History and epizootiology of Haplosporidium nelsoni (MSX), an oyster pathogen in Delaware Bay, 1957–1980

Between 1957 and 1959, a previously unknown sporozoan parasite, now designated as Haplosporidium nelsoni (formerly Minchinia nelsoni), or MSX, killed 90–95% of the oysters in lower Delaware Bay. Native oysters have been studied for more than 20 years since then to determine long-term disease and mortality patterns resulting from this host-parasite association. Development of resistance to MSX-kill in native oysters has reduced disease mortality to about half the original level, even though the pathogen continues to be very active in the bay. Since the initial epizootic, MSX levels have fluctuated in a cyclic pattern with peaks every 6 to 8 years. Periods of low disease pressure follow very cold winters, while average or above average winter temperatures correlate with high MSX activity. During peak years, every oyster in the lower bay may become infected. Although the parasite is salinity limited, salinities in the lower bay, the area from which oysters are marketed, are nearly always favorable for MSX, and fluctuations in river flow have almost no effect on MSX in this region. Infection periods recur each summer. Some oysters die soon after becoming infected; others survive through winter, but die in spring as the pathogen compounds normal overwinter stresses. Many survivors are able to suppress or rid themselves of infections when temperatures approach 20°C in late spring. Resistance to MSX-kill in native oysters is not correlated with an ability to prevent infection, but with restriction of parasites to localized, nonlethal lesions. The persistence of “hot spots” for infection in areas where oysters are sparse, the lack of spores in infected oysters, and failure to transmit the disease experimentally lead to the hypothesis that an alternate or reservoir host produces infective stages of MSX.     

Climate change, precipitation and impacts on an estuarine refuge from disease

Background

Oysters play important roles in estuarine ecosystems but have suffered recently due to overfishing, pollution, and habitat loss. A tradeoff between growth rate and disease prevalence as a function of salinity makes the estuarine salinity transition of special concern for oyster survival and restoration. Estuarine salinity varies with discharge, so increases or decreases in precipitation with climate change may shift regions of low salinity and disease refuge away from optimal oyster bottom habitat, negatively impacting reproduction and survival. Temperature is an additional factor for oyster survival, and recent temperature increases have increased vulnerability to disease in higher salinity regions.

Methodology/Principal Findings

We examined growth, reproduction, and survival of oysters in the New York Harbor-Hudson River region, focusing on a low-salinity refuge in the estuary. Observations were during two years when rainfall was above average and comparable to projected future increases in precipitation in the region and a past period of about 15 years with high precipitation. We found a clear tradeoff between oyster growth and vulnerability to disease. Oysters survived well when exposed to intermediate salinities during two summers (2008, 2010) with moderate discharge conditions. However, increased precipitation and discharge in 2009 reduced salinities in the region with suitable benthic habitat, greatly increasing oyster mortality. To evaluate the estuarine conditions over longer periods, we applied a numerical model of the Hudson to simulate salinities over the past century. Model results suggest that much of the region with suitable benthic habitat that historically had been a low salinity refuge region may be vulnerable to higher mortality under projected increases in precipitation and discharge.

Conclusions/Significance

Predicted increases in precipitation in the northeastern United States due to climate change may lower salinities past important thresholds for oyster survival in estuarine regions with appropriate substrate, potentially disrupting metapopulation dynamics and impeding oyster restoration efforts, especially in the Hudson estuary where a large basin constitutes an excellent refuge from disease.     

Behavioral aspects of cold tolerance in blackchin tilapia,Sarotherodon melanotheron,at different salinities

Synopsis Cold tolerance and behavioral responses of blackchin tilapia, Sarotherodon melanotheron, to rapidly decreasing temperatures were investigated at salinities of 5, 15, and 35 parts per thousand (ppt). Cold tolerance did not significantly differ with salinity or social rank. Mean temperatures were 10.7° C for beginning loss of equilibrium, 9.6° C for complete loss of equilibrium, and 6.9° C for death at all salinities. Behavioral activity declined with decreasing temperature and ceased between 10–12° C. Certain behavioral actions were significantly more frequent at 15 or 35 ppt salinity than at 5 ppt. The northward range expansion by introduced populations of the blackchin tilapia in the United States probably will be limited by its lower lethal temperature limits, but may also be affected by temperatures at which social behavior becomes disrupted.     

