Molecular confirmation of pearl formation in arctic mussels (Mytilus edulis) caused by Gymnophallus bursicola (Odhner 1900) metacercariae.

In recent field studies, suspected gymnophallid metacercariae were histologically located in the mantle of mussels from the Norwegian Sea. Mussels from the sites in which that infection was detected also presented abnormally high pearl numbers. It has been previously described that gymnophallid metacercariae could cause pearl formation processes in mussels, as a host reaction to encapsulate these metacercariae. Given the pathological host reaction these parasites elicit, a study was performed to identify gymnophallid metacercariae found in mussels collected from Tromsø at morphological and molecular level and to assess, by the use of molecular tools, the relationship between the parasite and the biological material inside the pearls. As a result, Gymnophallus bursicola metacercariae infecting Norwegian Mytilus edulis were identified according to morphological characters, along with the first 18S rDNA and COI sequences for this trematode species. In addition, parasite DNA from the core of the pearls was extracted and amplified for the first time, confirming the parasitological origin of these pearls. This procedure could allow identifying different parasitic organisms responsible for the generation of pearls in bivalves.

Infection cycle of Marteilia pararefringens in blue mussels Mytilus edulis in a heliothermic marine oyster lagoon in Norway.

Agapollen is a traditional heliothermic marine oyster lagoon in western Norway, representing the northernmost site of any Marteilia sp. protists detected in Europe. The semi-closed lagoon is a unique site to study the life cycle and development of M. pararefringens in naïve mussels. Two baskets with uninfected mussels were deployed in the lagoon outlet in May and October 2018, respectively, and sampled every 6 wk. The parasite was first detected in the mussels by PCR in early July and by histology in late August. By then, M. pararefringens had developed into mature stages, indicating a rapid development during mid-summer. Sporulation occurred during autumn. Mussels deployed in October never became infected, indicating that transmission was restricted to the warmest period of the year. Pronounced pathology was observed in infected mussels, including degenerated digestive tubules and infiltration of haemocytes. Mussel mortality was observed in the baskets, but whether this was due to infections of M. pararefringens or other environmental factors could not be determined. Plankton samples from the lagoon were also collected for PCR analysis. These samples, dominated by copepods, were positive for M. pararefringens in summer. In sorted samples, M. pararefringens was detected in the Acartia spp. and Paracartia grani fractions between July and October. These plankton copepods are therefore potentially involved in the life cycle of M. pararefringens.

Evaluation of real-time qPCR-based methods to detect the DNA of the three protozoan parasites Cryptosporidium parvum, Giardia duodenalis and Toxoplasma gondii in the tissue and hemolymph of blue mussels (M. edulis).

The protozoan parasites Cryptosporidium spp., Giardia duodenalis and Toxoplasma gondii can be transmitted to humans through shellfish consumption. No standardized methods are available for their detection in these foods, and the performance of the applied methods are rarely described in occurrence studies. Through spiking experiments, we characterized different performance criteria (e.g. sensitivity, estimated limit of detection (eLD95METH), parasite DNA recovery rates (DNA-RR)) of real-time qPCR based-methods for the detection of the three protozoa in mussel's tissues and hemolymph. Digestion of mussels tissues by trypsin instead of pepsin and the use of large buffer volumes was the most efficient for processing 50g-sample. Trypsin digestion followed by lipids removal and DNA extraction by thermal shocks and a BOOM-based technique performed poorly (e.g. eLD95METH from 30 to >3000 parasites/g). But trypsin digestion and direct DNA extraction by bead-beating and FastPrep homogenizer achieved higher performance (e.g. eLD95METH: 4-400 parasites/g, DNA-RR: 19-80%). Direct DNA recovery from concentrated hemolymph, by thermal shocks and cell lysis products removal was not efficient to sensitively detect the protozoa (e.g. eLD95METH: 10-1000 parasites/ml, DNA-RR ≤ 24%). The bead-beating DNA extraction based method is a rapid and simple approach to sensitively detect the three protozoa in mussels using tissues, that can be standardized to different food matrices. However, quantification in mussels remains an issue.

Temperature-parasite interaction: do trematode infections protect against heat stress?

