Managing marine disease emergencies in an era of rapid change

Testing a health baseline during a bivalve mollusc mortality event: An investigation into die-offs of pipi Paphies australis from Aotearoa New Zealand

Disease is a major threat to the economic, ecological and cultural services provided by wild bivalve populations. Over the past decade anecdotal reports on declining health of native bivalve populations around Aotearoa New Zealand have been supported by increasing observations of mass die-offs. Causes of declining health and mass die-offs of wild bivalves are not clear and could be due to a number of interactive and cumulative factors, including declining water quality, climate change, or disease. Pipi/kōkota (Paphies australis) within the Whangārei area (northern New Zealand) have suffered repeated die-offs and declining health since at least 2009. Baseline health data for wild native bivalve populations are scarce making it difficult to identify changes in pathogen infection prevalence and intensity and infer their importance to host health. This research aimed to examine and document the health of pipi in Whangārei with the objective of identifying factors that may contribute to their ill health and lack of population recovery. We sampled pipi from four sites within Whangārei, eight times across two years (total n = 640) to establish a health baseline using histopathology, general bacteriology, and qPCR for the intracellular bacteria Endozoicomonas spp. Three pipi mass die-offs occurred during the sampling window that were opportunistically sampled to compare against the health baseline established using healthy pipi. An increase in bacterial growth and a decrease in the abundance of Endozoicomonas spp. in mortality pipi was observed compared with the health baseline. Establishing a health baseline for pipi from Whangārei provided a benchmark to assess changes in a pipi population experiencing high mortality. Such data can help identify factors contributing to die-offs and to help inform what mitigation, if any, is possible in wild shellfish populations.

Recovering high-quality bacterial genomes from cross-contaminated cultures: a case study of marine Vibrio campbellii

BACKGROUND

Environmental monitoring of bacterial pathogens is critical for disease control in coastal marine ecosystems to maintain animal welfare and ecosystem function and to prevent significant economic losses. This requires accurate taxonomic identification of environmental bacterial pathogens, which often cannot be achieved by commonly used genetic markers (e.g., 16S rRNA gene), and an understanding of their pathogenic potential based on the information encoded in their genomes. The decreasing costs of whole genome sequencing (WGS), combined with newly developed bioinformatics tools, now make it possible to unravel the full potential of environmental pathogens, beyond traditional microbiological approaches. However, obtaining a high-quality bacterial genome, requires initial cultivation in an axenic culture, which is a bottleneck in environmental microbiology due to cross-contamination in the laboratory or isolation of non-axenic strains.

RESULTS

We applied WGS to determine the pathogenic potential of two Vibrio isolates from coastal seawater. During the analysis, we identified cross-contamination of one of the isolates and decided to use this dataset to evaluate the possibility of bioinformatic contaminant removal and recovery of bacterial genomes from a contaminated culture. Despite the contamination, using an appropriate bioinformatics workflow, we were able to obtain high quality and highly identical genomes (Average Nucleotide Identity value 99.98%) of one of the Vibrio isolates from both the axenic and the contaminated culture. Using the assembled genome, we were able to determine that this isolate belongs to a sub-lineage of Vibrio campbellii associated with several diseases in marine organisms. We also found that the genome of the isolate contains a novel Vibrio plasmid associated with bacterial defense mechanisms and horizontal gene transfer, which may offer a competitive advantage to this putative pathogen.

CONCLUSIONS

Our study shows that, using state-of-the-art bioinformatics tools and a sufficient sequencing effort, it is possible to obtain high quality genomes of the bacteria of interest and perform in-depth genomic analyses even in the case of a contaminated culture. With the new isolate and its complete genome, we are providing new insights into the genomic characteristics and functional potential of this sub-lineage of V. campbellii. The approach described here also highlights the possibility of recovering complete bacterial genomes in the case of non-axenic cultures or obligatory co-cultures.

Marine diseases and the Anthropocene: Understanding microbial pathogenesis in a rapidly changing world

Healthy marine ecosystems are paramount for Earth's biodiversity and are key to sustaining the global economy and human health. The effects of anthropogenic activity represent a pervasive threat to the productivity of marine ecosystems, with intensifying environmental stressors such as climate change and pollution driving the occurrence and severity of microbial diseases that can devastate marine ecosystems and jeopardise food security. Despite the potentially catastrophic outcomes of marine diseases, our understanding of host-pathogen interactions remains an understudied aspect of both microbiology and environmental research, especially when compared to the depth of information available for human and agricultural systems. Here, we identify three avenues of research in which we can advance our understanding of marine disease in the context of global change, and make positive steps towards safeguarding marine communities for future generations.

A new member of the Nudiviridae from the Florida stone crab (Menippe mercenaria)

Menippe mercenaria, the Florida stone crab, supports an unconventional fishery across the southern USA and Caribbean that involves claw-removal and the return of de-clawed animals to the sea. We provide pathological, ultrastructural, and genomic detail for a novel hepatopancreatic, nucleus-specific virus - Menippe mercenaria nudivirus (MmNV) - isolated from M. mercenaria, captured during fisheries-independent monitoring. The virus has a genome of 99,336 bp and encodes 84 predicted protein coding genes and shows greatest similarity to Aratus pisonii nudivirus (ApNV) (<60% protein similarity and 31 shared genes of greatest similarity), collected from the Florida Keys, USA. MmNV is a member of the Gammanudivirus genus (Naldaviricetes: Lefavirales: Nudiviridae). Comparisons of virus genome size, preferred host environment, and gene number revealed no clear associations between the viral traits and phylogenetic position. Evolution of the virus alongside the diversification of host taxa, with the potential for host-switching, remain more likely evolutionary pathways.

Isolation and characterization of Fusarium spp. From unhatched eggs of Caretta caretta in Tuscany (Italy)

Sea Turtle Egg Fusariosis (STEF) is a worldwide emergent fungal disease affecting eggs and causing embryos mortality in turtle's nests such as those of Caretta caretta. It is caused by a complex of species belonging to Fusarium genus, particularly those included in the Fusarium Solani Species Complex (FSSC). During the samplings carried out in summer 2020 along the Tuscany coastlines (Italy), C. caretta eggs showed clinical signs resembling those caused by STEF. A total of 32 fungal isolates were obtained from lesioned eggs whose molecular characterization allowing identifying as belonging to FSSC / Neocosmospora spp., Fusarium oxysporum Species Complex (FOSC) / F. oxysporum and Fusarium nodosum, i.e., fungal genera and speciesincluding also well-known plant pathogens. Isolates inoculated on several plant hosts did not result in any pathogenic activity but F. nodosum causing, on wheat spikes, disease symptoms.This is the first time F. nodosum has been isolated from portions of eggs showing evident signs of fungal infection. This work represents the first report of Fusarium spp. isolated from C. caretta eggs showing lesions resembling those caused by STEF on Tuscan coast thus posing a significant concern to loggerhead sea turtle conservation also in this region.