Comparing urban food system characteristics and actions in US and Indian cities from a multi‐environmental impact perspective: Toward a streamlined approach

Food action plans in many global cities articulate interest in multiple objectives including reducing in‐ and trans‐boundary environmental impacts (water, land, greenhouse gas (GHG)). However, there exist few standardized analytical tools to compare food system characteristics and actions across cities and countries to assess trade‐offs between multiple objectives (i.e., health, equity) with environmental outcomes. This paper demonstrates a streamlined model applied for analysis of four cities with varying characteristics across the United States and India, to quantify system‐wide water, energy/GHG, and land impacts associated with multiple food system actions to address health, equity, and environment. Baseline diet analysis finds key differences between countries in terms of meat consumption (Delhi 4; Pondicherry 16; United States 59, kg/capita/year), and environmental impact of processing of the average diet (21%, 19%, <1%, <1% of community‐wide GHG‐emissions for New York, Minneapolis, Delhi, and Pondicherry). Analysis of supply chains finds city average distance (food‐miles) varies (Delhi 420; Pondicherry 200; United States average 1,640 km/t‐food) and the sensitivity of GHG emissions of food demand to spatial variability of energy intensity of irrigation is greater in Indian than US cities. Analysis also finds greater pre‐consumer waste in India versus larger post‐consumer accumulations in the United States. Despite these differences in food system characteristics, food waste management and diet change consistently emerge as key strategies. Among diet scenarios, all vegetarian diets are not found equal in terms of environmental benefit, with the US Government's recommended vegetarian diet resulting in less benefit than other more focused targeted diet changes.     

Effect of environmental factors on survival and growth of quahog parasite unknown (QPX) in vitro

Quahog parasite unknown (QPX) is a protistan microorganism associated with mass mortalities of hard clams (Mercenaria mercenaria) along the northeastern coasts of the United States and maritime Canada. Because several studies indicate modulatory effects of prevailing environmental parameters on disease outbreaks, this study tested the effect of major environmental parameters (temperature, salinity and oxygen concentration; individually or combined) on QPX survival in artificial seawater and parasite growth in culture media in vitro. Three QPX isolates from two different geographic locations were compared. Results indicated that in vitro growth of QPX was optimal in standard culture medium at 34 ppt between 20 °C and 23 °C. Additionally, significant differences in temperature optima were observed for geographically distinct QPX isolates (p < 0.001) confirming previous studies suggesting the existence of different QPX strains (or ecotypes). When tested in seawater, QPX exhibited opposite trends with higher survival at 15 °C and 15 ppt. Results also demonstrated limited survival and growth of QPX under anoxic conditions. Additionally, results showed that the parasite is able to survive extreme temperatures (−12 °C to 32 °C) suggesting that QPX could overcome short periods of extreme conditions in the field. These results contribute to a better understanding of interactions between QPX and its environment, but potential impacts of environmental conditions on QPX disease development need further work as it also involves clam response to these factors.     

Molecular, Serological, and Virulence Characteristics of Vibrio parahaemolyticus Isolated from Environmental, Food, and Clinical Sources in North America and Asia

Potential virulence attributes, serotypes, and ribotypes were determined for 178 pathogenic Vibrio parahaemolyticus isolates from clinical, environmental, and food sources on the Pacific, Atlantic, and Gulf Coasts of the United States and from clinical sources in Asia. The food and environmental isolates were generally from oysters, and they were defined as being pathogenic by using DNA probes to detect the presence of the thermostable direct hemolysin (tdh) gene. The clinical isolates from the United States were generally associated with oyster consumption, and most were obtained from outbreaks in Washington, Texas, and New York. Multiplex PCR was used to confirm the species identification and the presence of tdh and to test for the tdh-related hemolysin trh. Most of the environmental, food, and clinical isolates from the United States were positive for tdh, trh, and urease production. Outbreak-associated isolates from Texas, New York, and Asia were predominantly serotype O3:K6 and possessed only tdh. A total of 27 serotypes and 28 ribogroups were identified among the isolates, but the patterns of strain distribution differed between the serotypes and ribogroups. All but one of the O3:K6 isolates from Texas were in a different ribogroup from the O3:K6 isolates from New York or Asia. The O3:K6 serotype was not detected in any of the environmental and food isolates from the United States, and none of the food or environmental isolates belonged to any of the three ribogroups that contained all of the O3:K6 and related clinical isolates. The combination of serotyping and ribotyping showed that the Pacific Coast V. parahaemolyticus population appeared to be distinct from that of either the Atlantic Coast or Gulf Coast. The fact that certain serotypes and ribotypes contained both clinical and environmental isolates while many others contained only environmental isolates implies that certain serotypes or ribotypes are more relevant for human disease.     