Blue mussels (Mytilus edulis) are important ecosystem engineers along Atlantic coastlines, where they are regularly subjected to rapid changes in temperature during the transition between tides. Global climate change and more frequent extreme weather events are expected to intensify this thermal stress even further. These increases in temperatures will not only affect intertidal mussels directly but also increase transmission dynamics of their parasites. Together, the effects of rises in temperature and parasitism will likely result in higher pressure on M. edulis and their ability to perform vital ecosystem services. In a set of experiments, we tested the effects of infections with the trematode Himasthla elongata and high temperatures during low tide air-exposure. Overall, we hypothesised that temperature and parasite infection intensity would each have significant negative effects on M. edulis survival, and that both stressors together would have a synergistic detrimental impact. Overall, high temperature levels had a strong negative effect on mussel survival. However, our results revealed a surprisingly more complex picture in infected individuals. While moderate parasite loads and increased temperature showed additive negative effects on mussel survival, high parasite infection intensities appeared to nullify the detrimental effects of temperature stress on mussels. Under climate warming, these benefits of parasites might actually outweigh the costs of infection and prove beneficial. Overall, these results suggest that the interactions between host-parasite systems and their changing environment are much more complex than a simple additive effect of multiple stressors.

Diseases and parasites of wild and cultivated mussels along the Patagonian coast of Argentina, southwest Atlantic Ocean.

A histological survey of the commercially edible mussels Mytilus platensis and M. chilensis from wild and cultivated populations along the coast of Patagonia, Argentina (42°00' to 54°47'S), was carried out to determine their health status. Diagnostic results included 3 types of inflammatory responses (infiltrative, nodular, and encapsulation), disseminated neoplasia disease, 2 abnormal reproductive conditions (gonadal atresia and intersex), prokaryotic inclusions, protozoans, and metazoans. Pathogen prevalence and infection intensity among mussels of different sampling sites and between those of wild and cultivated populations were compared. Inflammatory responses were recorded in all mussels from all sites, while disseminated neoplasia only occurred in the most southern cultivated M. chilensis. Intracellular prokaryotic inclusions were broadly distributed in the mussels from both northern and southern Patagonian coasts. Ciliates showed the highest prevalence among wild mussels from the colder waters of Bahía Brown. Turbellaria were recorded at higher prevalence in cultured mussels (41.7%), and trematode metacercariae occurred exclusively in intertidal wild mussels. None of the parasites found appears to be a problem to the fishery or farming, although disseminated neoplasia should be monitored. In addition, we found that mytilid species coexisting with M. platensis (Aulacomya atra and Perumytilus purpuratus) at one location shared the same pathological conditions and parasites, which differed from those of M. platensis at a distant locality. These results suggest that pathological conditions and parasites were influenced more by ecological habitat factors than by the species of mussels present, based on similar parasite assemblages found among closely related mytilid hosts in the same geographical area.

Effects of the DSP-toxic dinoflagellate Dinophysis acuta on clearance and respiration rate of the blue mussel, Mytilus edulis.

Diarrheic Shellfish Poisoning toxins (DST) are a severe health risk to shellfish consumers and can be a major problem for the shellfish industry. Bivalve molluscs can accumulate DST via ingestion of toxic dinoflagellates like Dinophysis spp., which are the most prominent producers of DST. The effects of DST-containing dinoflagellate Dinophysis acuta on bivalve clearance and respiration rate were investigated in the blue mussel (Mytilus edulis) exposed to different algal densities in a controlled laboratory study. Results showed that M. edulis exposed to D. acuta displayed a reduced clearance rate compared to M. edulis exposed to equivalent bio-volumes of the non-toxic cryptophyte Rhodomonas salina. Furthermore, M. edulis ceased to feed on D. acuta after 1 to 4 h, depending on D. acuta densities. The quickest response was observed at the highest densities of D. acuta. The estimated total amount of DST accumulated in the M. edulis exceeded the regulatory limit for human consumption and furthermore, intoxication of the M. edulis seemed to occur faster at high cell toxicity rather than at high cell density. However, respiration rates were, similar, irrespective of whether M. edulis were fed single diets of R. salina, D. acuta or a mixed diet of both algal species. In conclusion, the DST-containing D. acuta had a severe negative effect on the clearance of M. edulis, which can affect the conditions of the M. edulis negatively. Hence, DST may cause low quality M. edulis, due to reduced feeding when exposed to DST-containing D. acuta.

Reduced infectivity in Himasthla elongata (Trematoda, Himasthlidae) cercariae with deviant photoreaction.

