Intensive oyster aquaculture can reduce disease impacts on sympatric wild oysters

Risks associated with disease spread from fish and shellfish farming have plagued the growth and public perception of aquaculture worldwide. However, by processing nutrients and organic material from the water column, the culture of many suspension-feeding bivalves has been proposed as a novel solution toward mitigating problems facing coastal water quality, including the removal of disease-causing parasites. Here we developed and simulated an epidemiological model describing sympatric oyster Crassostrea virginica populations in aquaculture and the wild impacted by the protozoan parasite Perkinsus marinus. Our model captured the indirect interaction between wild and cultured populations that occurs through sharing water-borne P. marinus transmission stages, and we hypothesized that oyster aquaculture can enhance wild oyster populations through reduced parasitism as long as cultured oysters are harvested prior to spreading disease. We found that the density of oysters in aquaculture, which is commonly thought to lead to the spread of disease through farms and out to nearby populations in the wild, has only indirect effects on P. marinus transmission through its interaction with the rate of aquaculture harvests. Sufficient aquaculture harvest, which varies with the susceptibility of farmed oysters to P. marinus infection and their lifespan once infected, reduces disease by diluting parasites in the environment. Our modeling results offer new insights toward the broader epidemiological implications of oyster aquaculture and effective disease management

Deeper habitats and cooler temperatures moderate a climate-driven seagrass disease

Eelgrass creates critical coastal habitats worldwide and fulfills essential ecosystem functions as a foundation seagrass. Climate warming and disease threaten eelgrass, causing mass mortalities and cascading ecological impacts. Subtidal meadows are deeper than intertidal and may also provide refuge from the temperature-sensitive seagrass wasting disease. From cross-boundary surveys of 5761 eelgrass leaves from Alaska to Washington and assisted with a machine-language algorithm, we measured outbreak conditions. Across summers 2017 and 2018, disease prevalence was 16% lower for subtidal than intertidal leaves; in both tidal zones, disease risk was lower for plants in cooler conditions. Even in subtidal meadows, which are more environmentally stable and sheltered from temperature and other stressors common for intertidal eelgrass, we observed high disease levels, with half of the sites exceeding 50% prevalence. Models predicted reduced disease prevalence and severity under cooler conditions, confirming a strong interaction between disease and temperature. At both tidal zones, prevalence was lower in more dense eelgrass meadows, suggesting disease is suppressed in healthy, higher density meadows. These results underscore the value of subtidal eelgrass and meadows in cooler locations as refugia, indicate that cooling can suppress disease, and have implications for eelgrass conservation and management under future climate change scenarios

Tipping the balance: the impact of eelgrass wasting disease in a changing ocean

Infectious disease has the potential to cause devastating damage to valuable marine organisms and habitats. Eelgrass wasting disease (EGWD), caused by the pathogenic protist Labyrinthula zosterae (LZ), has caused mass die-offs in Zostera marina at regional and global scales. Despite this, little is known about the host-pathogen interaction or disease drivers in the Salish Sea. To determine the regional impact of EGWD, we measured summer prevalence and severity in the San Juan Islands, Padilla Bay, Hood Canal, South Puget Sound, and Willapa Bay. We used cultures and quantitative PCR to verify results, measuring LZ load in lesioned tissue from multiple sites. EGWD was present at all 16 sites surveyed, with prevalence ranging from 80% disease prevalence. Recent data suggest water temperature increases the virulence of LZ, indicating possible climate sensitivity. At our sites, water temperatures influenced both EGWD prevalence and severity, suggesting environmental conditions and climate change could impact the eelgrass-LZ relationship and lead to increased virulence. We ran a three-week controlled experiment to examine the impact of LZ infection on eelgrass shoots over time. We exposed half the eelgrass shoots to LZ infection and sampled shoots at seven time points. All exposed shoots showed signs of infection. EGWD severity and lesion number increased through time, corresponding with a measurable decrease in leaf and root growth and increased phenols. Our results show EGWD is widespread in Washington state eelgrass beds and suggests that EGWD severity is positively correlated with water temperature. Furthermore, EGWD has a detrimental effect on eelgrass health, potentially contributing to decreased density and meadow declines. While levels of EGWD in the field are variable, we identified four sites that are experiencing high prevalence. Further research is needed to understand the conditions leading to EGWD outbreaks

