Marteilia spp. parasites in bivalves: A revision of recent studies

Marteilia pararefringens infections are more frequent than revealed by the Norwegian surveillance programme, highlighting the need for its improvement

Norway had historically been considered free of marteiliosis in bivalves since the disease surveillance programme began in 1995. However, in 2016, Marteilia pararefringens, a protistan parasite of mussels Mytilus spp., was described in a heliothermic lagoon-a poll-previously used to produce flat oyster spat. To study whether the parasite was introduced, and possibly spread, via the historical flat oyster networks on the south and west coast, we sampled aquaculture polls that were part of different networks of farmers and wild, natural polls with no aquaculture activity. Additionally, we sampled mussel banks influenced by polls and sheltered bays that could have a similar environment to that of polls. We identified 7 sites with M. pararefringens-infected mussel populations: 5 were polls used in flat oyster production and 2 were in fjord areas with no known connection to any bivalve aquaculture. Prevalence ranged between 2 and 88%. At one site, Trysfjorden, we found M. pararefringens in atypical organs, including the gills, mantle, and intestine. Marteilia-like cells were also observed in the epithelium, lumen, and surrounding connective tissue of metanephridia and in the sinus of the anterior retractor muscle. Our results demonstrate that the parasite is more widespread than previously thought and is neither isolated to polls nor connected directly to aquaculture activity. Lastly, our findings highlight the need for an improved sampling strategy in surveillance programmes to detect marteiliosis in mussels.

Comparative genomics of Ascetosporea gives new insight into the evolutionary basis for animal parasitism in Rhizaria

BACKGROUND

Ascetosporea (Endomyxa, Rhizaria) is a group of unicellular parasites infecting aquatic invertebrates. They are increasingly being recognized as widespread and important in marine environments, causing large annual losses in invertebrate aquaculture. Despite their importance, little molecular data of Ascetosporea exist, with only two genome assemblies published to date. Accordingly, the evolutionary origin of these parasites is unclear, including their phylogenetic position and the genomic adaptations that accompanied the transition from a free-living lifestyle to parasitism. Here, we sequenced and assembled three new ascetosporean genomes, as well as the genome of a closely related amphizoic species, to investigate the phylogeny, origin, and genomic adaptations to parasitism in Ascetosporea.

RESULTS

Using a phylogenomic approach, we confirm the monophyly of Ascetosporea and show that Paramyxida group with Mikrocytida, with Haplosporida being sister to both groups. We report that the genomes of these parasites are relatively small (12-36 Mb) and gene-sparse (~ 2300-5200 genes), while containing surprisingly high amounts of non-coding sequence (~ 70-90% of the genomes). Performing gene-tree aware ancestral reconstruction of gene families, we demonstrate extensive gene losses at the origin of parasitism in Ascetosporea, primarily of metabolic functions, and little gene gain except on terminal branches. Finally, we highlight some functional gene classes that have undergone expansions during evolution of the group.

CONCLUSIONS

We present important new genomic information from a lineage of enigmatic but important parasites of invertebrates and illuminate some of the genomic innovations accompanying the evolutionary transition to parasitism in this lineage. Our results and data provide a genetic basis for the development of control measures against these parasites.

Identification of a new gregarine parasite associated with mass mortality events of freshwater pearl mussels (Margaritifera margaritifera) in Sweden

Freshwater bivalves play key ecological roles in lakes and rivers, largely contributing to healthy ecosystems. The freshwater pearl mussel, Margaritifera margaritifera, is found in Europe and on the East coast of North America. Once common in oxygenated streams, M. margaritifera is rapidly declining and consequently assessed as a threatened species worldwide. Deterioration of water quality has been considered the main factor for the mass mortality events affecting this species. Yet, the role of parasitic infections has not been investigated. Here, we report the discovery of three novel protist lineages found in Swedish populations of M. margaritifera belonging to one of the terrestrial groups of gregarines (Eugregarinorida, Apicomplexa). These lineages are closely related-but clearly separated-from the tadpole parasite Nematopsis temporariae. In one lineage, which is specifically associated with mortality events of M. margaritifera, we found cysts containing single vermiform zoites in the gills and other organs of diseased individuals using microscopy and in situ hybridization. This represents the first report of a parasitic infection in M. margaritifera that may be linked to the decline of this mussel species. We propose a tentative life cycle with the distribution of different developmental stages and potential exit from the host into the environment.

