Temperature correlates with annual changes in Hematodinium perezi prevalence in blue crab Callinectes sapidus in Florida, USA

Parasitic dinoflagellate Hematodinium in marine decapod crustaceans: a review on current knowledge and future perspectives

Parasitic dinoflagellates of the genus Hematodinium are known to infect various marine crustaceans worldwide, especially crabs and several species of shrimp and lobster. Some of these species are new host species and components of commercial fishery products. These parasitic species are predominantly found in the hemolymph of the host and cause pathological changes and functional damage to organs and tissues, leading to death. In recent years, these parasites have infected important commercially valuable species, particularly in European waters, US waters, Australian waters, and recently in Shandong Peninsula in China. These Hematodinium pathogens were also reported to affect wild shrimp in Chinese waters and in the English North Sea. These rapid spreads affect crustacean aquaculture industries, where they are indeed a significant threat to the sustainability of the aquaculture of important crustaceans. The fishery products industries are also under pressure from the invasion of this pathogen, as the crab meat produced has a bitter taste, which may reduce its marketability. In response to these threats, this review was aimed at providing a broader understanding of the development of parasite distribution and ecological aspects of Hematodinium. In addition, the interaction of these pathogens with their hosts, the environmental drivers of Hematodinium disease, and future research perspectives were discussed.

Salinity and temperature affect the symbiont profile and host condition of Florida USA blue crabs Callinectes sapidus

Subtropical Florida blue crabs, Callinectes sapidus, exhibit differing life history traits compared to their temperate counterparts, likely influencing symbiont infection dynamics. Little information exists for Florida C. sapidus symbiont profiles, their distribution among various habitats, and influence on crab condition. Using histopathology, genomics, and transmission electron microscopy, we describe the first symbiont profiles for Florida C. sapidus occupying freshwater to marine habitats. Twelve symbiont groups were identified from 409 crabs including ciliophorans, digenean, microsporidian, Haplosporidia, Hematodinium sp., Nematoda, filamentous bacteria, gregarine, Callinectes sapidus nudivirus, Octolasmis sp., Cambarincola sp., and putative microcell. Overall, 78% of C. sapidus were documented with one or more symbiont groups demonstrating high infection rates in wild populations. Environmental variables water temperature and salinity explained 48% of the variation in symbiont groups among Florida habitats, and salinity was positively correlated with C. sapidus symbiont diversity. This suggests freshwater C. sapidus possess fewer symbionts and represent healthier individuals compared to saltwater populations. Crab condition was examined using the reflex action mortality predictor (RAMP) to determine if reflex impairment could be linked to symbiont prevalence. Symbionts were found positively correlated with crab condition, and impaired crabs were more likely to host symbionts, demonstrating symbiont inclusion may boost predictive ability of the RAMP application. The microsporidian symbiont group had a particularly strong effect on C. sapidus reflex response, and impairment was on average 1.57 times higher compared to all other symbiont groups. Our findings demonstrate the importance of considering full symbiont profiles and their associations with a spatially and temporally variable environment to fully assess C. sapidus population health.

Natural Transmission of Hematodinium perezi in Juvenile Blue Crabs (Callinectes sapidus) in the Laboratory

Hematodinium perezi is a dinoflagellate endoparasitic in marine crustaceans, primarily decapods. It occurs in juvenile blue crabs, Callinectes sapidus, at high prevalence levels and has severe pathogenic effects in this host. The life history outside the host has not been experimentally investigated and, until now, transmission using dinospores has not been successful. We investigated the natural transmission dynamics of H. perezi in the laboratory using small juvenile crabs, which are highly susceptible to infection in the field, and elevated temperatures, which are known to stimulate dinospore production. Natural water-borne transmission to naïve crabs varied between 7-100% and was not correlated with dinospore densities measured from their aquaria water. Infections appeared to develop quickly in naïve hosts at 25 °C, suggesting that elevated temperatures as seen in the late summer and early autumn have a strong influence on the transmission of H. perezi in natural systems.

The relationship of blue crab (Callinectes sapidus) size class and molt stage to disease acquisition and intensity of Hematodinium perezi infections