Predicting the Growth of Vibrio parahaemolyticus in Oysters under Varying Ambient Temperature

Temperature is a critical factor that influences the proliferation of pathogens in hosts. One example of this is the human pathogen Vibrio parahaemolyticus (V. parahaemolyticus) in oysters. Here, a continuous time model was developed for predicting the growth of Vibrio parahaemolyticus in oysters under varying ambient temperature. The model was fit and evaluated against data from previous experiments. Once evaluated, the V. parahaemolyticus dynamics in oysters were estimated at different post-harvest varying temperature scenarios affected by water and air temperature and different ice treatment timing. The model performed adequately under varying temperature, reflecting that (i) increasing temperature, particularly in hot summers, favors a rapid V. parahaemolyticus growth in oysters, resulting in a very high risk of gastroenteritis in humans after consumption of a serving of raw oysters, (ii) pathogen inactivation due to day/night oscillations and, more evidently, due to ice treatments, and (iii) ice treatment is much more effective, limiting the risk of illness when applied immediately onboard compared to dockside. The model resulted in being a promising tool for improving the understanding of the V. parahaemolyticus-oyster system and supporting studies on the public health impact of pathogenic V. parahaemolyticus associated with raw oyster consumption. Although robust validation of the model predictions is needed, the initial results and evaluation showed the potential of the model to be easily modified to match similar systems where the temperature is a critical factor shaping the proliferation of pathogens in hosts.

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. This article is part of the theme issue 'Infectious disease ecology and evolution in a changing world'.

Assigning cause for emerging diseases of aquatic organisms

Resolving the cause of disease (= aetiology) in aquatic organisms is a challenging but essential goal, heightened by increasing disease prevalence in a changing climate and an interconnected world of anthropogenic pathogen spread. Emerging diseases play important roles in evolutionary ecology, wildlife conservation, the seafood industry, recreation, cultural practices, and human health. As we emerge from a global pandemic of zoonotic origin, we must focus on timely diagnosis to confirm aetiology and enable response to diseases in aquatic ecosystems. Those systems' resilience, and our own sustainable use of seafood, depend on it. Synchronising traditional and recent advances in microbiology that span ecological, veterinary, and medical fields will enable definitive assignment of risk factors and causal agents for better biosecurity management and healthier aquatic ecosystems.

Environmental and pathological factors affecting the hatching success of the two northernmost loggerhead sea turtle (Caretta caretta) nests

In recent years, the report of loggerhead sea turtle (Caretta caretta) Mediterranean nesting range has expanded together with new records of nests becoming northward on the Italian coastline of the Tyrrhenian and Adriatic seas. These areas are characterized by intensive human activities, such as tourism, fishery, and marine traffic, all possibly involved in the influence of the use of coastal habitat by marine species. These anthropic threats, in addition to the natural ones and the changing environmental characteristics of the beach, may influence the growth of microorganisms causing hatching failures. Among microorganisms, fungal infection by the genus Fusarium (Link, 1809) is considered one of the main causes of globally declining sea turtle populations. In summer 2021, the two northernmost worldwide loggerhead sea turtle nests were monitored along the Northern Adriatic coastline (Veneto, Italy). These first records may potentially candidate this area as suitable for a large part of the loggerhead turtle's life cycle and it could represent a minor sea turtle nesting area that, according to Prato and colleagues, remained unnoticed due to the lack of specific monitoring. Sea Turtle Egg Fusariosis (STEF) was deemed to have deeply compromised the hatching success of the northmost one. Climate change and anthropogenic impacts have been scored as one of the highest hazards to sea turtle health and could have played a role in the STEF development. Environmental changes, human activities, and emerging pathogens deserve the highest attention in terms of health research, and conservation management.

Cross-Hemispheric Genetic Diversity and Spatial Genetic Structure of Callinectes sapidus Reovirus 1 (CsRV1)

The movement of viruses in aquatic systems is rarely studied over large geographic scales. Oceanic currents, host migration, latitude-based variation in climate, and resulting changes in host life history are all potential drivers of virus connectivity, adaptation, and genetic structure. To expand our understanding of the genetic diversity of Callinectes sapidus reovirus 1 (CsRV1) across a broad spatial and host life history range of its blue crab host (Callinectes sapidus), we obtained 22 complete and 96 partial genomic sequences for CsRV1 strains from the US Atlantic coast, Gulf of Mexico, Caribbean Sea, and the Atlantic coast of South America. Phylogenetic analyses of CsRV1 genomes revealed that virus genotypes were divided into four major genogroups consistent with their host geographic origins. However, some CsRV1 sequences from the US mid-Atlantic shared high genetic similarity with the Gulf of Mexico genotypes, suggesting potential human-mediated movement of CsRV1 between the US mid-Atlantic and Gulf coasts. This study advances our understanding of how climate, coastal geography, host life history, and human activity drive patterns of genetic structure and diversity of viruses in marine animals and contributes to the capacity to infer broadscale host population connectivity in marine ecosystems from virus population genetic data.

Artificial shelters and marine infectious disease: no detectable effect of the use of casitas to enhance juvenile Panulirus argus in shelter-poor habitats on a viral disease dynamics

Casitas, low-lying artificial shelters that mimic large crevices, are used in some fisheries for Caribbean spiny lobsters (Panulirus argus). These lobsters are highly gregarious and express communal defense of the shelter. Scaled-down casitas have been shown to increase survival, persistence, and foraging ranges of juveniles. Therefore, the use of casitas has been suggested to help enhance local populations of juvenile P. argus in Caribbean seagrass habitats, poor in natural crevice shelters, in marine protected areas. Following the emergence of Panulirus argus virus 1 (PaV1), which is lethal to juveniles of P. argus, concern was raised about the potential increase in PaV1 transmission with the use of casitas. It was then discovered that lobsters tend to avoid shelters harboring diseased conspecifics, a behavior which, alone or in conjunction with predatory culling of diseased lobsters, has been proposed as a mechanism reducing the spread of PaV1. However, this behavior may depend on the ecological context (i.e., availability of alternative shelter and immediacy of predation risk). We conducted an experiment in a lobster nursery area to examine the effect of the use of casitas on the dynamics of the PaV1 disease. We deployed 10 scaled-down casitas per site on five 1-ha sites over a reef lagoon (casita sites) and left five additional sites with no casitas (control sites). All sites were sampled 10 times every 3-4 months. Within each site, all lobsters found were counted, measured, and examined for clinical signs of the PaV1 disease. Mean density and size of lobsters significantly increased on casita sites relative to control sites, but overall prevalence levels remained similar. There was no relationship between lobster density and disease prevalence. Dispersion parameters (m and k of the negative binomial distribution) revealed that lobsters tended to avoid sharing natural crevices, but not casitas, with diseased conspecifics. These results confirm that casitas provide much needed shelter in seagrass habitats and that their large refuge area may allow distancing between healthy and diseased lobsters. On eight additional sampling times over two years, we culled all diseased lobsters observed on casita sites. During this period, disease prevalence did not decrease but rather increased and varied with site, suggesting that other factors (e.g., environmental) may be influencing the disease dynamics. Using scaled-down casitas in shelter-poor habitats may help efforts to enhance juvenile lobsters for conservation purposes, but monitoring PaV1 prevalence at least once a year during the first few years would be advisable.