Ecotypic differentiation and phenotypic plasticity in reproductive traits of Armadillidium vulgare (Isopoda: Oniscidea)

Armadillidium vulgare differed in growth and survivorship on two field sites. Growth rates were higher at a site with consistently higher quality food than at the other site where less high-quality food was produced and which was less predictably accessible. Survivorship was higher at the second site where temperature fluctuations were consistently smaller. Individuals from the two populations were kept for 6 months under the same food and temperature conditions and patterns of resource allocation to reproductive traits analysed. Members of the population from the site with good growth conditions had significantly higher reproductive allocation, by 13.5%, and larger broods, by 9.1%, than those from the site with poor growth conditions. Contrary to theoretical predictions, they also had significantly larger offspring, by 7.5% dry mass. Larger offspring survived better than small ones. This differential survivorship, by 20% for a 3.4% difference in live mass, was much more pronounced under conditions of moisture stress and under fluctuating temperature regimes. Larger offspring would therefore be at a selective advantage on the site with more severe temperature fluctuations. Phenotypic plasticity in reproductive traits in response to experimental changes in food quality, temperature and crowding were monitored. Reproductive allocation was increased by 20.8% under conditions of higher food quality, by 14.7% at higher temperatures, and by 12.5% under less crowded conditions. Brood size, but not offspring dry mass, increased when food quality increased. When crowding increased by 25.0%, the size of broods remained the same but the dry mass of individual offspring decreased by 11.2%. Members of the population from the site with more variable access to high-quality food showed more plasticity in reproductive traits in response to changes in food supply than members of the population from the site with the more predictable food supply. Members of the population from the site with more stable temperatures showed less plasticity to temperature changes than members of the population from the site with greater temperature fluctuations. It is concluded that the observed microevolutionary processes and phenotypic plasticity have adaptative value as responses to spatial and temporal heterogeneity in environmental conditions.     

Transmission of the haplosporidian parasite MSX Haplosporidium nelsoni to the eastern oyster Crassostrea virginica in an upweller system

The haplosporidian oyster parasite MSX (Multinucleated Sphere X) Haplosporidium nelsoni was transmitted to eastern oysters Crassostrea virginica. Hatchery-raised, MSX-free juvenile oysters were placed in upweller tanks. Water to the tanks was filtered through a screen with 1 mm2 openings and originated from the water column overlaying naturally infected oysters beds (MSX prevalence 17 to 57%). MSX was diagnosed by histopathological analysis. MSX-disease (57% prevalence) with increased mortality (19%) was observed 11 wk after the beginning of the exposure and mortality of 80% after 16 wk. The study demonstrates transmission of MSX via water-borne infectious agents capable of passing through a 1 mm filter.     

Molecular,serological, and virulence characteristics of Vibrio parahaemolyticus isolated from environmental,food, and clinical sources in North America and Asia

Potential virulence attributes, serotypes, and ribotypes were determined for 178 pathogenic Vibrio parahaemolyticus isolates from clinical, environmental, and food sources on the Pacific, Atlantic, and Gulf Coasts of the United States and from clinical sources in Asia. The food and environmental isolates were generally from oysters, and they were defined as being pathogenic by using DNA probes to detect the presence of the thermostable direct hemolysin (tdh) gene. The clinical isolates from the United States were generally associated with oyster consumption, and most were obtained from outbreaks in Washington, Texas, and New York. Multiplex PCR was used to confirm the species identification and the presence of tdh and to test for the tdh-related hemolysin trh. Most of the environmental, food, and clinical isolates from the United States were positive for tdh, trh, and urease production. Outbreak-associated isolates from Texas, New York, and Asia were predominantly serotype O3:K6 and possessed only tdh. A total of 27 serotypes and 28 ribogroups were identified among the isolates, but the patterns of strain distribution differed between the serotypes and ribogroups. All but one of the O3:K6 isolates from Texas were in a different ribogroup from the O3:K6 isolates from New York or Asia. The O3:K6 serotype was not detected in any of the environmental and food isolates from the United States, and none of the food or environmental isolates belonged to any of the three ribogroups that contained all of the O3:K6 and related clinical isolates. The combination of serotyping and ribotyping showed that the Pacific Coast V. parahaemolyticus population appeared to be distinct from that of either the Atlantic Coast or Gulf Coast. The fact that certain serotypes and ribotypes contained both clinical and environmental isolates while many others contained only environmental isolates implies that certain serotypes or ribotypes are more relevant for human disease.     