A common garden experiment supports a genetic component underlying the increased resilience of common cockle (Cerastoderma edule) to the parasite Marteilia cochillia

The common cockle is a valuable bivalve species inhabiting the Atlantic European coasts. The parasite Marteilia cochillia has devastated cockle beds in the southern Galician (NW Spain) rias since 2012. Previous data suggested that cockles from Ría de Arousa acquired some resilience to this parasite through natural selection after consecutive annual marteiliosis outbreaks and candidate markers associated with marteiliosis resilience were identified using population genomics and transcriptomics approaches. Here, a common garden experiment was performed using a naïve stock (from Ría de Muros-Noia) and an affected stock (from Ría de Arousa) to test this hypothesis. Breeders from both stocks were used to produce seed cohorts at hatchery, which were pre-grown in a raft (outdoor nursery stage) and deployed in two shellfish beds affected by marteiliosis in Ría de Arousa (growing-out stage). In both beds, the naïve stock showed high marteiliosis prevalence and was fully depleted in a short period, while the affected stock barely showed evidence of marteiliosis. A set of 45 SNPs putatively associated with marteiliosis resilience were fitted for MassARRAY genotyping to check their role in the differential resilience detected between both stocks. Though no significant differentiation was found between the naïve and the affected stocks with neutral markers, 28 SNPs showed significant divergence between them, suggesting that these SNPs were involved in directional selection during eight generations (to the most) of marteiliosis pressure (long-term selection). Furthermore, signals of selection were also detected in the naïve stock along the marteiliosis outbreak in the growing-out stage (short-term selection) and six SNPs, all shared with the long-term evaluation, showed consistent signals of differentiation according to the infection severity. Some of these SNPs were located within immune genes pertaining to families such as proteasome, ubiquitin, tumor necrosis factor, and glutathione S-transferase. These resilience-associated markers will be useful to recover cockle production in Galicia.

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.

The impact of climate change and pollution on trematode-bivalve dynamics

Coastal ecosystems and their marine populations are increasingly threatened by global environmental changes. Bivalves have emerged as crucial bioindicators within these ecosystems, offering valuable insights into biodiversity and overall ecosystem health. In particular, bivalves serve as hosts to trematode parasites, making them a focal point of study. Trematodes, with their life cycles intricately linked to external factors, provide excellent indicators of environmental changes and exhibit a unique ability to accumulate pollutants beyond ambient levels. Thus, they act as living sentinels, reflecting the ecological condition of their habitats. This paper presents a comprehensive review of recent research on the use of bivalve species as hosts for trematodes, examining the interactions between these organisms. The study also investigates the combined impact of trematode infections and other pollutants on bivalve molluscs. Trematode infections have multifaceted consequences for bivalve species, influencing various aspects of their physiology and behavior, including population-wide mortality. Furthermore, the coexistence of trematode infections and other sources of pollution compromises host resistance, disrupts parasite transmission, and reduces the abundance of intermediate hosts for complex-living parasites. The accumulation process of these parasites is influenced not only by external factors but also by host physiology. Consequently, the implications of climate change and environmental factors, such as temperature, salinity, and ocean acidification, are critical considerations. In summary, the intricate relationship between bivalves, trematode parasites, and their surrounding environment provides valuable insights into the health and sustainability of coastal ecosystems. A comprehensive understanding of these interactions, along with the influence of climate change and environmental parameters, is essential for effective management and conservation strategies aimed at preserving these delicate ecosystems and the diverse array of species that rely on them.

