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Abstract Salinity plays an important role in the physiology of marine organisms and their pathogens. As the climate continues to change, marine and estuarine salinities may become more extreme and irregular, potentially altering host–pathogen interactions. In this review, we explore and highlight the role of salinity on a range of pathosystems, outlining impacts to the host, pathogen and their interactions. We present fisheries and aquaculture-focused case studies, including behavioural preferences for freshwater in sea louse-infested trout and the use of freshwater as a delouser in salmon aquaculture, the effects of rainfall and drought on prevalence of a lethal parasitic infection of crustaceans, as well as low saline environments as refugia from oyster disease. We also present a case study of human health risks examining how changing coastal salinities can affect seafood-associated illnesses, and a conservation-directed case study examining the role of salinity and disease in defining the distribution of a habitat-forming species. Changing climatic conditions drive diverse salinity-altered epidemiological outcomes in these pathosystems. As salinities become more variable and less predictable, and fisheries management and marine conservation require increasing priority, future studies investigating multiple stressors, salinity included, on pathosystems will provide insights into organism and ecosystem resilience under future climate scenarios. This article is part of the theme issue ‘Managing infectious marine diseases in wild populations’. 

Abstract Infectious disease plays a key role in shaping marine communities, including in seagrass meadows, which form biodiverse coastal habitats. Eelgrass (Zostera marina) is the most widespread seagrass species and is susceptible to seagrass wasting disease, caused by the protist Labyrinthula zosterae. As a foundation species, eelgrass strongly influences ecosystem structure, function and services; recent work has begun to explore the links between critical community interactions and seagrass wasting disease. Here, we highlight recent advances about how the eelgrass community regulates and responds to seagrass wasting disease, from the microbiome to herbivores and filter feeders. We further show how efforts to model seagrass wasting disease progression can build on prior efforts to predict eelgrass growth and productivity and can inform our understanding of ecosystem health, resilience and vulnerability. As climate change alters environmental conditions, potentially favouring the wasting disease pathogen, efforts to integrate community interactions with disease ecology will be critical to forecast ecosystem dynamics and to develop effective coastal management strategies. We offer guidance on addressing major knowledge gaps in the study of eelgrass wasting disease in order to deepen both ecological theory and applied practices and identify how an integrated marine-disease-community ecology can inform a broader, cross-cutting understanding of disease. 

Abstract As a mechanism of the dilution effect, predation and filter feeding on parasitic propagules are hypothesized to reduce transmission to susceptible hosts and alter host–parasite interactions. In marine systems, the effect of other community members on the disease dynamics of microparasites in their suitable hosts is poorly known. In a coastal estuarine host–parasite system, we examined how eastern oysters,Crassostrea virginica, affect the transmission of a parasitic dinoflagellate,Hematodinium perezi, to juvenile blue crabs,Callinectes sapidus. We deployed juvenile blue crabs in custom mesh bags that were sandwiched by oysters into holo‐endemic areas, or areas with high endemic transmission for the parasite in juvenile hosts. Controls consisted of juvenile crabs deployed with an equivalent number of oyster shells to test for the effect of rugosity on transmission and crabs deployed alone. Deployments lasted 7–13 days and were done over different temporal and spatial scales. Results from the field deployments suggest that oysters, not shells, reduced the probability of infection to crab hosts. To investigate consumption in the laboratory, single oysters in 1 L aquaria were fed dinospores ofH. perezireleased from infected crabs. Oysters reduced parasite densities in the water at rates similar to those observed for a common phytoplankton,Tetraselmis chui, that is grown specifically as oyster food. Our results jointly support that oysters benefit adjacent community members through feeding on transmissive stages of their pathogens and highlight the need for additional field‐based approaches addressing environmental heterogeneity in pathogen transmission. 

IntroductionSeagrass 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 opportunisticLabyrinthulaspp., 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. MethodsEelgrass (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 infectiousLabyrinthula 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. ResultsTwo days after exposure, more than 90% of plants had visible lesions andLzDNA was detectable in 100% percent of sampled plants in theLzexposed treatment. Concentrations of total phenolic compounds were lower after 5 days of combined exposure to warmer temperatures andLz, but were unaffected in other treatments. Concentrations of condensed tannins were not affected byLzor temperature, and did not change over time. Analysis of the eelgrass transcriptome revealed 540 differentially expressed genes in response toLzexposure, 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. DiscussionThe lack of a significant temperature signal was unexpected but suggests a more pronounced physiological response toLzinfection 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.