Oysters and Eelgrass: Potential Partners in a High PCO2 Ocean

Ocean acidification (OA) threatens calcifying organisms such as the Pacific oyster, Crassostrea gigas. In contrast, eelgrass, Zostera marina, can benefit from the increase in available carbon for photosynthesis found at a lower seawater pH. Seagrasses can remove dissolved inorganic carbon from OA environments, creating local daytime pH refugia. Pacific oysters may improve the health of eelgrass by filtering out pathogens such as Labyrinthula zosterae, which causes eelgrass wasting disease (EWD). Using a laboratory experiment, we found that co-culture of eelgrass with oysters reduced the severity of EWD. EWD was also reduced in more acidic waters, which negatively affect oyster growth

Characterizing host-pathogen interactions between Zostera marina and Labyrinthula zosterae

Introduction Seagrass meadows serve as an integral component of coastal ecosystems but are declining rapidly due to numerous anthropogenic stressors including climate change. Eelgrass wasting disease, caused by opportunistic Labyrinthula spp., is an increasing concern with rising seawater temperature. To better understand the host-pathogen interaction, we paired whole organism physiological assays with dual transcriptomic analysis of the infected host and parasite. Methods Eelgrass (Zostera marina) shoots were placed in one of two temperature treatments, 11° C or 18° C, acclimated for 10 days, and exposed to a waterborne inoculation containing infectious Labyrinthula zosterae (Lz) or sterile seawater. At two- and five-days post-exposure, pathogen load, visible disease signs, whole leaf phenolic content, and both host- and pathogen- transcriptomes were characterized. Results Two days after exposure, more than 90% of plants had visible lesions and Lz DNA was detectable in 100% percent of sampled plants in the Lz exposed treatment. Concentrations of total phenolic compounds were lower after 5 days of combined exposure to warmer temperatures and Lz, but were unaffected in other treatments. Concentrations of condensed tannins were not affected by Lz or temperature, and did not change over time. Analysis of the eelgrass transcriptome revealed 540 differentially expressed genes in response to Lz exposure, but not temperature. Lz-exposed plants had gene expression patterns consistent with increased defense responses through altered regulation of phytohormone biosynthesis, stress response, and immune function pathways. Analysis of the pathogen transcriptome revealed up-regulation of genes potentially involved in breakdown of host defense, chemotaxis, phagocytosis, and metabolism. Discussion The lack of a significant temperature signal was unexpected but suggests a more pronounced physiological response to Lz infection as compared to temperature. Pre-acclimation of eelgrass plants to the temperature treatments may have contributed to the limited physiological responses to temperature. Collectively, these data characterize a widespread physiological response to pathogen attack and demonstrate the value of paired transcriptomics to understand infections in a host-pathogen system

Examining the Roles of Environment, Host, and Pathogen in the Host-Pathogen Relationship Between the Oyster Herpesvirus and the Pacific Oyster

The Pacific oyster, Crassostrea gigas, an oyster species indigenous to Japan, has been introduced globally becoming the primary species of oyster cultured in many areas of the world. In Tomales Bay, California, seed mortalities have occurred in Pacific oysters nearly annually since 1993, and the oyster herpesvirus (OsHV), a virulent pathogen of larval and juvenile bivalves (particularly known in Pacific oysters) was first detected in 2002. Sentinel field studies (2000-2003) were conducted in Tomales Bay in order to better understand the role of environmental factors (temperature, phytoplankton, and salinity) and oyster health (measured using histology and/or OsHV-specific Polymerase Chain Reaction (PCR)) on Pacific oyster survival. Elevated temperatures were the only environmental factor consistently related to mortalities (2000-2003), and in 2003, elevated temperature means predicted OsHV presence (p < 0.005); OsHV presence predicted mortality (p=0.01). A separate survey conducted in 2003 detected OsHV in multiple species of bivalves grown in Tomales Bay (C. gigas, Ostrea edulis, C. virginica, C. sikamea, Mytilus galloprovincialis, and Venerupis phillipinarum) and C. gigas grown in nearby Drakes Bay using OsHV-specific PCR and/or quantitative PCR (qPCR); qPCR copy numbers were low in each species tested but were significantly greater in C. gigas (p < 0.0001) the only species that appear to be impacted by mortalities. To confirm the infectious etiology of OsHV detected in Tomales Bay, Pacific oyster larvae were exposed to either filtered homogenates from OsHV-infected Pacific oysters in Tomales Bay or uninfected oyster tissue. OsHV was detected and quantified only in oyster larvae exposed to OsHV using qPCR and reverse transcriptase qPCR, and confirmed using transmission electron microscopy. Taken together, data from field and lab-based experiments indicates an infectious disease (OsHV) acts in synergy with temperature to kill Pacific oysters in Tomales Bay, California. Preliminary gene identification of both upregulated host and OsHV genes in larvae exposed to OsHV was conducted using SOLiD sequencing and GeneFishing PCR. Genes identified may provide a foundation to better understand both host response and virus infection, ultimately better defining the hostpathogen relationship between OsHV and Pacific oysters