Digenean larvae of hermaphroditic generation - cercariae - are known to be polymorphic at genetic and behavioural levels. Cercariae arise as a result of parthenogenetic reproduction of intramolluscan stages, and represent a clone if a snail was infected with a single miracidium. Here we investigated cercarial clones of Himasthla elongata - namely, the infectivity of cercariae with normal (negative) and deviant (positive) photoreaction. In our study, most H. elongata clones showed intraclonal variance in their response to light. The proportion of photopositive cercariae ranged between 0.2% and 60% in different H. elongata clones. Photopositive larvae demonstrated significantly reduced rates of encystment in Mytilus edulis haemolymph in vitro and in young mussels. We discuss the possible mechanisms behind intraclonal variations, such as non-specific genomic rearrangements.

High mortality of mussels in northern Brittany - Evaluation of the involvement of pathogens, pathological conditions and pollutants.

In 2014, a high and unusual mass mortality of mussels occurred in several important production areas along the French coasts of the Atlantic and English Channel. In the first quarter of 2016, mass mortalities hit farms on the west coast of the country once again. These heterogeneous mortality events elicited a multi-parametric study conducted during the 2017 mussel season in three sites in northern Brittany (Brest, Lannion and St. Brieuc). The objective was to assess the health status of these mussels, follow mortality and attempt to identify potential causes of the abnormal high mortality of farmed mussels in northern Brittany. Brest was the most affected site with 70% cumulative mortality, then Lannion with 40% and finally St. Brieuc with a normal value of 15%. We highlighted a temporal 'mortality window' that opened throughout the spring season, and concerned the sites affected by mortality of harmful parasites (including pathogenic bacteria), neoplasia, metal contamination, and tissue alterations. Likely, the combination of all these factors leads to a weakening of mussels that can cause death.

Influence of Himasthla elongata (Trematoda: Echinostomatidae) metacercariae on heart rate in blue mussels (Mytilus edulis).

Whether metacercariae of the digenean Himasthla elongata (Himasthlidae) are harmful to their secondary intermediate hosts, the blue mussel, Mytilus edulis, is a disputable question. To shed light on this issue, we performed field monitoring of the heart rate (HR) in mussels infected with H. elongata over a period of 1.5 years. We observed a significant rise in HR in infected mussels, but only 1 year after infection and at temperatures of 15-17 °C. HR variance also grew sharply in infected mussels within the same temperature range. We also detected a decreased ability of infected mussels to compensate for the rise in the environment temperature. Finally, growth rate in the infected mussels was slower compared to the control group. We provide evidence for measurable pathogenic effects caused by H. elongata metacercariae in their secondary intermediate mussel hosts.

Detection and genetic characterization of Toxoplasma gondii in market-sold mussels (Mytilus edulis) in certain provinces of China.

Mussels, randomly collected from fish markets in China, were analyzed by a semi-nested PCR to detect B1 gene of Toxoplasma gondii. Out of the 2215 samples, fifty-five (2.48%) were detected T. gondii-positive. The prevalence in Shandong province, Liaoning province and Zhejiang province were 2.51%, 2.26% and 2.69%, respectively. T. gondii oocysts were more frequently detected in digestive glands (1.04%) and haemolymph (1.49%) when compared with gills (0.23%). Of the 55 positive DNA, only two samples showed complete genotype at 11 locus and were authenticated as ToxoDB Genotype #9. To our knowledge, the present study is the first to confirm the presence of T. gondii in market-sold mussels in China. The findings point to the risk of humans acquiring T. gondii infection by consuming mussels bought in the aquatic product market.

Toxoplasma gondii and Other Zoonotic Protozoans in Mediterranean Mussel ( Mytilus galloprovincialis) and Blue Mussel ( Mytilus edulis): A Food Safety Concern?

Mediterranean mussels ( Mytilus galloprovincialis) and blue mussels ( Mytilus edulis) are among the most consumed fishery products, but they are frequent vehicles of foodborne infection worldwide. In this study, we investigated the occurrence and seasonality of zoonotic protozoans in mussels farmed or sold at retail outlets in Italy. We collected and tested 1,440 M. galloprovincialis and 180 M. edulis. Pooled samples were molecularly tested for Giardia duodenalis, Cryptosporidium spp., and Toxoplasma gondii and then sequenced. Sixty-two (45.9%; 95% confidence interval, 37.5 to 54.3%) mussel pools tested positive for one or more of the investigated pathogens. Both Mytilus species and samples from all the investigated areas harbored pathogens. Mussels were statistically more contaminated by Cryptosporidium spp., followed by T. gondii and G. duodenalis assemblage A, and M. galloprovincialis was more contaminated than M. edulis ( P < 0.01). Contamination was more likely in mussels at retail outlets ( P < 0.05) than in those from farms and in mussels collected in spring ( P < 0.01) than in other seasons. This is the first report of T. gondii found in M. galloprovincialis in Italy and in M. edulis in Europe. The detection of zoonotic protozoans in a widely consumed food source indicates the need for a more detailed microbiological risk analysis, especially considering that bivalve mollusks are often consumed raw worldwide.

