Dynamics of the parasite Marteilia refringens (Paramyxea) in Mytilus galloprovincialis and zooplankton populations in Alfacs Bay (Catalonia, Spain)

Long-term survey discloses a shift in the dynamics pattern of an emerging disease of cockles Cerastoderma edule, marteiliosis, and raises hypotheses to explain it

Drivers of marine disease outbreaks are poorly understood in spite of their growing impact. We present here results from a unique case study examining how cockles Cerastoderma edule have responded to the introduction of the novel protistan Marteilia cochillia, which led in 2012 to cockle fishery collapse in Galician rias. Based on intensive survey for eight years (2011-2019) of two affected shellfish beds, inner and outer in the Ría de Arousa, involving monthly evaluation of cockle health status and estimation of mortality, detailed information is provided of the declining impact of marteiliosis over a wild cockle population with evidence suggesting its increasing resistance. Disease dynamics involved an annual "breaking wave" of prevalence and subsequent cockle mass mortality, causing the near extinction of every recruited cohort. A shift in this pattern, from a severe epidemic towards an endemic profile, was observed in the inner shellfish bed since the cohort that was recruited in 2016, suggesting the hypothesis of increasing marteiliosis resistance through natural selection. Risk factors that may contribute to trigger marteiliosis outbreaks were analysed. Host age and sex did not influence susceptibility to marteiliosis. No clear relationships between environmental conditions (temperature, salinity and upwelling index) or cockle density and disease dynamics were found. Spatial differences in disease dynamics could be due to differences in the abundance of infective stages hypothetically linked to spatial differences in the population dynamics of a putative planktonic intermediate host. All these findings have potential implications for the management of diseased populations.

Species which may act as vectors or reservoirs of diseases covered by the Animal Health Law: Listed pathogens of molluscs

Vector or reservoir species of five mollusc diseases listed in the Animal Health Law were identified, based on evidence generated through an extensive literature review, to support a possible updating of Regulation (EU) 2018/1882. Mollusc species on or in which Mikrocytos mackini, Perkinsus marinus, Bonamia exitiosa, Bonamia ostreae and Marteilia refringens were detected, in the field or during experiments, were classified as reservoir species with different levels of certainty depending on the diagnostic tests used. Where experimental evidence indicated transmission of the pathogen from a studied species to another known susceptible species, this studied species was classified as a vector species. Although the quantification of the risk of spread of the pathogens by the vectors or reservoir species was not part of the terms of reference, such risks do exist for the vector species, since transmission from infected vector species to susceptible species was proven. Where evidence for transmission from infected molluscs was not found, these were defined as reservoir. Nonetheless, the risk of the spread of the pathogens from infected reservoir species cannot be excluded. Evidence identifying conditions that may prevent transmission by vectors or reservoir mollusc species during transport was collected from scientific literature. It was concluded that it is very likely to almost certain (90-100%) that M. mackini, P. marinus, B. exitiosa B. ostreae and M. refringens will remain infective at any possible transport condition. Therefore, vector or reservoir species that may have been exposed to these pathogens in an affected area in the wild or at aquaculture establishments or through contaminated water supply can possibly transmit these pathogens. For transmission of M. refringens, the presence of an intermediate host, a copepod, is necessary.

Environmental distribution and seasonal dynamics of Marteilia refringens and Bonamia ostreae, two protozoan parasites of the European flat oyster, Ostrea edulis

Introduction

Marteilia refringens and Bonamia ostreae are protozoan parasites responsible for mortalities of farmed and wild flat oysters Ostrea edulis in Europe since 1968 and 1979, respectively. Despite almost 40 years of research, the life-cycle of these parasites is still poorly known, especially regarding their environmental distribution.

Methods

We carried out an integrated field study to investigate the dynamics of M. refringens and B. ostreae in Rade of Brest, where both parasites are known to be present. We used real-time PCR to monitor seasonally over four years the presence of both parasites in flat oysters. In addition, we used previously developed eDNA based-approaches to detect parasites in planktonic and benthic compartments for the last two years of the survey.

Results

M. refringens was detected in flat oysters over the whole sampling period, sometimes with a prevalence exceeding 90%. It was also detected in all the sampled environmental compartments, suggesting their involvement in parasite transmission and overwintering. In contrast, B. ostreae prevalence in flat oysters was low and the parasite was almost never detected in planktonic and benthic compartments. Finally, the analysis of environmental data allowed describing the seasonal dynamics of both parasites in Rade of Brest: M. refringens was more detected in summer and fall than in winter and spring, contrary to B. ostreae which showed higher prevalence in winter and spring.

Discussion

The present study emphasizes the difference between M. refringens and B. ostreae ecology, the former presenting a wider environmental distribution than the latter, which seems closely associated to flat oysters. Our findings highlight the key role of planktonic and benthic compartments in M. refringens transmission and storage or potential overwintering, respectively. More generally, we provide here a method that could be useful not only to further investigate non cultivable pathogens life-cycle, but also to support the design of more integrated surveillance programs.