Mixtures of genotypes increase disease resistance in a coral nursery

Marine infectious diseases are a leading cause of population declines globally due, in large part, to challenges in diagnosis and limited treatment options. Mitigating disease spread is particularly important for species targeted for conservation. In some systems, strategic arrangement of organisms in space can constrain disease outbreaks, however, this approach has not been used in marine restoration. Reef building corals have been particularly devastated by disease and continue to experience catastrophic population declines. We show that mixtures of genotypes (i.e., diversity) increased disease resistance in the critically endangered Acropora cervicornis, a species that is frequently targeted for restoration of degraded reefs in the broader Caribbean region. This finding suggests a more generalized relationship between diversity and disease and offers a viable strategy for mitigating the spread of infectious diseases in corals that likely applies to other foundation species targeted for restoration.

Strategies for managing marine disease

The incidence of emerging infectious diseases (EIDs) has increased in wildlife populations in recent years and is expected to continue to increase with global environmental change. Marine diseases are relatively understudied compared with terrestrial diseases but warrant parallel attention as they can disrupt ecosystems, cause economic loss, and threaten human livelihoods. Although there are many existing tools to combat the direct and indirect consequences of EIDs, these management strategies are often insufficient or ineffective in marine habitats compared with their terrestrial counterparts, often due to fundamental differences between marine and terrestrial systems. Here, we first illustrate how the marine environment and marine organism life histories present challenges and opportunities for wildlife disease management. We then assess the application of common disease management strategies to marine versus terrestrial systems to identify those that may be most effective for marine disease outbreak prevention, response, and recovery. Finally, we recommend multiple actions that will enable more successful management of marine wildlife disease emergencies in the future. These include prioritizing marine disease research and understanding its links to climate change, improving marine ecosystem health, forming better monitoring and response networks, developing marine veterinary medicine programs, and enacting policy that addresses marine and other wildlife diseases. Overall, we encourage a more proactive rather than reactive approach to marine wildlife disease management and emphasize that multidisciplinary collaborations are crucial to managing marine wildlife health.

Bacteriology & bivalves: Assessing diagnostic tools for geographically remote bivalve populations

Two sampling approaches for the growth of common or dominant bacteria from bivalve haemolymph were compared: (1) samples processed in the field immediately after collection (field samples), and (2) samples processed in the laboratory at least 24 h after collection (laboratory samples). The sampling approaches were compared on 210 marine bivalve molluscs Paphies subtriangulata and P. australis from two shallow intertidal sites in North Island New Zealand. The approaches were evaluated for the amount of bacterial growth, type of growth, and diversity of growth. Differences in amount and type of growth between the two sampling approaches were observed. Samples processed in the field from P. subtriangulata had significantly more bacterial growth, and a higher diversity of bacteria, including more common or dominant bacterial species. Laboratory samples had a higher proportion of samples with no growth, however common or dominant bacteria were still isolated from these samples. For P. australis, field samples more often had no bacterial growth and laboratory samples had a significantly higher number of common or dominant growth present. Field samples did however contain a higher diversity of bacteria. By conducting bacteriology on bivalves in either the field or the laboratory only, there may be limitations to determining the significance of a bacterial agent isolated. Sampling of both field and laboratory samples should be carried out where possible to optimise detection of important bacteria.

Monitoring of microbiological dynamics in beach sand and seawater samples from recreational and non-recreational beaches over a two-year period

The frequencies of Escherichia coli, Staphylococcus aureus and Methicillin-resistant S. aureus (MRSA) and 'aerobic mesophilic bacteria' (AMB) counts in different seasons and marine sources were compared to understand the microbiological dynamics at beaches in N. Cyprus. Also, antibiotic resistance patterns were evaluated. The characterization and AMB enumeration studies were performed by conventional microbiological methods. AMB counts increased from winter to summer significantly (45.5*10⁴ CFU/mL to 2.5*10⁶ CFU/mL). Similarly, percentage detection frequencies of the bacteria were higher in summer compared to winter and were significant particularly for E. coli in both sand and seawater samples in 2019 and 2020 (p = 0.0181, p = 0.0142, p = 0.1257, p = 0.0446, respectively). However, a significant difference was not detected in percentage detection frequencies in terms of different sources or recreational status of beaches. The highest resistance percentages were detected against beta-lactam and lincosamides group of antibiotics. Results of the study signified that regular microbiological monitoring for beaches is essential.

Evaluating the effect of nuclear inclusion X (NIX) infections on Pacific razor clam populations

Nuclear inclusion X (NIX), the etiological agent of bacterial gill disease in Pacific razor clams Siliqua patula, was associated with host mortality events in coastal Washington State, USA, during the mid-1980s. Ongoing observations of truncated razor clam size distributions in Kalaloch Beach, Washington, raised concerns that NIX continues to impact populations. We conducted a series of spatial and longitudinal NIX surveillances, examined archived razor clam gill tissue, and used population estimates from stock assessments to test whether (1) the prevalence and intensity of NIX infections is higher at Kalaloch Beach relative to nearby beaches, (2) infected gill tissue has features consistent with historical descriptions of NIX-associated histopathology, and (3) annual clam survival is inversely related to NIX infection prevalence and intensity. NIX prevalence exceeded 85% at all sampled locations, and infection intensity was the highest at Kalaloch Beach by 0.9-2.6 orders of magnitude. Kalaloch Beach clams revealed histopathology consistent with previous NIX epidemics, including enlarged and/or rupturing branchial epithelial cells, branchial necrosis, and high hemocyte densities. Estimated annual survival was 22% at Kalaloch Beach, and ranged between 57 and 99% at other study sites. NIX infection intensity (via quantitative PCR) was not significantly correlated with annual survival; however, annual survival was lowest at Kalaloch Beach, where infection intensities were highest, suggesting that clams can tolerate infections up to a lethal threshold. Collectively these data support the hypothesis that high NIX intensities are associated with host mortality. NIX-associated mortality appears to be more pronounced at Kalaloch Beach relative to other Washington beaches.

A Review of Asteroid Biology in the Context of Sea Star Wasting: Possible Causes and Consequences

AbstractSea star wasting-marked in a variety of sea star species as varying degrees of skin lesions followed by disintegration-recently caused one of the largest marine die-offs ever recorded on the west coast of North America, killing billions of sea stars. Despite the important ramifications this mortality had for coastal benthic ecosystems, such as increased abundance of prey, little is known about the causes of the disease or the mechanisms of its progression. Although there have been studies indicating a range of causal mechanisms, including viruses and environmental effects, the broad spatial and depth range of affected populations leaves many questions remaining about either infectious or non-infectious mechanisms. Wasting appears to start with degradation of mutable connective tissue in the body wall, leading to disintegration of the epidermis. Here, we briefly review basic sea star biology in the context of sea star wasting and present our current knowledge and hypotheses related to the symptoms, the microbiome, the viruses, and the associated environmental stressors. We also highlight throughout the article knowledge gaps and the data needed to better understand sea star wasting mechanistically, its causes, and potential management.

Impact of climate driven temperature increase on inland aquaculture: application to land-based production of common carp (Cyprinus carpio L.)