Interspecific transmission of Seagrass Wasting Disease from Pacific oysters, Crassostrea gigas, to eelgrass, Zostera marina

In order to feed the growing population of the world in more sustainable ways, more governments and other actors are turning to the aquaculture of marine species to provide for the increasing need for sustenance. Oysters, specifically Crassostrea gigas, are an important aquaculture species, and have been introduced into many areas for this purpose, however welack understanding of their direct interactions with cohabiting species. One such species that commonly coccurs in areas with C. gigas is Zostera marina, eelgrass, which is affected by Seagrass Wasting Disease (SWD). The transmission dynamics of SWD from oysters to eelgrass needs more study. A laboratory experiment was conducted where Pacific oysters, C. gigas, were exposed to Labyrinthula zosterae and transferred over to tanks containing naive eelgrass to test if oysters were a vector for Labyrinthula zosterae. The resulting data shows that disease prevalence of directly inoculated treatments and oyster vectoring of disase were similar. Disease severity was also highest in the inoculated treatments, with direct inoculation being highest. Overall, the findings show that transfer of oysters from infected waters to non-infected waters can introduce disease and subsequent infection. These findings should be used to inform general aquaculture practices and the management and land use practices of oyster beds and possibly other farmed shellfish species

Immune response of the Caribbean sea fan, Gorgonia ventalina, exposed to an Aplanochytrium parasite as revealed by transcriptome sequencing

Coral reef communities are undergoing marked declines due to a variety of stressors including disease. The sea fan coral, Gorgonia ventalina, is a tractable study system to investigate mechanisms of immunity to a natural occurring pathogen. Functional studies in Gorgonia ventalina immunity indicate that several key pathways and cellular responses are involved in response to natural microbial invaders, although to date the functional and regulatory pathways remain largely un-described. This study used short-read sequencing (Illumina GAIIx) to identify genes involved in the response of G. ventalina to a naturally occurring Aplanochytrium spp. parasite. De novo assembly of the G. ventalina transcriptome yielded 90,230 contigs of which 40,142 were annotated. RNA-Seq analysis revealed 210 differentially expressed genes in sea fans exposed to the Aplanochytrium parasite. Differentially expressed genes involved in immunity include pattern recognition molecules, anti-microbial peptides, and genes involved in wound repair and reactive oxygen species formation. Gene enrichment analysis indicated eight biological processes were enriched representing 36 genes, largely involved with protein translation and energy production. This is the first report using high-throughput sequencing to characterize the host response of a coral to a natural pathogen. Furthermore, we have generated the first transcriptome for a soft (octocoral or non-scleractinian) coral species. Expression analysis revealed genes important in invertebrate innate immune pathways, as well as those whose role is previously un-described in cnidarians. This resource will be valuable in characterizing G. ventalina immune response to infection and co-infection of pathogens in the context of environmental change

Seawater carbonate chemistry and pH of the extracellular calcifying fluid and skeletal concentrations of trace metals of three coral species (Pocillopora eydouxi, Porites lobata, and Porites rus)

Ocean acidification (OA) threatens the growth and function of coral reef ecosystems. A key component to coral health is the microbiome, but little is known about the impact of OA on coral microbiomes. A submarine CO2 vent at Maug Island in the Northern Marianas Islands provides a natural pH gradient to investigate coral responses to long-term OA conditions. Three coral species (Pocillopora eydouxi, Porites lobata, and Porites rus) were sampled from three sites where mean seawater pH is 8.04, 7.98, and 7.94. We characterized coral bacterial communities (using 16S rRNA gene sequencing) and determined pH of the extracellular calcifying fluid (ECF) (using skeletal boron isotopes) across the seawater pH gradient. Bacterial communities of both Porites species stabilized (decreases in community dispersion) with decreased seawater pH, coupled with large increases in the abundance of Endozoicomonas, an endosymbiont. P. lobata experienced a significant decrease in ECF pH near the vent, whereas P. rus experienced a trending decrease in ECF pH near the vent. By contrast, Pocillopora exhibited bacterial community destabilization (increases in community dispersion), with significant decreases in Endozoicomonas abundance, while its ECF pH remained unchanged across the pH gradient. Our study shows that OA has multiple consequences on Endozoicomonas abundance and suggests that Endozoicomonas abundance may be an indicator of coral response to OA. We reveal an interesting dichotomy between two facets of coral physiology (regulation of bacterial communities and regulation of calcification), highlighting the importance of multidisciplinary approaches to understanding coral health and function in a changing ocean