Discovery of the parasite Marteilia cocosarum sp. nov. In common cockle (Cerastoderma edule) fisheries in Wales, UK and its comparison with Marteilia cochillia

Diseases of bivalve molluscs caused by paramyxid parasites of the genus Marteilia have been linked to mass mortalities and the collapse of commercially important shellfish populations. Until recently, no Marteilia spp. have been detected in common cockle (Cerastoderma edule) populations in the British Isles. Molecular screening of cockles from ten sites on the Welsh coast indicates that a Marteilia parasite is widespread in Welsh C. edule populations, including major fisheries. Phylogenetic analysis of ribosomal DNA (rDNA) gene sequences from this parasite indicates that it is a closely related but different species to Marteilia cochillia, a parasite linked to mass mortality of C. edule fisheries in Spain, and that both are related to Marteilia octospora, for which we provide new rDNA sequence data. Preliminary light and transmission electron microscope (TEM) observations support this conclusion, indicating that the parasite from Wales is located primarily within areas of inflammation in the gills and the connective tissue of the digestive gland, whereas M. cochillia is found mainly within the epithelium of the digestive gland. The impact of infection by the new species, here described as Marteilia cocosarum n. sp., upon Welsh fisheries is currently unknown.

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.

Marteilia cochillia is released into seawater via cockle Cerastoderma edule faeces

Outbreaks of Marteilia cochillia have caused massive mortalities of common cockle, Cerastoderma edule, in some natural beds in Galicia (NW Spain) since 2012. The life cycle of Marteilia spp. is still unresolved and the most accepted hypothesis suggests that an additional host is involved. Researchers have assumed that sporangia are shed into the environment in the faeces, but details about this process have not been reported previously. Here, we report the massive liberation of Marteilia cochillia sporangia through the exhalant siphon into the environment, packaged as faeces. Using light microscopy observations on fresh samples, imprints and histology, we also describe a thick (ca. 5 µm) transparent envelope covering the sporangia that has not been reported previously. The massive release of encapsulated sporangia reported here ensures that millions of infective stages of M. cochillia cycle through the environment and become available for infection. The elucidation of the role played by the sporangia envelope would be of utmost importance for the understanding M. cochillia life cycle.

Field studies on the effects of Marteilia sp. on growth of mussel Mytilus galloprovincialis in Thermaikos Gulf

Global warming may accelerate growth and distribution of pathogens influencing aquatic organisms' diseases and human health. Despite the extensive research, the biology, cellular development and life cycle and of Marteilia sp. parasites as well as the influence of parasitic infection on the hosts are not fully understood. The aim of this study was to investigate the effect of Marteilia sp. prevalence and infection intensity on mussels' growth rate and morphometric characteristics under natural conditions in Thermaikos Gulf, a major bivalve production area in Greece, during a five-month growth period. The length, width, height and weight of the infected mussels were significantly lower compared to non-infected and the decrease was proportional to the intensity of mussel infection by the parasite. Moreover, the estimation of allometric relations between length, height, width and weight revealed significantly lower growth of mussel wet weight in relation to shell length for infected mussels compared to healthy ones. The negative effect of marteiliosis on the shell length growth rate of infected mussels was also confirmed by von Bertalanffy equations.

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.

Moving from Histopathology to Molecular Tools in the Diagnosis of Molluscs Diseases of Concern under EU Legislation

One of the main factors limiting molluscs production is the presence of pathogens and diseases. Disease agent transfer via transfers of live molluscs has been a major cause of disease outbreaks and epizootics. Because of that, the European Union has adopted several decisions and directives, the last in 2006 (2006/88/EC) to control movements of marine organisms over the European countries. Once the disease is established in a determined area its eradication is a complicated task because life cycle of pathogens are not completely known and only a good and early diagnosis of the disease could be the most appropriate way to deal with it. Besides, molluscs do not have an adaptive immune response and vaccination strategies are not possible. Molluscs listed diseases under EU legislation are mainly protozoan parasites, that's why histological techniques are recognized for their diagnosis. However, molecular techniques are being increasingly used primarily as confirmatory techniques of the presence of the pathogens but also in disease monitoring programs. Research perspectives are mainly focussed in the optimization, of the already described techniques to gain in sensitivity and sensibility and in the development of new molecular biology techniques (quantitative real time PCRs), that are faster and easier to apply and that allow a positive diagnosis even in early stages of infection. However, molecular tools detect DNA sequences of the pathogen which does not imply that pathogen is viable in the cell host and the infection is established. Consequently, it needs to be validated against other techniques, such as histology or in situ hybridization, so that its reliability can be determined.