Climate change will expose the food producing sector to a range of challenges. Inland aquaculture farms are particularly vulnerable, due to the difficulty in changing their location, and therefore require specific tools to predict the influence of direct and indirect effects on production, environment, and economic feasibility. The objective of our study was to apply a simple set of models to produce a set of growth, risk and suitability maps for stakeholders within the common carp sector in Poland, to assist decision making under two different scenarios of climate change: a moderate situation (RCP 4.5) and an extreme situation (RCP 8.5). We used present (2000-2019) and future projections (2080-2099) for water surface temperature based on land surface temperature data from regionally downscaled climate models to draw maps to: i) show optimal temperature conditions for carp growth, ii) assess risk of disease outbreak caused by three important common carp pathogens: Cyprinid herpesvirus 3 (CyHV-3), carp edema virus (CEV) and spring viremia of carp (SVCV), and iii) predict potential suitability changes of carp farming in Poland. The study identified areas with the most and least favourable temperature conditions for carp growth, as well as those areas with the highest/lowest number of days with suitable temperatures for virus infection. These suitability maps showed the combined effect of direct and indirect effects of climate change projections under RCP 8.5 and RCP 4.5 scenarios. The approach applied herein will be of use worldwide for analysing the risks of temperature increase to land-based aquaculture, and the results presented are important for carp farmers in Poland and elsewhere, industry in general, and government stakeholders, to understand the direct and indirect effects of climate change on the triple bottom line of people, planet, and profit. This article is protected by copyright. All rights reserved.

Keeping up with advances in qPCR pathogen detection: an example for QPX disease in hard clams

With marine diseases on the rise and increased reliance on molecular tools for disease surveillance, validated pathogen detection capabilities are important for effective management, mitigation, and response to disease outbreaks. At the same time, in an era of continual evolution and advancement of molecular tools for pathogen detection, it is critical to regularly reassess previously established assays to incorporate improvements of common practices and procedures, such as the minimum information for publication of quantitative real-time PCR experiments (MIQE) guidelines. Here, we reassessed, re-optimized, and improved the quantitative PCR (qPCR) assay routinely used for Quahog Parasite Unknown (QPX) disease monitoring. We made 19 significant changes to the qPCR assay, including improvements to PCR amplification efficiency, DNA extraction efficiency, inhibition testing, incorporation of linearized standards for absolute quantification, an inter-plate calibration technique, and improved conversion from copy number to number of cells. These changes made the assay a more effective and efficient tool for disease monitoring and pathogen detection, with an improved linear relationship with histopathology compared to the previous version of the assay. To support the wide adoption of validated qPCR assays for marine pathogens, we provide a simple workflow that can be applied to the development of new assays, re-optimization of old or suboptimal assays, or assay validation after changes to the protocol and a MIQE-compliant checklist that should accompany any published qPCR diagnostic assay to increase experimental transparency and reproducibility amongst laboratories.

Histopathology of a threatened surf clam, toheroa (Paphies ventricosa) from Aotearoa New Zealand

The toheroa (Paphies ventricosa) is endemic to Aotearoa (New Zealand). Following decades of overfishing in the 1900 s, commercial and recreational fishing of toheroa is now prohibited. For unknown reasons, protective measures in place for over 40 years have not ensured the recovery of toheroa populations. For the first time, a systematic pathology survey was undertaken to provide a baseline of toheroa health in remaining major populations. Using histopathology, parasites and pathologies in a range of tissues are assessed and quantified spatio-temporally. Particular focus is placed on intracellular microcolonies of bacteria (IMCs). Bayesian ordinal logistic regression is used to model IMC infection and several facets of toheroa health. Model outputs show condition to be the most important predictor of IMC intensity in toheroa tissues. The precarious state of many toheroa populations around Aotearoa should warrant greater attention from scientists, conservationists, and regulators. It is hoped that this study will provide some insight into the current health status of a treasured and iconic constituent of several expansive surf beaches in Aotearoa.

Transcriptomic Response to Perkinsus marinus in Two Crassostrea Oysters Reveals Evolutionary Dynamics of Host-Parasite Interactions

Infectious disease outbreaks are causing widespread declines of marine invertebrates including corals, sea stars, shrimps, and molluscs. Dermo is a lethal infectious disease of the eastern oyster Crassostrea virginica caused by the protist Perkinsus marinus. The Pacific oyster Crassostrea gigas is resistant to Dermo due to differences in the host-parasite interaction that is not well understood. We compared transcriptomic responses to P. marinus challenge in the two oysters at early and late infection stages. Dynamic and orchestrated regulation of large sets of innate immune response genes were observed in both species with remarkably similar patterns for most orthologs, although responses in C. virginica were stronger, suggesting strong or over-reacting immune response could be a cause of host mortality. Between the two species, several key immune response gene families differed in their expansion, sequence variation and/or transcriptional response to P. marinus, reflecting evolutionary divergence in host-parasite interaction. Of note, significant upregulation of inhibitors of apoptosis (IAPs) was observed in resistant C. gigas but not in susceptible C. virginica, suggesting upregulation of IAPs is an active defense mechanism, not a passive response orchestrated by P. marinus. Compared with C. gigas, C. virginica exhibited greater expansion of toll-like receptors (TLRs) and positive selection in P. marinus responsive TLRs. The C1q domain containing proteins (C1qDCs) with the galactose-binding lectin domain that is involved in P. marinus recognition, were only present and significantly upregulated in C. virginica. These results point to previously undescribed differences in host defense genes between the two oyster species that may account for the difference in susceptibility, providing an expanded portrait of the evolutionary dynamics of host-parasite interaction in lophotrochozoans that lack adaptive immunity. Our findings suggest that C. virginica and P. marinus have a history of coevolution and the recent outbreaks may be due to increased virulence of the parasite.

Little evidence for genetic variation associated with susceptibility to sea star wasting syndrome in the keystone species Pisaster ochraceus

The keystone species Pisaster ochraceus suffered mass mortalities along the northeast Pacific Ocean from Sea Star Wasting Syndrome (SSWS) outbreaks in 2013-2016. SSWS causation remains of debate, leading to concerns as to whether outbreaks will continue to impact this species. Considering the apparent link between ocean temperature and SSWS, the future of this species and intertidal communities remains uncertain. Surveys of co-occurring apparently normal and wasting P. ochraceus along the central Oregon coast in 2016 allowed us to address whether variation in disease status showed genetic variation that may be associated with differences in susceptibility to SSWS. We performed restriction site-associated DNA sequencing (2bRAD-seq) to genotype ~72,000 single nucleotide polymorphism (SNP) loci across apparently normal and wasting sea stars. Locus-specific analyses of differentiation (F_ST ) between disease-status groups revealed no signal of genetic differences separating the two groups. Using a multivariate approach, we observed weak separation between the groups, but identified 18 SNP loci showing highest discriminatory power between the groups and scanned the genome annotation for linked genes. A total of 34 protein-coding genes were found to be located within 15 kb (measured by linkage disequilibrium decay) of at least one of the 18 SNPs, and 30 of these genes had homologies to annotated protein databases. Our results suggest that the likelihood of developing SSWS symptoms does not have a strong genetic basis. The few genomic regions highlighted had only modest levels of differentiation, but the genes associated with these regions may form the basis for functional studies aiming to understand disease progression.

