2 new Marteilia host species were proposed: the copepods Oithona sp. (Cyclopoida) and an indeterminate Harpaticoida. Consequently, the objective of the present work was to study the dynamics of Marteilia in the zooplankton community from one of the bays, Alfacs Bay, as well as the dynamics of the parasite in cultivated mussels during 1 complete year. Six different zooplankton taxa appeared to be parasitized by M. refringens, including copepods (3 Calanoida, Acartia discaudata, A. clausi and A. italica; 1 Cyclopoida, Oithona sp.; and 1 Harpacticoida, Euterpina acutifrons), and larval stages of decapod crustaceans (zoea larvae of Brachyura, probably Portumnus sp.). These taxa are thus proposed as new subjects for study, since they could be intermediate hosts in the infection process of mussels by Marteilia.

Marteilia refringens is a paramyxean parasite which infects the flat oyster Ostrea edulis and mussels (Mytilus galloprovincialis), where it has been attributed to a separate species, Marteilia maurini, by several authors. Doubts persist though as to the existence or not of two species of Marteilia in Europe. We have devised a molecular method for the diagnosis of M. refringens based on 358 bp nested-PCR of the rDNA intergene spacer (rDNA IGS) which is capable of detecting 0·5 fg of M. refringens DNA. Molecular characterization of this spacer indicates that the Marteilia parasites which infect oysters and mussels are two different strains of the same species.

Oil spills are an important source of PAHs in marine and coastal areas and comprise a short- and long-term threat for aquatic organisms. Some PAHs are known to be toxic, in particular mutagenic and/or carcinogenic, and their toxicological effects must be evaluated. Here, the impact of the “Erika” oil spill, which occurred at the end of 1999, was studied by combining chemical (PAH analyses) and toxicological approaches (biological effect assessment). “Erika” elutriates have been found to be more toxic than the elutriate obtained with a crude oil, Bal 250, inducing deleterious effects in Mytilus galloprovincialis and Crassostrea gigas embryos and in Isochrysis galbana algae. The embryotoxicity test in mussel is more sensitive than growth test in I. galbana. Naphthalenic compounds make up more than 95% of total PAHs quantified in elutriates. “Erika” elutriates are enriched with naphthalene, methyl-naphthalene, anthracene and higher-molecular-weight compounds. On the contrary, Bal 250 elutriate is characterized by the highest dibenzothiophene, methyl-dibenzothiophene and dimethyl-dibenzothiophene levels. Weathering does not highly affect the toxicity of the “Erika” oil. This study also confirms the potential impact of the “Erika” fuel on the biological quality of sea water and sediments from Traict du Croisic on the Atlantic coast of France.

Domoic acid is a neurotoxin responsible for the Amnesic Shellfish Poisoning (ASP). With the aim of determining its depuration kinetics in raft mussels Mytilus galloprovincialis and the effects of body weight, salinity and temperature on it, an experiment involving these factors was carried out. Mussels which had incorporated this toxin to a concentration of 153 μg·g–1 of soft tissue during a bloom of the pennate diatom Pseudo nitzschia australis, were collected from the Galician Rías and placed at temperatures of 18 and 22 ºC and salinities of 12.5 and 31. Mussel samples were taken at the start of the experiment and daily during the four subsequent days and the domoic acid contents of the soft tissues were analysed. The simple one-compartment kinetics, that is a kinetics in which all toxin depurates at the same rate, was fitted to the data, with good quantitative results. Notwithstanding, the deviations of the model from the actual data were dependent on time, suggesting that such a simple model is not enough to correctly describe the data, and that a more complex kinetics may be more adequate. A two-compartment kinetic model, in which two pools of toxins exist (compartments), each one depurating at different rate, described qualitatively better the depuration but its quantitative contribution to the fit was not statistically significant. The parameters of the model, obtained by least squares fitting, suggest the possibility of a small second compartment of very small or null depuration rate, as detected in other species. This kind of model would explain the reduced quantitative contribution of the second (slowly depurating) compartment.Low salinity was shown to reduce the depuration rate. The two other factors checked, temperature and body weight, nor any interaction had significant effect on depuration rate.

The impact of Mediterranean mussels (Mytilus galloprovincialis Lamarck) on particle removal from the water column under different flow velocities was measured in the laboratory within a recirculating flume. At 20 °C, the filtration rates increased with current velocity up to 20−25 cm·s−1. Filtration rates remained the same at 26 °C. At high phytoplankton concentration (Cryptophyceae: 8 500−11 000 Cryptomonas sp. cells·mL−1), filtration rates were low (0.2−0.4 L·h−1 per standard individual of 1 g dry weight), whereas higher filtration rates (0.5−2.5 L·h−1 per standard individual) were observed with lower concentrations of the algae Prorocentrum minimum (Dinophyceae : 1 300−5 800 cells·mL−1). The filtration rates of M. galloprovincialis were affected by high phytoplankton loads, and exhibited a maximum related to current velocity. Our results suggest that mussels, feeding over a wide range of seston concentrations and flow velocities, show an adaptive behaviour in an area where environmental conditions are known to vary drastically.

Most recent authors have called the Chilean blue mussel (formerly Mytilus chilensis) Mytilus edulis. Mussels from four nominal species (Mytilus edulis, M. trossulus, M. galloprovincialis and M. chilensis) were collected from the eastern coast of Newfoundland, Canada, New Zealand and the southern coast of Chile. Canonical discriminant analysis of selected shell morphometrical characters showed that although there is some overlap in canonical variates, the three nominal species, M. edulis, M. trossulus and M. chilensis can be morphologically divided into three distinct groups. The two nuclear-DNA markers (ITS and Glu-5) and the mtDNA marker studied indicate that the nuclear and mitochondrial genomes of M. edulis and M. trossulus are different. M. edulis, M. galloprovincialis and M. chilensis share similar restriction fragment patterns for the mtDNA and for the ITS nuclear-DNA marker, while the Glu-5 polymerase chain reaction (PCR) assay showed that the M. edulis and M. chilensis genotypes disagree, producing species-specific banding patterns that can be used as a diagnostic marker between the two nominal species. The molecular and morphometric data reported here give more evidence to support the view of other authors that the Chilean blue mussel should be included as M. edulis subspecies: Mytilus edulis chilensis.

Mussel (Mytilus galloprovincialis) plasma contains cytotoxic activity against both vertebrate (erythrocytes and mouse tumour) and protozoan cells. Procaryotes (Escherichia coli and Vibrio alginolyticus) were not sensitive to the cytotoxicity. The activity was still present in dialyzed samples but was inhibited by heating at 45 °C. Large individual variability which was not correlated with protein concentration and an increasing number of reactive specimens following injection, suggested that naive mussels were in various stages of immune response. Purification by anion exchange chromatography followed by gel filtration revealed a 320 kDa cytotoxic polymeric protein that acts through a polymerization process after binding onto target cell membranes as revealed by ultrastructural observation.

European and Pacific oysters (Ostrea edulis and Crassostrea gigas) expressed antibacterial activity against both Gram negative and Gram positive bacteria which was most probably due to small proteins. When tested against the marine pathogenic Vibrio alginolyticus, hemocyte lysates of both species were more active than cell-free plasma. Antibacterial activity showed significant individual variability that was dramatically reduced by stimulation through mechanical stress or injection. The number of spontaneously active Pacific oysters increased from 50 to 100% following a single injection of bacteria.

These results strongly support the view that bivalve molluscs possess sensitive immuno-defense mechanisms that will greatly aid the development of aquaculture systems by employing refined techniques of transgenesis.