Elucidating the life cycle of Marteilia sydneyi, the aetiological agent of QX disease in the Sydney rock oyster (Saccostrea glomerata)

Marteilia sydneyi (Phylum Paramyxea, Class Marteiliidea, Order Marteiliida) (the causative agent of QX disease) is recognised as the most severe parasite to infect Saccostrea glomerata, the Sydney rock oyster, on the east coast of Australia. Despite its potential impact on industry (>95% mortality), research towards lessening these effects has been hindered by the lack of an experimental laboratory model of infection as a consequence of our incomplete understanding of the life cycle of this parasite. Here, we explored the presence of this parasite in hosts other than a bivalve mollusc from two study sites on the Hawkesbury River, New South Wales, Australia. We employed PCR-based in situ hybridisation and sequence analysis of a portion of the first internal transcribed spacer of rDNA in an attempt to detect M. sydneyi DNA in 21 species of polychaete worm. Marteilia DNA was detected in 6% of 1247 samples examined by PCR; the analysis of all amplicons defined one distinct sequence type for first internal transcribed spacer, representing M. sydneyi. Of the polychaete operational taxonomic units test-positive in PCR, we examined 116 samples via in situ hybridisation DNA probe staining and identified M. sydneyi DNA in the epithelium of the intestine of two specimens of Nephtys australiensis. Two differing morphological forms were identified: a 'primordial' cell that contained a well-defined nucleus but had little differentiation in the cytoplasm, and a 'plasmodial' cell that showed an apparent syncytial structure. This finding represents the first known record of the identification of M. sydneyi being parasitic in an organism other than an oyster, and only the third record of any species of Marteilia identified from non-molluscan hosts. Future work aims at determining if N. australiensis and S. glomerata are the only hosts in the life cycle of this paramyxean, and the development of experimental models to aid the production of QX disease-resistant oysters.

Marteiliosis in mussels: a rare disease?

Among 1280 cultured and wild adult Mediterranean mussels, Mytilus galloprovincialis, collected over a 1-year surveillance period from the Slovene Adriatic Sea, 0.3% were histologically positive for the presence of Marteilia spp. The infection was concentrated in winter. Employing the molecular methods of PCR, cloning, DNA restriction and sequencing, only Marteilia refringens type M was detected in all the infected mussels. Although all life-cycle stages of M. refringens severely infected digestive glands, only sporadic disruption of epithelial cells of digestive tubules and focal destruction of digestive tubules were observed in the infected mussels. This was the first detection of M. refringens in M. galloprovincialis from the Slovene Adriatic Sea with the lowest prevalence reported to date. In addition, our results highlight the need for sequencing to complement the established PCR-RFLP analysis for correct parasite typing.

Contribution to the understanding of the cycle of the protozoan parasite Marteilia refringens

The paramyxean parasite Marteilia refringens infects several bivalve species including European flat oysters Ostrea edulis and Mediterranean mussels Mytilus galloprovincialis. Sequence polymorphism allowed definition of three parasite types ‘M’, ‘O’ and ‘C’ preferably detected in oysters, mussels and cockles respectively. Transmission of the infection from infected bivalves to copepods Paracartia grani could be experimentally achieved but assays from copepods to bivalves failed. In order to contribute to the elucidation of the M. refringens life cycle, the dynamics of the infection was investigated in O. edulis, M. galloprovincialis and zooplankton over one year in Diana lagoon, Corsica (France). Flat oysters appeared non-infected while mussels were infected part of the year, showing highest prevalence in summertime. The parasite was detected by PCR in zooplankton particularly after the peak of prevalence in mussels. Several zooplanktonic groups including copepods, Cladocera, Appendicularia, Chaetognatha and Polychaeta appeared PCR positive. However, only the copepod species Paracartia latisetosa showed positive signal by in situ hybridization. Small parasite cells were observed in gonadal tissues of female copepods demonstrating for the first time that a copepod species other than P. grani can be infected with M. refringens. Molecular characterization of the parasite infecting mussels and zooplankton allowed the distinguishing of three Marteilia types in the lagoon.

Bonamia exitiosa (Haplosporidia) observed infecting the European flat oyster Ostrea edulis cultured on the Spanish Mediterranean coast

Bonamia exitiosa and Bonamia ostreae are parasites that reproduce within the haemocytes of several oyster species. In Europe, the host species is the flat oyster Ostrea edulis. The parasite B. ostreae has been responsible for mortalities since the late 1970s throughout the European Atlantic coast. B. exitiosa was first detected, in 2007, on this continent in flat oysters cultured in Galicia (NW Spain). Since then, the parasite has also been detected in France, Italy and the United Kingdom. The bays of the Ebro Delta in the south of Catalonia represent the main bivalve culture area in the Mediterranean coast of Spain. Previous information from the area includes reports of several flat oyster pathogens, including the notifiable parasite Marteilia refringens. However, the status with regard to Bonamia parasites was uncertain. In the present study, a Bonamia parasite was observed in flat oysters cultured in the Alfacs Bay of the Ebro Delta by histology and real-time PCR. PCR-RFLP and sequencing suggested the presence of B. exitiosa. Finally, phylogenetic analyses of the studied Bonamia isolates corroborated B. exitiosa infection. M. refringens was also observed in the same oyster batch, and co-infection with both parasites was also detected. This is the first detection of B. exitiosa, in Catalonia and the Spanish Mediterranean coast. The impact of the parasite on the Mediterranean flat oyster activity needs to be urgently addressed.