Mediterranean Aquaculture in a Changing Climate: Temperature Effects on Pathogens and Diseases of Three Farmed Fish Species

Climate change is expected to have a drastic effect on aquaculture worldwide. As we move forward with the agenda to increase and diversify aquaculture production, rising temperatures will have a progressively relevant impact on fish farming, linked to a multitude of issues associated with fish welfare. Temperature affects the physiology of both fish and pathogens, and has the potential to lead to significant increases in disease outbreaks within aquaculture systems, resulting in severe financial impacts. Significant shifts in future temperature regimes are projected for the Mediterranean Sea. We therefore aim to review and discuss the existing knowledge relating to disease outbreaks in the context of climate change in Mediterranean finfish aquaculture. The objective is to describe the effects of temperature on the physiology of both fish and pathogens, and moreover to list and discuss the principal diseases of the three main fish species farmed in the Mediterranean, namely gilthead seabream (Sparus aurata), European seabass (Dicentrarchus labrax), and meagre (Argyrosomus regius). We will attempt to link the pathology of each disease to a specific temperature range, while discussing potential future disease threats associated with the available climate change trends for the Mediterranean Sea.

The current situation and potential effects of climate change on the microbial load of marine bivalves of the Greek coastlines: an integrative review

Global warming affects the aquatic ecosystems, accelerating pathogenic microorganisms' and toxic microalgae's growth and spread in marine habitats, and in bivalve molluscs. New parasite invasions are directly linked to oceanic warming. Consumption of pathogen-infected molluscs impacts human health at different rates, depending, inter alia, on the bacteria taxa. It is therefore necessary to monitor microbiological and chemical contamination of food. Many global cases of poisoning from bivalve consumption can be traced back to Mediterranean regions. This article aims to examine the marine bivalve's infestation rate within the scope of climate change, as well as to evaluate the risk posed by climate change to bivalve welfare and public health. Biological and climatic data literature review was performed from international scientific sources, Greek authorities and State organizations. Focusing on Greek aquaculture and bivalve fisheries, high-risk index pathogenic parasites and microalgae were observed during summer months, particularly in Thermaikos Gulf. Considering the climate models that predict further temperature increases, it seems that marine organisms will be subjected in the long term to higher temperatures. Due to the positive linkage between temperature and microbial load, the marine areas most affected by this phenomenon are characterized as 'high risk' for consumer health.

Variation in Immune-Related Gene Expression Provides Evidence of Local Adaptation in Porites astreoides (Lamarck, 1816) between Inshore and Offshore Meta-Populations Inhabiting the Lower Florida Reef Tract, USA

Coral communities of the Florida Reef Tract (FRT) have changed dramatically over the past 30 years. Coral cover throughout the FRT is disproportionately distributed; >70% of total coral cover is found within the inshore patch reef zone (<2 km from shore) compared to 30% found within the offshore bank reef zone (>5 km from shore). Coral mortality from disease has been differentially observed between inshore and offshore reefs along the FRT. Therefore, differences between the response of inshore and offshore coral populations to bacterial challenge may contribute to differences in coral cover. We examined immune system activation in Porites astreoides (Lamarck, 1816), a species common in both inshore and offshore reef environments in the FRT. Colonies from a representative inshore and offshore site were reciprocally transplanted and the expression of three genes monitored biannually for two years (two summer and two winter periods). Variation in the expression of eukaryotic translation initiation factor 3, subunit H (eIF3H), an indicator of cellular stress in Porites astreoides, did not follow annual patterns of seawater temperatures (SWT) indicating the contribution of other stressors (e.g., irradiance). Greater expression of tumor necrosis factor (TNF) receptor associated factor 3 (TRAF3), a signaling protein of the inflammatory response, was observed among corals transplanted to, or located within the offshore environment indicating that an increased immune response is associated with offshore coral more so than the inshore coral (p < 0.001). Corals collected from the offshore site also upregulated the expression of adenylyl cyclase associated protein 2 (ACAP2), increases which are associated with decreasing innate immune system inflammatory responses, indicating a counteractive response to increased stimulation of the innate immune system. Activation of the innate immune system is a metabolically costly survival strategy. Among the two reefs studied, the offshore population had a smaller mean colony size and decreased colony abundance compared to the inshore site. This correlation suggests that tradeoffs may exist between the activation of the innate immune system and survival and growth. Consequently, immune system activation may contribute to coral community dynamics and declines along the FRT.

Expansion of intertidal mussel beds following disease-driven reduction of a keystone predator

Disease shapes community composition by removing species with strong interactions. To test whether the absence of keystone predation due to disease produced changes to the species composition of rocky intertidal communities, we leverage a natural experiment involving mass mortality of the keystone predator Pisaster ochraceus from Sea Star Wasting Syndrome. Over four years, we measured dimensions of mussel beds, sizes of Mytilus californianus, mussel recruitment, and species composition on vertical rock walls at six rocky intertidal sites on the central California coast. We also assessed the relationship between changes in mussel cover and changes in sea star density across 33 sites along the North American Pacific coast using data from long-term monitoring. After four years, the lower boundary of the central California mussel beds shifted downward toward the water 18.7 ± 15.8 cm (SD) on the rock and 11.7 ± 11.0 cm in elevation, while the upper boundary remained unchanged. In central California, downward expansion and total area of the mussel bed were positively correlated with mussel recruitment but were not correlated with pre-disease sea star density or biomass. At a multi-region scale, changes in mussel percent cover were positively correlated with pre-disease sea star densities but not change in densities. Species composition of primary substrate holders and epibionts below the mussel bed remained similar across years. Extirpation of the community below the bed did not occur. Instead, this community became limited to a smaller spatial extent while the mussel bed expanded.

Family I84 protease inhibitors likely constitute a Mollusca-specific protein family functioning in host defense

Protease inhibitors are proteins or small polypeptides functioning in numerous biological processes in all organisms. The I84 family of protease inhibitors in the MEROPS database represents a novel protease inhibitor family that has been reported in 2 bivalves, Crassostrea virginica and Sinonovacula constricta, and is believed to play a role in host defense. In the present study, 7 new members of Family I84 were identified in 2 bivalves, Meretrix meretrix and Mytilus galloprovincialis, and 1 gastropod, Haliotis discus hannai, at the mRNA level via cDNA cloning. The expression patterns of the newly identified genes varied in response to salinity stresses and pathogen-associated molecular pattern stimulations, suggesting their involvement in the host defense of related species. Additionally, analyses of sequence data in public databases did not reveal any Family I84 protease inhibitor molecules in non-molluscan animals. The results indicated that Family I84 protease inhibitors are likely mollusk specific, constituting a unique host defense mechanism in molluscan species.