Understanding Crassostrea virginica tolerance of Perkinsus marinus through global gene expression analysis

Disease tolerance, a host's ability to limit damage from a given parasite burden, is quantified by the relationship between pathogen load and host survival or reproduction. Dermo disease, caused by the protozoan parasite P. marinus, negatively impacts survival in both wild and cultured eastern oyster (C. virginica) populations. Resistance to P. marinus has been the focus of previous studies, but tolerance also has important consequences for disease management in cultured and wild populations. In this study we measured dermo tolerance and evaluated global expression patterns of two sensitive and two tolerant eastern oyster families experimentally challenged with distinct doses of P. marinus (0, 10⁶, 10⁷, and 10⁸ parasite spores per gram wet weight, n = 3-5 individuals per family per dose). Weighted Gene Correlation Network Analysis (WGCNA) identified several modules correlated with increasing parasite dose/infection intensity, as well as phenotype. Modules positively correlated with dose included transcripts and enriched GO terms related to hemocyte activation and cell cycle activity. Additionally, these modules included G-protein coupled receptor, toll-like receptor, and tumor necrosis factor pathways, which are important for immune effector molecule and apoptosis activation. Increased metabolic activity was also positively correlated with treatment. The module negatively correlated with infection intensity was enriched with GO terms associated with normal cellular activity and growth, indicating a trade-off with increased immune response. The module positively correlated with the tolerant phenotype was enriched for transcripts associated with "programmed cell death" and contained a large number of tripartite motif-containing proteins. Differential expression analysis was also performed on the 10⁸ dosed group using the most sensitive family as the comparison reference. Results were consistent with the network analysis, but signals for "programmed cell death" and serine protease inhibitors were stronger in one tolerant family than the other, suggesting that there are multiple avenues for disease tolerance. These results provide new insight for defining dermo response traits and have important implications for applying selective breeding for disease management.

Molecular and microscopic identification of Eomarteilia granula infection in Manila clam Ruditapes philippinarum off the south coast of Korea

A report on the new species Eomarteilia (=Marteilia) granula infecting Manila clam Ruditapes philippinarum from Japan in 2014 suggests the possibility of E. granula infecting other Manila clam populations in the Northwest Pacific region, including Korea. In this study, we report the first infections by E. granula in Manila clams off the south coast of Korea. Histology revealed Marteilia-like plasmodia in the digestive tubule epithelia. Tissue imprints demonstrated that each parasite sporangium enclosed 4 spores and transmission electron microscopy (TEM) revealed ultrastructure of primary cells enclosing secondary cells, which contained spores. Mature spores consisted of 3 sporoplasms: outermost, intermediate, and innermost. The outermost sporoplasm showed a peripheral electron-dense monolayer characteristic of E. granula. The 18S rDNA amplified from the Marteilia-like parasite yielded 1784-bp PCR amplicon sequences which were 99.8% similar to that of E. granula previously reported (as M. granula) from Japan. In the molecular phylogenetic analysis, the novel Marteilia-like organism formed a well-supported clade with E. granula. Accordingly, we concluded that the novel Marteilia-like parasite that we found infecting some Korean Manila clams is Eomarteilia granula. Field surveys revealed that the infection was limited to clams of the south coast of Korea, with the prevalence ranging from 3.3 to 5.0%.

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.

How Safe to Eat Are Raw Bivalves? Host Pathogenic and Public Health Concern Microbes within Mussels, Oysters, and Clams in Greek Markets

Raw-bivalves consumption is a wide trend in Mediterranean countries. Despite the unambiguous nutritional value of seafood, raw consumption of bivalves may involve risks that could pose a significant threat to consumers' health. Their filter-feeding behavior is responsible for the potential hosting of a wide variety of microorganisms, either pathogenic for the bivalves or public health threats. Under this prism, the current study was conducted in an effort to evaluate the risk of eating raw bivalves originating from the two biggest seafood markets in Thessaloniki, the largest production area of bivalves in Greece. Both microbiological and molecular methodologies were applied in order to assess the presence of various harmful microbes, including noroviruses, Bonamia, Marteilia, Esherichia coli, Salmonella, and Vibrio. Results indicated the presence of several Vibrio strains in the analyzed samples, of which the halophilic Vibrio harveyi was verified by 16S rRNA sequencing; other than this, no enteropathogenic Vibrio spp. was detected. Furthermore, although Esherichia coli was detected in several samples, it was mostly below the European Union (EU) legislation thresholds. Interestingly, the non-target Photobacterium damselae was also detected, which is associated with both wound infections in human and aquatic animals. Regarding host pathogenic microorganisms, apart from Vibrio harveyi, the protozoan parasite Marteilia refrigens was identified in oysters, highlighting the continuous infection of this bivalve in Greece. In conclusion, bivalves can be generally characterized as a safe-to-eat raw food, hosting more bivalve pathogenic microbes than those of public health concern.