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

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

Sex-biased maternal investment in voles: importance of environmental conditions

Adaptive bias in sex allocation is traditionally proposed to be related to the condition of mothers as well as to the unequal fitness values of produced sexes. A positive relationship between mother condition and investment into male offspring is often predicted. This relationship was also recently found to depend on environmental conditions. We studied these causalities experimentally using a design where winter food supply was manipulated in eight outdoor-enclosed populations of field voles Microtus agrestis. At the beginning of the breeding season in spring, food-supplemented mothers seemed to be in a similar condition, measured as body mass, head width, body condition index and parasite load (blood parasite Trypanosoma), to non-supplemented mothers. Food supplements affected neither the litter size, the reproductive effort of mothers, nor the litter sex ratios at birth. However, food supplementation significantly increased the birth size of male offspring and improved their condition, as indicated by reduced parasite loads (intestinal Eimeria). Interestingly, mothers in good body condition produced larger male offspring only when environmental conditions were improved by food supplements. Although the adaptiveness of variation in mammalian sex ratios is still questionable, our study indicates that mothers in good condition bias their investment towards male offspring, but only when environmental conditions are favourable.     

Use of Competitive PCR to Detect and Quantify Haplosporidium nelsoni Infection (MSX disease) in the Eastern Oyster (Crassostrea virginica)

This study was undertaken to develop a quantitative polymerase chain reaction assay that would improve the utility of PCR for detecting Haplosporidium nelsoni (MSX), a serious parasite of the eastern oyster Crassostrea virginica. A competitive PCR sequence was generated from the H. nelsoni small subunit ribosomal DNA fragment, originally described by Stokes and colleagues, that was amplified by the same PCR primers and had similar amplification performance. Assays performed using competitor dilutions ranging from 0.05 to 500 pg/μl DNA were used to test oyster samples designated using histological techniques as having ``light' or ``heavy' MSX infections. Visual diagnoses were confirmed equally well with three methods: densitometry of ethidium-bromide-stained agarose, densitometry of SYBRGreen-stained polyacrylamide gels, and analysis by GeneScan 3.0 of fluorescent products detected in ultrathin gels. Oysters diagnosed as negative for MSX tested as negative or light by PCR. Oysters with light MSX infections generally had less than 5 pg/μl infectious DNA. Oysters with heavy infections generally corresponded to 5 pg/μl or greater competitor dilutions. Received September 3, 1999; accepted March 3, 2000.     

Northward expansion of a marine parasite: Testing the role of temperature adaptation

The known range of the eastern oyster (Crassostrea virginica) parasite, Perkinsus marinus, expanded into the northeastern United States in the early 1990s. We used both in vitro and in vivo data to test the hypothesis that the northward expansion was associated with a low-temperature adapted strain of the parasite. In vitro proliferation of nine P. marinus isolates from three geographic sites, Massachusetts and New Jersey in the new range, and South Carolina in the historic southern range, was measured at seven temperatures (5 to 35 °C) using a tetrazolium blue dye assay. We wanted to determine if there were between- and within-geographic location differences in the P. marinus proliferation rate, and if so, whether they were associated with temperature. We found no evidence of low-temperature adaptation based on the fact that net proliferation rates for isolates from all three geographic locations were similar at temperatures from 5 to 20 °C. On the other hand, at temperatures of 25 to 35 °C, the South Carolina isolates exhibited higher proliferation rates than the northern isolates suggesting possible high-temperature adaptation of parasite strains that are routinely exposed to higher temperatures. Although there was significant within-location variation among isolates, the data tended to group together by geographic location supporting the hypothesis that there is an important regional component to the proliferation rate of P. marinus isolates. A survey of published data showed that the temperature at which in vivo proliferation was first observed in oysters at sites from the Gulf of Mexico to Massachusetts was typically between 20 and 23 °C with no evidence of a geographic cline. These results lend support to the hypothesis that the recent warming trend in the northeastern US is the most likely explanation for the P. marinus range extension.     