Emerging infectious disease: An underappreciated area of strategic concern for food security

Emerging infectious diseases (EIDs) increasingly threaten global food security and public health. Despite technological breakthroughs, we are losing the battle with (re)emerging diseases as treatment costs and production losses rise. A horizon scan of diseases of crops, livestock, seafood and food-borne illness suggests these costs are unsustainable. The paradigm of coevolution between pathogens and particular hosts teaches that emerging diseases occur only when pathogens evolve specific capacities that allow them to move to new hosts. EIDs ought to be rare and unpredictable, so crisis response is the best we can do. Alternatively, the Stockholm Paradigm suggests that the world is full of susceptible but unexposed hosts that pathogens could infect, given the opportunity. Global climate change, globalized trade and travel, urbanization and land-use changes (often associated with biodiversity loss) increase those opportunities, making EID frequent. We can, however, anticipate their arrival in new locations and their behaviour once they have arrived. We can 'find them before they find us', mitigating their impacts. The DAMA (Document, Assess, Monitor, Act) protocol alters the current reactive stance and embodies proactive solutions to anticipate and mitigate the impacts of EID, extending human and material resources and buying time for development of new vaccinations, medications and control measures.

Invertebrate health in marine protected areas (MPAs)

Marine protected areas (MPAs) consist of various categories of safeguarded areas in the marine environment, from semi-protected areas to total no take zones. The reported effects of MPAs are overwhelmingly positive, with numerous reports of fish size (biomass), abundance (recovery) and diversity increases, however, literature is lacking on the role and consequences of MPAs on parasite and disease dynamics, and in particular, invertebrate health. The implementation of MPAs has been known to alter trophic cascades and community dynamics, and with invertebrates commonly at the base of these systems, it is vital that their status is investigated. Overcrowding in areas closed to fishing is known to have parasitological consequences in some scenarios, and land/water use change has been known to alter host and vector communities, possibly elevating disease risk. Equally, reserves can be used as tools for alleviating impacts of marine disease. This review aims to consolidate extant literature and provide a comprehensive viewpoint on how invertebrates (and their health status) can be affected by MPAs, which are increasingly being implemented based on the relative urgency now being placed on protecting global biodiversity. In highlighting the paucity of knowledge surrounding MPAs and disease, especially that of the unenigmatic invertebrate groups, this review, published in the Special Issue on 'Invertebrates as One Health Sentinels', provides an opportunity for wide dissemination and provocation of further research in this area.

Increasing temperatures accentuate negative fitness consequences of a marine parasite

Infectious diseases are key drivers of wildlife populations and agriculture production, but whether and how climate change will influence disease impacts remains controversial. One of the critical knowledge gaps that prevents resolution of this controversy is a lack of high-quality experimental data, especially in marine systems of significant ecological and economic consequence. Here, we performed a manipulative experiment in which we tested the temperature-dependent effects on Atlantic salmon (Salmo salar) of sea lice (Lepeophtheirus salmonis)—a parasite that can depress the productivity of wild-salmon populations and the profits of the salmon-farming industry. We explored sea-louse impacts on their hosts across a range of temperatures (10, 13, 16, 19, and 22 °C) and infestation levels (zero, ‘low’ (mean abundance ± SE = 1.6 ± 0.1 lice per fish), and ‘high’ infestation (6.8 ± 0.4 lice per fish)). We found that the effects of sea lice on the growth rate, condition, and survival of juvenile Atlantic salmon all worsen with increasing temperature. Our results provide a rare empirical example of how climate change may influence the impacts of marine disease in a key social-ecological system. These findings underscore the importance of considering climate-driven changes to disease impacts in wildlife conservation and agriculture.

Post-mortem examination of the Caribbean spiny lobster (Panulirus argus, Latreille 1804) and pathology in a fishery of the Lesser Antilles

The Caribbean spiny lobster, Panulirus argus (Latreille, 1804) is a highly commercial species and comprises the largest spiny lobster fishery in the world. Although populations have declined throughout its range, there is little known regarding its diseases and pathogens. The objectives of this study were to provide illustrated and standardized methods for postmortem examination, and to describe baseline gross and microscopic pathology for P. argus. From July 2017-March 2019, a postmortem examination including comprehensive histological assessment was performed on 313 fishery-caught lobsters. Epibionts and lesions observed include branchial cirriped infestation (69%), branchial encysted nemertean worm larvae (23%), tail fan necrosis (11%), skeletal muscle necrosis (7%), antennal gland calculi (6%), branchial infarction (2%), and microsporidiosis (0.6%). This report confirms the rare prevalence of microsporidiosis in P. argus and describes nemertean worm larvae in the gill. This study also reports a condition resembling excretory calcinosis in spiny lobster. The methods and data produced by this study facilitate disease diagnosis and sustainable stock management of P. argus.

Modeling Pathogen Dispersal in Marine Fish and Shellfish

In marine ecosystems, oceanographic processes often govern host contacts with infectious agents. Consequently, many approaches developed to quantify pathogen dispersal in terrestrial ecosystems have limited use in the marine context. Recent applications in marine disease modeling demonstrate that physical oceanographic models coupled with biological models of infectious agents can characterize dispersal networks of pathogens in marine ecosystems. Biophysical modeling has been used over the past two decades to model larval dispersion but has only recently been utilized in marine epidemiology. In this review, we describe how biophysical models function and how they can be used to measure connectivity of infectious agents between sites, test hypotheses regarding pathogen dispersal, and quantify patterns of pathogen spread, focusing on fish and shellfish pathogens.

Newly emerging diseases of marine turtles, especially sea turtle egg fusariosis (SEFT), caused by species in the Fusarium solani complex (FSSC)

Sea turtles are presently considered severely endangered species that are historically threatened by many environmental factors. Recently, additional threats to sea turtles from two pathogenic species of fungi in the Fusarium solani species complex (F. falciforme and F. keratoplasticum) have been identified. These species infect marine turtle eggs, causing sea turtle egg fusariosis, and kill their embryos, with recent reports of hatch-failure in seven globally distributed species of endangered sea turtles (Caretta caretta, Chelonia mydas, Dermochelys coriaceae, Eretmochelys imbricata, Lepidochelys olivacea, Lepidochelys kempi and Natator depressus). Mycelia and spores of pathogenic species of Fusarium are produced in disturbed terrestrial soils and are transported to the ocean in coastal run off. We propose that these fungi grow on floating particles of plant tissues (leaves and wood), animal tissues, silt and plastics, which are carried by wind and currents and the turtles themselves to the beaches where the turtles lay their eggs.

Confirming validity measures of visual assessment of PaV1 infection in Caribbean spiny lobsters

Panulirus argus virus 1 (PaV1) affects wild populations of Caribbean spiny lobsters. PaV1 can be lethal but shows predilection for juvenile lobsters. Because P. argus is one of the most valuable fisheries around the wider Caribbean region, monitoring disease prevalence in local populations is desirable. Diseased lobsters are easily recognized by their milky hemolymph, but this sign only becomes evident in advanced stages of infection. Other methods have been developed to detect PaV1, but are less practical for long-term monitoring of patterns of infection in populations. A previous study estimated the validity measures (sensitivity and specificity) of detection of PaV1 infection by observed clinical signs against endpoint PCR assays, using a representative sample of lobsters comprising mainly subadults and adults from a commercial fishing area. In the present study, these validity measures were estimated in a similar manner for a different population comprising mainly juveniles from a protected nursery area. We obtained virtually the same sensitivity and specificity values (0.48 and 1, respectively) for observed clinical signs as in the previous study (0.51 and 1, respectively), confirming the validity of applying a simple 2× correction factor to monitor the patterns of PaV1 infection over time based on more easily conducted visual assessments of a representative sample of the population.