Effect of tidal level on abundance of symbionts in the White Sea blue mussel.

In the White Sea, the blue mussel Mytilus edulis occupies a wide range of biotopes and is associated with numerous symbiotic organisms. At some sites, mussel cover spreads continuously from the intertidal to the subtidal zone. We checked whether the patterns of infection by different associated organisms differed among the upper subtidal, zero-depth and lower intertidal zones at 3 sites in the Kandalaksha Gulf and the Onega Bay of the White Sea. Organisms belonging to 13 taxa were found in mantle cavities and tissues of blue mussels. Parasitic green algae, a sporocyst and metacercariae of 5 species of digenean trematodes occupied mussel tissues; commensal ciliates, rhabdocoelans and some free-living invertebrates were found in mantle cavities. Quantitative composition of symbiotic communities of mussels was not the same at different tidal levels: Urastoma cyprinae (commensal rhabdocoelans) were more abundant in the subtidal and zero-depth zones, while encysted metacercariae of Renicola roscovita and Himasthla sp. were more abundant at the zero-depth and intertidal zones. We suggested several hypotheses to explain this heterogeneity.

Lipid profiles in Himasthla elongata and their intermediate hosts, Littorina littorea and Mytilus edulis.

In this research, the lipid profiles in rediae and free-living larvae (cercariae) of the trematodes Himasthla elongata were studied. It was shown that the lipid profiles of cercariae and rediae reflect the selective accumulation of membrane phospholipids and polyunsaturated fatty acids. In addition, infected tissues' lipid compositions in the intermediate hosts Littorina littorea (whole organism) and Mytilus edulis (foot) were studied. Modifications in the molluscs' lipid compositions were mainly caused by the parasites' metabolic requirements, as the parasites are unable to engage in de novo lipid biosynthesis, and thus, they utilise the host tissues' metabolites. The assumption that changes in the lipid unsaturation of infected intermediate hosts may affect their temperature resistance was discussed.

Marteilia refringens and Marteilia pararefringens sp. nov. are distinct parasites of bivalves and have different European distributions.

Marteilia refringens causes marteiliosis in oysters, mussels and other bivalve molluscs. This parasite previously comprised two species, M. refringens and Marteilia maurini, which were synonymized in 2007 and subsequently referred to as M. refringens 'O-type' and 'M-type'. O-type has caused mass mortalities of the flat oyster Ostrea edulis. We used high throughput sequencing and histology to intensively screen flat oysters and mussels (Mytilus edulis) from the UK, Sweden and Norway for infection by both types and to generate multi-gene datasets to clarify their genetic distinctiveness. Mussels from the UK, Norway and Sweden were more frequently polymerase chain reaction (PCR)-positive for M-type (75/849) than oysters (11/542). We did not detect O-type in any northern European samples, and no histology-confirmed Marteilia-infected oysters were found in the UK, Norway and Sweden, even where co-habiting mussels were infected by the M-type. The two genetic lineages within 'M. refringens' are robustly distinguishable at species level. We therefore formally define them as separate species: M. refringens (previously O-type) and Marteilia pararefringens sp. nov. (M-type). We designed and tested new Marteilia-specific PCR primers amplifying from the 3' end of the 18S rRNA gene through to the 5.8S gene, which specifically amplified the target region from both tissue and environmental samples.

The blue mussel Mytilus edulis is vulnerable to the toxic dinoflagellate Karlodinium armiger-Adult filtration is inhibited and several life stages killed.

Blooms of the toxic dinoflagellates Karlodinium armiger and K. veneficum are frequently observed in Alfacs Bay, Spain, causing mass mortality to wild and farmed mussels. An isolate of K. armiger from Alfacs Bay was grown in the laboratory and exposed to adults, embryos and trochophore larvae of the blue mussel, Mytilus edulis. Adult mussels rejected to filter K. armiger at cell concentrations >1.5·103 cells ml-1. Exposure of adult mussels (23-33 mm shell length) to a range of K. armiger cell concentrations led to mussel mortality with LC50 values of 9.4·103 and 6.1·103 cells ml-1 after 24 and 48 h exposure to ~3.6·104 K. armiger cells ml-1, respectively. Karlodinium armiger also affected mussel embryos and trochophore larvae and feeding by K. armiger on both embryos and larvae was observed under the microscope. Embryos exposed to low K. armiger cell concentrations suffered no measurable mortality. However, at higher K. armiger cell concentrations the mortality of the embryos increased significantly with cell concentration and reached 97% at 1.8·103 K. armiger cells ml-1 after 29 h of exposure. Natural K. armiger blooms may not only have serious direct effects on benthic communities, but may also affect the recruitment of mussels in affected areas.