Effects of marine harmful algal blooms on bivalve cellular immunity and infectious diseases: A review

Bivalves were long thought to be "symptomless carriers" of marine microalgal toxins to human seafood consumers. In the past three decades, science has come to recognize that harmful algae and their toxins can be harmful to grazers, including bivalves. Indeed, studies have shown conclusively that some microalgal toxins function as active grazing deterrents. When responding to marine Harmful Algal Bloom (HAB) events, bivalves can reject toxic cells to minimize toxin and bioactive extracellular compound (BEC) exposure, or ingest and digest cells, incorporating nutritional components and toxins. Several studies have reported modulation of bivalve hemocyte variables in response to HAB exposure. Hemocytes are specialized cells involved in many functions in bivalves, particularly in immunological defense mechanisms. Hemocytes protect tissues by engulfing or encapsulating living pathogens and repair tissue damage caused by injury, poisoning, and infections through inflammatory processes. The effects of HAB exposure observed on bivalve cellular immune variables have raised the question of possible effects on susceptibility to infectious disease. As science has described a previously unrecognized diversity in microalgal bioactive substances, and also found a growing list of infectious diseases in bivalves, episodic reports of interactions between harmful algae and disease in bivalves have been published. Only recently, studies directed to understand the physiological and metabolic bases of these interactions have been undertaken. This review compiles evidence from studies of harmful algal effects upon bivalve shellfish that establishes a framework for recent efforts to understand how harmful algae can alter infectious disease, and particularly the fundamental role of cellular immunity, in modulating these interactions. Experimental studies reviewed here indicate that HABs can modulate bivalve-pathogen interactions in various ways, either by increasing bivalve susceptibility to disease or conversely by lessening infection proliferation or transmission. Alteration of immune defense and global physiological distress caused by HAB exposure have been the most frequent reasons identified for these effects on disease. Only few studies, however, have addressed these effects so far and a general pattern cannot be established. Other mechanisms are likely involved but are under-studied thus far and will need more attention in the future. In particular, the inhibition of bivalve filtration by HABs and direct interaction between HABs and infectious agents in the seawater likely interfere with pathogen transmission. The study of these interactions in the field and at the population level also are needed to establish the ecological and economical significance of the effects of HABs upon bivalve diseases. A more thorough understanding of these interactions will assist in development of more effective management of bivalve shellfisheries and aquaculture in oceans subjected to increasing HAB and disease pressures.

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.

Blue mussels (Mytilus edulis spp.) as sentinel organisms in coastal pollution monitoring: A review

The blue mussel (Mytilus spp.) is widely used as a bioindicator for monitoring of coastal water pollution (mussel watch programs). Herein we provide a review of this study field with emphasis on: the suitability of Mytilus spp. as environmental sentinels; uptake and bioaccumulation patterns of key pollutant classes; the use of Mytilus spp. in mussel watch programs; recent trends in Norwegian mussel monitoring; environmental quality standards and background concentrations of key contaminants; pollutant effect biomarkers; confounding factors; particulate contaminants (microplastics, engineered nanomaterials); climate change; harmonization of monitoring procedures; and the use of deployed mussels (transplant caging) in pollution monitoring. Lastly, the overall state of the art of blue mussel pollution monitoring is discussed and some important issues for future research and development are highlighted.