Sea star wasting disease demography and etiology in the brooding sea star Leptasterias spp

Sea star wasting disease (SSWD) describes a suite of disease signs believed to have led to catastrophic die-offs in many asteroid species, beginning in 2013. While most studies have focused on large, easily visible sea stars with widely-dispersing larvae, less information is available on the effect of this disease outbreak on smaller sea star species, such as the six-armed sea star Leptasterias spp. Unlike many larger sea stars, Leptasterias brood non-feeding young instead of broadcast-spawning planktonic larvae. Limited dispersal and thus limited gene flow may make these sea stars more vulnerable to local selective pressures, such as disease outbreaks. Here, we examined Leptasterias populations at sites along the California coast and documented abundance changes coincident with recent Pacific coast SSWD in 2014. Detection of Leptasterias in central California declined, and Leptasterias were not detected at multiple sites clustered around the San Francisco Bay outflow in the most recent surveys. Additionally, we categorized disease signs in Leptasterias in the field and laboratory, which mirrored those seen in larger sea stars in both settings. Finally, we found that magnesium chloride (MgCl2) slowed the progression of physical deterioration related to SSWD when applied to sea stars in the laboratory, suggesting that MgCl2 may prolong the survival of diseased individuals.

Pathogen proteases and host protease inhibitors in molluscan infectious diseases

The development of infectious diseases represents an outcome of dynamic interactions between the disease-producing agent's pathogenicity and the host's self-defense mechanism. Proteases secreted by pathogenic microorganisms and protease inhibitors produced by host species play an important role in the process. This review aimed at summarizing major findings in research on pathogen proteases and host protease inhibitors that had been proposed to be related to the development of mollusk diseases. Metalloproteases and serine proteases respectively belonging to Family M4 and Family S8 of the MEROPS system are among the most studied proteases that may function as virulence factors in mollusk pathogens. On the other hand, a mollusk-specific family (Family I84) of novel serine protease inhibitors and homologues of the tissue inhibitor of metalloprotease have been studied for their potential in the molluscan host defense. In addition, research at the genomic and transcriptomic levels showed that more proteases of pathogens and protease inhibitor of hosts are likely involved in mollusk disease processes. Therefore, the pathological significance of interactions between pathogen proteases and host protease inhibitors in the development of molluscan infectious diseases deserves more research efforts.

Bacterial Community Dynamics in an Oyster Hatchery in Response to Probiotic Treatment

Larval oysters in hatcheries are susceptible to diseases caused by bacterial pathogens, including Vibrio spp. Previous studies have shown that daily addition of the probiotic Bacillus pumilus RI06-95 to water in rearing tanks increases larval survival when challenged with the pathogen Vibrio coralliilyticus. We propose that the presence of probiotics causes shifts in bacterial community structure in rearing tanks, leading to a net decrease in the relative abundance of potential pathogens. During three trials spanning the 2012-2015 hatchery seasons, larvae, tank biofilm, and rearing water samples were collected from control and probiotic-treated tanks in an oyster hatchery over a 12-day period after spawning. Samples were analyzed by 16S rRNA sequencing of the V4 or V6 regions followed by taxonomic classification, in order to determine bacterial community structures. There were significant differences in bacterial composition over time and between sample types, but no major effect of probiotics on the structure and diversity of bacterial communities (phylum level, Bray-Curtis k = 2, 95% confidence). Probiotic treatment, however, led to a higher relative percent abundance of Oceanospirillales and Bacillus spp. in water and oyster larvae. In the water, an increase in Vibrio spp. diversity in the absence of a net increase in relative read abundance suggests a likely decrease in the abundance of specific pathogenic Vibrio spp., and therefore lower chances of a disease outbreak. Co-occurrence network analysis also suggests that probiotic treatment had a systemic effect on targeted members of the bacterial community, leading to a net decrease in potentially pathogenic species.

Modelling sea lice control by lumpfish on Atlantic salmon farms: interactions with mate limitation, temperature and treatment rules

Atlantic salmon farming is one of the largest aquaculture sectors in the world. A major impact on farm economics, fish welfare and, potentially, nearby wild salmonid populations, is the sea louse ectoparasite Lepeophtheirus salmonis. Sea louse infestations are most often controlled through application of chemicals, but in most farming regions, sea lice have evolved resistance to the small set of available chemicals. Therefore, alternative treatment methodologies are becoming more widely used. One increasingly common alternative treatment involves the co-culture of farmed salmon with cleaner fish, which prey on sea lice. However, despite their wide use, little is understood about the situations in which cleaner fish are most effective. For example, previous work suggests that a low parasite density results in sea lice finding it difficult to acquire mates, reducing fecundity and population growth. Other work suggests that environmental conditions such as temperature and external sea louse pressure have substantial impact on this mate limitation threshold and may even remove the effect entirely. We used an Agent-Based Model (ABM) to simulate cleaner fish on a salmon farm to explore interactions between sea louse mating behaviour, cleaner fish feeding rate, temperature and external sea louse pressure. We found that sea louse mating has a substantial effect on sea louse infestations under a variety of environmental conditions. Our results suggest that cleaner fish can control sea louse infestations most effectively by maintaining the population below critical density thresholds.

El Niño drives a widespread ulcerative skin disease outbreak in Galapagos marine fishes

Climate change increases local climatic variation and unpredictability, which can alter ecological interactions and trigger wildlife disease outbreaks. Here we describe an unprecedented multi-species outbreak of wild fish disease driven by a climate perturbation. The 2015–16 El Niño generated a +2.5 °C sea surface temperature anomaly in the Galapagos Islands lasting six months. This coincided with a novel ulcerative skin disease affecting 18 teleost species from 13 different families. Disease signs included scale loss and hemorrhagic ulcerated patches of skin, fin deterioration, lethargy, and erratic behavior. A bacterial culture isolated from skin lesions of two of the affected fish species was identified by sequencing of the 16S rRNA gene as a Rahnella spp. Disease prevalence rates were linearly correlated with density in three fish species. In January 2016, disease prevalence reached 51.1% in the ring-tailed damselfish Stegastes beebei (n = 570) and 18.7% in the king angelfish Holacanthus passer (n = 318), corresponding to 78% and 86% decreases in their populations relative to a 4.5-year baseline, respectively. We hypothesize that this outbreak was precipitated by the persistent warm temperatures and lack of planktonic productivity that characterize extreme El Niño events, which are predicted to increase in frequency with global warming.

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.