Cryptic invasion of a parasitic copepod: Compromised identification when morphologically similar invaders co-occur in invaded ecosystems.

Despite their frequent occurrence and strong impacts on native biota, biological invasions can long remain undetected. One reason for this is that an invasive species can be morphologically similar to either native species or introduced species previously established in the same region, and thus be subject to mistaken identification. One recent case involves congeneric invasive parasites, copepods that now infect bivalve hosts along European Atlantic coasts, after having been introduced independently first from the Mediterranean Sea (Mytilicola intestinalis Steuer, 1902) and later from Japan (Mytilicola orientalis Mori, 1935). At least one report on M. intestinalis may have actually concerned M. orientalis, and M. orientalis thus qualifies as a "cryptic invader". Because these two parasitic copepods are morphologically similar, knowledge about their distribution, impact and interactions depends crucially on reliable species identification. In this study, we evaluated the reliability of morphological identification of these two species in parts of their invasive range in Europe (Dutch Delta and Wadden Sea) in comparison with molecular methods of well-established accuracy based on COI gene sequences and ITS1 restriction fragment length polymorphism. Based on seven easily measured or scored macro-morphological variables that were recorded for 182 individual copepods isolated from blue mussels (Mytilus edulis Linnaeus, 1758), principal component analysis showed two relatively distinct but overlapping morphological species groups for females, but no clear separation in males. Discriminant function analysis showed that the females can be discriminated reasonably well based on some of the morphological characteristics (identification error rate of 7%) while males cannot (error rate of 25%). The direction of the dorsolateral thoracic protuberances was identified as the most important trait for species discrimination, but among the morphological features checked, none could flawlessly discriminate between both species. We recommend the use of molecular techniques in future studies of invasive Mytilicola to reliably discriminate between the species. The morphological similarity of these two invaders suggests a more general problem of cryptic invasions and compromised identification of parasites in invaded ecosystems. This problem should be borne in mind whenever invasive parasites are investigated.

Characterisation of the intracellular protozoan MPX in Scottish mussels, Mytilus edulis Linnaeus, 1758.

Ciliates have been reported as pathogens of many species of economically important bivalves. Mussel protozoan X (MPX), is an uncharacterised intracellular ciliate of mussels and has been widely reported in Mytilus spp. around the world. In order to characterise this ciliate, Mytilus edulis samples were collected from a site on the West coast of Scotland, and four different fixatives for histological examination were tested. Fresh preparations of mussel digestive glands were also examined by laser scanning confocal microscopy. Intracellular ciliates were prepared by laser capture microdissection and partial sequences of small subunit ribosomal RNA gene and of large subunit ribosomal RNA gene were generated, using Phyllopharyngea primers. Methacarn solution proved to be the best fixative for both histological and molecular characterisation. The morphological and molecular investigations confirmed that this ciliate belongs to the class Phyllopharyngea, order Rhynchodida. However, this organism does not belong to any known family, genus or species, therefore, a new description is necessary, following further morphological analyses. Most mussel samples containing MPX displayed mild to moderate infections, with no signs of necrosis or haemocytic response, although a single sample displayed a severe infection (∼103 ciliates per section). The localisation of this ciliate in tissues other than the digestive gland, the presence of necrosis in infected tissue of the most severely infected mussel and the binary fission of this ciliate have been observed here for the first time. We also report the first observation of the live ciliate isolated from tissue. Although MPX remains of unknown significance to the mussel industry, tools and protocols described here will be useful in further characterising these and other ciliates (subclass Rhynchodia) known as pathogens for bivalves.

Dual transcriptomics reveals co-evolutionary mechanisms of intestinal parasite infections in blue mussels Mytilus edulis.