Identification of a rosette-like agent as Sphaerothecum destruens, a multi-host fish pathogen

A recent threat to European fish diversity was attributed to an infectious pathogen, a rosette-like intracellular parasite carried by invasive cyprinids. Here we show that the emerging rosette-like agent is Sphaerothecum destruens, originally found to be responsible for disease outbreaks in salmon in the United States. Sequencing of the ribosomal internal transcribed spacer (ITS) DNA highlights some level of geographical isolation. Unlike the situation in the United States, its occurrence in invasive fishes presents a risk of spread from wild invasive populations to sympatric populations of susceptible native fish and as such represents a risk for fisheries, as movement of fish for stocking purposes is common practice.     

Epizootiology of Minchinia nelsoni in susceptible wild oysters in Virginia, 1959 to 1971

Twelve years after the haplosporidian parasite Minchinia nelsoni erupted causing severe epizootics of oysters in lower Chesapeake Bay, regular patterns of mortalities and disease prevalence persisted. Distribution changed with salinity and weather patterns, but in mesohaline areas (about 15–25 ‰) infective pressure remained high and relatively stable despite scarcity of oysters.Susceptible disease-free oytsers from low-salinity areas of the James River, the major seed area in Virginia, were transplanted annually to disease-prevalent areas for monitoring in trays. Mortalities were usually over 50% the first year and almost as high in the second year. Prevalences of the pathogen, called MSX, ranged from 35 to 50% in live oysters. Seasonal patterns of disease activity are depicted from 1960 through 1971, and they exhibit exceptional regularity for open-water conditions. Source and history of oysters, and timing of exposure are important variables that affect disease activity as well as size and age. The disease caused by MSX appears to be not contagious in trays.The patterns of disease and mortality obtained from susceptible wild oysters without previous exposure provided a basis for evaluating other stocks including genetic strains selected for disease resistance.     

Climate adaptation is not enough: warming does not facilitate success of southern tundra plant populations in the high Arctic

Rapidly rising temperatures are expected to cause latitudinal and elevational range shifts as species track their optimal climate north and upward. However, a lack of adaptation to environmental conditions other than climate – for example photoperiod, biotic interactions, or edaphic conditions – might limit the success of immigrants in a new location despite hospitable climatic conditions. Here, we present one of the first direct experimental tests of the hypothesis that warmer temperatures at northern latitudes will confer a fitness advantage to southern immigrants relative to native populations. As rates of warming in the Arctic are more than double the global average, understanding the impacts of warming in Arctic ecosystems is especially urgent. We established experimentally warmed and nonwarmed common garden plots at Alexandra Fiord, Ellesmere Island in the Canadian High Arctic with seeds of two forb species (Oxyria digyna and Papaver radicatum) originating from three to five populations at different latitudes across the Arctic. We found that plants from the local populations generally had higher survival and obtained a greater maximum size than foreign individuals, regardless of warming treatment. Phenological traits varied with latitude of the source population, such that southern populations demonstrated substantially delayed leaf‐out and senescence relative to northern populations. Our results suggest that environmental conditions other than temperature may influence the ability of foreign populations and species to establish at more northerly latitudes as the climate warms, potentially leading to lags in northward range shifts for some species.     

The biogeography of parasitism in sticklebacks: distance,habitat differences and the similarity in parasite occurrence and abundance

Similarity in parasite community composition often decreases with both increasing geographic distance and environmental dissimilarity between localities, though it is unknown whether similarity in local abundance of selected parasite species follows similar rules. We tested this using data on metazoan parasites in 126 stickleback (Gasterosteus aculeatus) populations, with locations from Eurasia, eastern North America, and western North America treated separately. Similarity values were regressed against pairwise distances between localities; after correcting for distance, the effect of environmental dissimilarity was assessed by splitting similarity values into those between pairs of localities with either similar, moderately different or very different salinity (freshwater, marine or brackish). For selected parasite species, pairwise similarity in abundance (mean no. parasites per host) were computed across all localities, and treated as above. Similarity in parasite community composition decreased with increasing distance between localities in all three geographic regions. A significant effect of environmental difference was found in all regions: for a given distance between two sites, their parasite communities were more similar if they were of the same salinity. Slopes for distance decay in similarity were consistently higher for eastern North America than for Eurasia. Among the 12 parasite species for which sufficient data were available, only 4 showed the expected relationship, i.e. the greater the geographical separation between host populations, the greater the difference in parasite abundance; also, significant effects of environmental differences in salinity were only found for 3 of these species. Our findings show that parasite communities of sticklebacks are structured by geographical distance and local salinity conditions. The results indicate that strong effects at the community level do not translate into corresponding effects at the population level, suggesting that parasite dispersal and population dynamics are controlled by different processes.     