The Use of Kernel Density Estimation With a Bio-Physical Model Provides a Method to Quantify Connectivity Among Salmon Farms: Spatial Planning and Management With Epidemiological Relevance

Connectivity in an aquatic setting is determined by a combination of hydrodynamic circulation and the biology of the organisms driving linkages. These complex processes can be simulated in coupled biological-physical models. The physical model refers to an underlying circulation model defined by spatially-explicit nodes, often incorporating a particle-tracking model. The particles can then be given biological parameters or behaviors (such as maturity and/or survivability rates, diel vertical migrations, avoidance, or seeking behaviors). The output of the bio-physical models can then be used to quantify connectivity among the nodes emitting and/or receiving the particles. Here we propose a method that makes use of kernel density estimation (KDE) on the output of a particle-tracking model, to quantify the infection or infestation pressure (IP) that each node causes on the surrounding area. Because IP is the product of both exposure time and the concentration of infectious agent particles, using KDE (which also combine elements of time and space), more accurately captures IP. This method is especially useful for those interested in infectious agent networks, a situation where IP is a superior measure of connectivity than the probability of particles from each node reaching other nodes. Here we illustrate the method by modeling the connectivity of salmon farms via sea lice larvae in the Broughton Archipelago, British Columbia, Canada. Analysis revealed evidence of two sub-networks of farms connected via a single farm, and evidence that the highest IP from a given emitting farm was often tens of kilometers or more away from that farm. We also classified farms as net emitters, receivers, or balanced, based on their structural role within the network. By better understanding how these salmon farms are connected to each other via their sea lice larvae, we can effectively focus management efforts to minimize the spread of sea lice between farms, advise on future site locations and coordinated treatment efforts, and minimize any impact of farms on juvenile wild salmon. The method has wide applicability for any system where capturing infectious agent networks can provide useful guidance for management or preventative planning decisions.

Dermal mycobacteriosis and warming sea surface temperatures are associated with elevated mortality of striped bass in Chesapeake Bay

Temperature is hypothesized to alter disease dynamics, particularly when species are living at or near their thermal limits. When disease occurs in marine systems, this can go undetected, particularly if the disease is chronic and progresses slowly. As a result, population-level impacts of diseases can be grossly underestimated. Complex migratory patterns, stochasticity in recruitment, and data and knowledge gaps can hinder collection and analysis of data on marine diseases. New tools enabling quantification of disease impacts in marine environments include coupled biogeochemical hydrodynamic models (to hindcast key environmental data), and multievent, multistate mark-recapture (MMSMR) (to quantify the effects of environmental conditions on disease processes and assess population-level impacts). We used MMSMR to quantify disease processes and population impacts in an estuarine population of striped bass (Morone saxatilis) in Chesapeake Bay from 2005 to 2013. Our results supported the hypothesis that mycobacteriosis is chronic, progressive, and, frequently, lethal. Yearly disease incidence in fish age three and above was 89%, suggesting that this disease impacts nearly every adult striped bass. Mortality of diseased fish was high, particularly in severe cases, where it approached 80% in typical years. Severely diseased fish also had a 10-fold higher catchability than healthy fish, which could bias estimates of disease prevalence. For both healthy and diseased fish, mortality increased with the modeled average summer sea surface temperature (SST) at the mouth of the Rappahannock River; in warmer summers (average SST ≥ 29°C), a cohort is predicted to experience >90% mortality in 1 year. Regression of disease signs in mildly and moderately diseased fish was <2%. These results suggest that these fish are living at their maximum thermal tolerance and that this is driving increased disease and mortality. Management of this fishery should account for the effects of temperature and disease on impacted populations.

Review: Host-pathogen dynamics of seagrass diseases under future global change

Human-induced global change is expected to amplify the disease risk for marine biota. However, the role of disease in the rapid global decline of seagrass is largely unknown. Global change may enhance seagrass susceptibility to disease through enhanced physiological stress, while simultaneously promoting pathogen development. This review outlines the characteristics of disease-forming organisms and potential impacts of global change on three groups of known seagrass pathogens: labyrinthulids, oomycetes and Phytomyxea. We propose that hypersalinity, climate warming and eutrophication pose the greatest risk for increasing frequency of disease outbreaks in seagrasses by increasing seagrass stress and lowering seagrass resilience. In some instances, global change may also promote pathogen development. However, there is currently a paucity of information on these seagrass pathosystems. We emphasise the need to expand current research to better understand the seagrass-pathogen relationships, serving to inform predicative modelling and management of seagrass disease under future global change scenarios.

Environment, dosage, and pathogen isolate moderate virulence in eelgrass wasting disease

Eelgrass wasting disease, caused by the marine pathogen Labyrinthula zosterae, has the potential to devastate important eelgrass habitats worldwide. Although this host-pathogen interaction may increase under certain environmental conditions, little is known about how disease severity is impacted by multiple components of a changing environment. In this study, we investigated the effects of variation in 3 different L. zosterae isolates, pathogen dosage, temperature, and light on severity of infections. Severity of lesions on eelgrass varied among the 3 different isolates inoculated in laboratory trials. Our methods to control dosage of inoculum showed that disease severity increased with pathogen dosage from 104 to 106 cells ml-1. In a dosage-controlled light and temperature 2-way factorial experiment consisting of 2 light regimes (diel light cycle and complete darkness) and 2 temperatures (11 and 18°C), L. zosterae cell growth rate in vitro was higher at the warmer temperature. In a companion experiment that tested the effects of light and temperature in in vivo inoculations, disease severity was higher in dark treatments and temperature was marginally significant. We suggest that the much greater impact of light in the in vivo inoculation experiment indicates an important role for plant physiology and the need for photosynthesis in slowing severity of infections. Our work with controlled inoculation of distinct L. zosterae isolates shows that pathogen isolate, increasing dosage of inoculum, increasing temperature, and diminishing light increase disease severity, suggesting L. zosterae will cause increased damage to eelgrass beds with changing environmental conditions.

Large-scale impacts of sea star wasting disease (SSWD) on intertidal sea stars and implications for recovery

Disease outbreaks can have substantial impacts on wild populations, but the often patchy or anecdotal evidence of these impacts impedes our ability to understand outbreak dynamics. Recently however, a severe disease outbreak occurred in a group of very well-studied organisms–sea stars along the west coast of North America. We analyzed nearly two decades of data from a coordinated monitoring effort at 88 sites ranging from southern British Columbia to San Diego, California along with 2 sites near Sitka, Alaska to better understand the effects of sea star wasting disease (SSWD) on the keystone intertidal predator, Pisaster ochraceus. Quantitative surveys revealed unprecedented declines of P. ochraceus in 2014 and 2015 across nearly the entire geographic range of the species. The intensity of the impact of SSWD was not uniform across the affected area, with proportionally greater population declines in the southern regions relative to the north. The degree of population decline was unrelated to pre-outbreak P. ochraceus density, although these factors have been linked in other well-documented disease events. While elevated seawater temperatures were not broadly linked to the initial emergence of SSWD, anomalously high seawater temperatures in 2014 and 2015 might have exacerbated the disease’s impact. Both before and after the onset of the SSWD outbreak, we documented higher recruitment of P. ochraceus in the north than in the south, and while some juveniles are surviving (as evidenced by transition of recruitment pulses to larger size classes), post-SSWD survivorship is lower than during pre-SSWD periods. In hindsight, our data suggest that the SSWD event defied prediction based on two factors found to be important in other marine disease events, sea water temperature and population density, and illustrate the importance of surveillance of natural populations as one element of an integrated approach to marine disease ecology. Low levels of SSWD-symptomatic sea stars are still present throughout the impacted range, thus the outlook for population recovery is uncertain.