On theoretical grounds, antagonistic co-evolution between hosts and their parasites should be a widespread phenomenon but only received little empirical support so far. Consequently, the underlying molecular mechanisms and evolutionary steps remain elusive, especially in nonmodel systems. Here, we utilized the natural history of invasive parasites to document the molecular underpinnings of co-evolutionary trajectories. We applied a dual-species transcriptomics approach to experimental cross-infections of blue mussel Mytilus edulis hosts and their invasive parasitic copepods Mytilicola intestinalis from two invasion fronts in the Wadden Sea. We identified differentially regulated genes from an experimental infection contrast for hosts (infected vs. control) and a sympatry contrast (sympatric vs. allopatric combinations) for both hosts and parasites. The damage incurred by Mytilicola infection and the following immune response of the host were mainly reflected in cell division processes, wound healing, apoptosis and the production of reactive oxygen species (ROS). Furthermore, the functional coupling of host and parasite sympatry contrasts revealed the concerted regulation of chitin digestion by a Chitotriosidase 1 homolog in hosts with several cuticle proteins in the parasite. Together with the coupled regulation of ROS producers and antagonists, these genes represent candidates that mediate the different evolutionary trajectories within the parasite's invasion. The host-parasite combination-specific coupling of these effector mechanisms suggests that underlying recognition mechanisms create specificity and local adaptation. In this way, our study demonstrates the use of invasive species' natural history to elucidate molecular mechanisms of host-parasite co-evolution in the wild.

Evaluation of Rapid, Early Warning Approaches to Track Shellfish Toxins Associated with Dinophysis and Alexandrium Blooms.

Marine biotoxin-contaminated seafood has caused thousands of poisonings worldwide this century. Given these threats, there is an increasing need for improved technologies that can be easily integrated into coastal monitoring programs. This study evaluates approaches for monitoring toxins associated with recurrent toxin-producing Alexandrium and Dinophysis blooms on Long Island, NY, USA, which cause paralytic and diarrhetic shellfish poisoning (PSP and DSP), respectively. Within contrasting locations, the dynamics of pelagic Alexandrium and Dinophysis cell densities, toxins in plankton, and toxins in deployed blue mussels (Mytilus edulis) were compared with passive solid-phase adsorption toxin tracking (SPATT) samplers filled with two types of resin, HP20 and XAD-2. Multiple species of wild shellfish were also collected during Dinophysis blooms and used to compare toxin content using two different extraction techniques (single dispersive and double exhaustive) and two different toxin analysis assays (liquid chromatography/mass spectrometry and the protein phosphatase inhibition assay (PP2A)) for the measurement of DSP toxins. DSP toxins measured in the HP20 resin were significantly correlated (R² = 0.7-0.9, p < 0.001) with total DSP toxins in shellfish, but were detected more than three weeks prior to detection in deployed mussels. Both resins adsorbed measurable levels of PSP toxins, but neither quantitatively tracked Alexandrium cell densities, toxicity in plankton or toxins in shellfish. DSP extraction and toxin analysis methods did not differ significantly (p > 0.05), were highly correlated (R² = 0.98-0.99; p < 0.001) and provided complete recovery of DSP toxins from standard reference materials. Blue mussels (Mytilus edulis) and ribbed mussels (Geukensia demissa) were found to accumulate DSP toxins above federal and international standards (160 ng g-1) during Dinophysis blooms while Eastern oysters (Crassostrea virginica) and soft shell clams (Mya arenaria) did not. This study demonstrated that SPATT samplers using HP20 resin coupled with PP2A technology could be used to provide early warning of DSP, but not PSP, events for shellfish management.

Trophic relationship between the invasive parasitic copepod Mytilicola orientalis and its native blue mussel (Mytilus edulis) host.

Invasive parasites can spill over to new hosts in invaded ecosystems with often unpredictable trophic relationships in the newly arising parasite-host interactions. In European seas, the intestinal copepod Mytilicola orientalis was co-introduced with Pacific oysters (Magallana gigas) and spilled over to native blue mussels (Mytilus edulis), with negative impacts on the condition of infected mussels. However, whether the parasite feeds on host tissue and/or stomach contents is yet unknown. To answer this question, we performed a stable isotope analysis in which we included mussel host tissue and the primary food sources of the mussels, microphytobenthos (MPB) and particulate organic matter (POM). The copepods were slightly enriched in δ15N (mean Δ15N ± s.d.; 1·22 ± 0·58‰) and δ13C (Δ13C 0·25 ± 0·32‰) with respect to their host. Stable isotope mixing models using a range of trophic fractionation factors indicated that host tissue was the main food resource with consistent additional contributions of MPB and POM. These results suggest that the trophic relationship of the invasive copepod with its mussel host is parasitic as well as commensalistic. Stable isotope studies such as this one may be a useful tool to unravel trophic relationships in new parasite-host associations in the course of invasions.