Stochastic environmental fluctuations drive epidemiology in experimental host–parasite metapopulations

Environmental fluctuations are important for parasite spread and persistence. However, the effects of the spatial and temporal structure of environmental fluctuations on host–parasite dynamics are not well understood. Temporal fluctuations can be random but positively autocorrelated, such that the environment is similar to the recent past (red noise), or random and uncorrelated with the past (white noise). We imposed red or white temporal temperature fluctuations on experimental metapopulations of Paramecium caudatum, experiencing an epidemic of the bacterial parasite Holospora undulata. Metapopulations (two subpopulations linked by migration) experienced fluctuations between stressful (5°C) and permissive (23°C) conditions following red or white temporal sequences. Spatial variation in temperature fluctuations was implemented by exposing subpopulations to the same (synchronous temperatures) or different (asynchronous temperatures) temporal sequences. Red noise, compared with white noise, enhanced parasite persistence. Despite this, red noise coupled with asynchronous temperatures allowed infected host populations to maintain sizes equivalent to uninfected populations. It is likely that this occurs because subpopulations in permissive conditions rescue declining subpopulations in stressful conditions. We show how patterns of temporal and spatial environmental fluctuations can impact parasite spread and host population abundance. We conclude that accurate prediction of parasite epidemics may require realistic models of environmental noise.     

The effect of salinity on experimental infections of a Hematodinium sp. in blue crabs, Callinectes sapidus

The parasitic dinoflagellate Hematodinium sp. parasitizes blue crabs along the Atlantic seaboard of the United States. Infections in blue crabs have only been reported from waters where salinity is >11 practical salinity units (psu). Blue crabs maintain a hyperosmotic internal concentration at low salinities (0-5 psu), roughly comparable to 24 psu, and should be capable of maintaining an infection in low-salinity waters even if Hematodinium spp. cells are intolerant of low salinities. We tested this notion by observing the effect of low salinity on the progression of disease in crabs experimentally infected with the parasite. Blue crabs were acclimated to 5 psu or 30 psu salinity treatments. They were inoculated with Hematodinium sp. and necropsied 3, 7, 10, and 15 days post-inoculation. The low-salinity treatment did not have an effect on the proliferation of Hematodinium sp. infections in blue crabs; moreover, a greater proportion of infections in crabs in the low-salinity treatment developed dinospore stages than did those in the high-salinity treatment, indicating that salinity may affect the development of the parasite. However, dinospores from in vitro cultures rapidly became inactive when held in salinities <15 psu. Our experiments indicate that Hematodinium spp. can develop in blue crabs at low salinities, but that the parasite is incapable of transmission in this environment, which explains the lack of natural infections in crabs at low salinities.     

Simulated climate change,epidemic size,and host evolution across host–parasite populations

Climate change is causing warmer and more variable temperatures as well as physical flux in natural populations, which will affect the ecology and evolution of infectious disease epidemics. Using replicate seminatural populations of a coevolving freshwater invertebrate‐parasite system (host: Daphnia magna, parasite: Pasteuria ramosa), we quantified the effects of ambient temperature and population mixing (physical flux within populations) on epidemic size and population health. Each population was seeded with an identical suite of host genotypes and dose of parasite transmission spores. Biologically reasonable increases in environmental temperature caused larger epidemics, and population mixing reduced overall epidemic size. Mixing also had a detrimental effect on host populations independent of disease. Epidemics drove parasite‐mediated selection, leading to a loss of host genetic diversity, and mixed populations experienced greater evolution due to genetic drift over the season. These findings further our understanding of how diversity loss will reduce the host populations’ capacity to respond to changes in selection, therefore stymying adaptation to further environmental change.