Infectious diseases of marine molluscs and host responses as revealed by genomic tools

More and more infectious diseases affect marine molluscs. Some diseases have impacted commercial species including MSX and Dermo of the eastern oyster, QPX of hard clams, withering syndrome of abalone and ostreid herpesvirus 1 (OsHV-1) infections of many molluscs. Although the exact transmission mechanisms are not well understood, human activities and associated environmental changes often correlate with increased disease prevalence. For instance, hatcheries and large-scale aquaculture create high host densities, which, along with increasing ocean temperature, might have contributed to OsHV-1 epizootics in scallops and oysters. A key to understanding linkages between the environment and disease is to understand how the environment affects the host immune system. Although we might be tempted to downplay the role of immunity in invertebrates, recent advances in genomics have provided insights into host and parasite genomes and revealed surprisingly sophisticated innate immune systems in molluscs. All major innate immune pathways are found in molluscs with many immune receptors, regulators and effectors expanded. The expanded gene families provide great diversity and complexity in innate immune response, which may be key to mollusc's defence against diverse pathogens in the absence of adaptive immunity. Further advances in host and parasite genomics should improve our understanding of genetic variation in parasite virulence and host disease resistance.

Experimental infection of European flat oyster Ostrea edulis with ostreid herpesvirus 1 microvar (OsHV-1μvar): Mortality, viral load and detection of viral transcripts by in situ hybridization

Ostreid herpesvirus 1 (OsHV-1) infections have been reported in several bivalve species. Mortality of Pacific oyster Crassostrea gigas spat has increased considerably in Europe since 2008 linked to the spread of a variant of OsHV-1 called μvar. In the present study we demonstrated that O. edulis juveniles can be infected by OsHV-1μvar when administered as an intramuscular injection. Mortality in the oysters injected with OsHV-1μvar was first detected 4 days after injection and reached 25% mortality at day 10. Moreover, the high viral load observed and the detection of viral transcripts by in situ hybridization in several tissues of dying oysters suggested that OsHV-1μvar was the cause of mortality in the O. edulis juveniles. This is therefore the first study to provide evidence about the pathogenicity of OsHV-1μvar in a species that does not belong to the Crassostrea genus. Additionally, we present a novel method to detect OsHV-1 transcripts in infected individuals' using in situ hybridization.

The ecology, evolution, impacts and management of host-parasite interactions of marine molluscs

Molluscs are economically and ecologically important components of aquatic ecosystems. In addition to supporting valuable aquaculture and wild-harvest industries, their populations determine the structure of benthic communities, cycling of nutrients, serve as prey resources for higher trophic levels and, in some instances, stabilize shorelines and maintain water quality. This paper reviews existing knowledge of the ecology of host-parasite interactions involving marine molluscs, with a focus on gastropods and bivalves. It considers the ecological and evolutionary impacts of molluscan parasites on their hosts and vice versa, and on the communities and ecosystems in which they are a part, as well as disease management and its ecological impacts. An increasing number of case studies show that disease can have important effects on marine molluscs, their ecological interactions and ecosystem services, at spatial scales from centimeters to thousands of kilometers and timescales ranging from hours to years. In some instances the cascading indirect effects arising from parasitic infection of molluscs extend well beyond the temporal and spatial scales at which molluscs are affected by disease. In addition to the direct effects of molluscan disease, there can be large indirect impacts on marine environments resulting from strategies, such as introduction of non-native species and selective breeding for disease resistance, put in place to manage disease. Much of our understanding of impacts of molluscan diseases on the marine environment has been derived from just a handful of intensively studied marine parasite-host systems, namely gastropod-trematode, cockle-trematode, and oyster-protistan interactions. Understanding molluscan host-parasite dynamics is of growing importance because: (1) expanding aquaculture; (2) current and future climate change; (3) movement of non-native species; and (4) coastal development are modifying molluscan disease dynamics, ultimately leading to complex relationships between diseases and cultivated and natural molluscan populations. Further, in some instances the enhancement or restoration of valued ecosystem services may be contingent on management of molluscan disease. The application of newly emerging molecular tools and remote sensing techniques to the study of molluscan disease will be important in identifying how changes at varying spatial and temporal scales with global change are modifying host-parasite systems.