In the blue crab, Callinectes sapidus, early studies suggested a relationship between smaller crabs, which molt more frequently, and higher rates of infection by the dinoflagellate parasite, Hematodinium perezi. In order to better explore the influence of size and molting on infections, blue crabs were collected from the Maryland coastal bays and screened for the presence of H. perezi in hemolymph samples using a quantitative PCR assay. Molt stage was determined by a radioimmunoassay which measured ecdysteroid concentrations in blue crab hemolymph. Differences were seen in infection prevalence between size classes, with the medium size class (crabs 61 to 90 mm carapace width) and juvenile crabs (≤ 30 mm carapace width) having the highest infection prevalence at 47.2% and 46.7%, respectively. All size classes were susceptible to infection, although fall months favored disease acquisition by juveniles, whereas mid-sized animals (31-90 mm carapace width) acquired infection predominantly in summer. Disease intensity was also most pronounced in the summer, with blue crabs > 61 mm being primary sources of proliferation. Molt status appeared to be influenced by infection, with infected crabs having significantly lower concentrations of ecdysteroids than uninfected crabs in the spring and the fall. We hypothesize that infection by H. perezi may increase molt intervals, with a delay in the spring molt cycle as an evolutionary adaptation functioning to coincide with increased host metabolism, providing optimal conditions for H. perezi propagation. Regardless of season, postmolt crabs harbored significantly higher proportions of moderate and heavy infections, suggesting that the process of ecdysis, and the postmolt recovery period, has a positive effect on parasite proliferation.

Effects of temperature and salinity on prevalence and intensity of infection of blue crabs, Callinectes sapidus, by Vibrio cholerae, V. parahaemolyticus, and V. vulnificus in Louisiana

Coastal marine and estuarine environments are experiencing higher average temperatures, greater frequency of extreme temperature events, and altered salinities. These changes are expected to stress organisms and increase their susceptibility to infectious diseases. However, beyond these generalities, little is known about how environmental factors influence host-pathogen relationships in the marine realm. We investigated the prevalence and intensity of infections by Vibrio cholerae, V. parahaemolyticus, and V. vulnificus in blue crabs, Callinectes sapidus, from Louisiana saltmarshes in relation to temperature and salinity. We evaluated relationships for single measurements taken at the time of collection and for more complex measurements representing accumulated exposure to physiologically-stressful environmental conditions for up to 31 days prior to collection. We found that: (1) prevalence of infection varied across the Louisiana coast, (2) prevalence of all three Vibrio species was influenced by temperature and salinity, and (3) measurements that represent accumulated exposure to extreme conditions are useful predictors of infection prevalence and can provide insights into underlying biological mechanisms.

Differential host mortality explains the effect of high temperature on the prevalence of a marine pathogen

Infectious diseases threaten marine populations, and the extent of their impacts is often assessed by prevalence of infection (the proportion of infected individuals). Changes in prevalence are often attributed to altered rates of transmission, although the rates of birth, recovery, and mortality also determine prevalence. The parasitic dinoflagellate Hematodinium perezi causes a severe, often fatal disease in blue crabs. It has been speculated that decreases in prevalence associated with high temperatures result from lower rates of infection. We used field collections, environmental sensor data, and high-temperature exposure experiments to investigate the factors that change prevalence of infections in blue crab megalopae (post-larvae). These megalopae migrate from offshore waters, where temperatures are moderate, to marshes where temperatures may be extremely high. Within a few days of arriving in the marsh, the megalopae metamorphose into juvenile crabs. We found a strong negative association between prevalence of Hematodinium infection in megalopae and the cumulative time water temperatures in the marsh exceeded 34°C over the preceding two days. Temperatures this high are known to be lethal for blue crabs, suggesting that higher mortality of infected megalopae could be the cause of reduced prevalence. Experimental exposure of megalopae from the marsh to a temperature of 34°C resulted in higher mortality for infected than uninfected individuals, and decreased the prevalence of infection among survivors from 18% to 3%.

Hematodinium spp. infections in wild and cultured populations of marine crustaceans along the coast of China

The parasitic dinoflagellate Hematodinium spp. infects a broad range of marine crustaceans. Its epidemics have impacted wild populations of various commercial fishery species around the world and the sustainability of mariculture in China. To study the epidemiology of Hematodinium spp. in marine crustaceans along the coast of China, we conducted a broad survey of wild and cultured stocks of major crustacean species in 2013 to 2015. Hematodinium sp. infections were identified in wild stocks of Portunus trituberculatus from Huludao, Laizhou, Qingdao, Yangtze River Estuary and Zhoushan, and Scylla paramamosain from Shantou; and cultured stocks of Portunus trituberculatus and Penaeus monodon from a polyculture pond in Qingdao. In the polyculture pond, Hematodinium sp. infections were observed in Portunus trituberculatus from June until October, with peak prevalence (up to 90%) observed in late July to early August. Furthermore, Hematodinium sp. infection was identified for the first time in the giant tiger prawn Penaeus monodon in the polyculture system during the disease outbreak. Phylogenetic analysis indicated that the Hematodinium isolate infecting Penaeus monodon was identical to the isolate infecting the co-cultured Portunus trituberculatus, and it was grouped into H. perezi genotype II together with the other isolates reported in China. The Hematodinium sp. isolated from Portunus trituberculatus appeared to have similar life stages as the H. perezi genotype III isolated from the American blue crab Callinectes sapidus. Our study indicates that outbreaks of Hematodinium disease can be a significant threat to the widely used polyculture system for decapods in China that may be particularly vulnerable to such generalist pathogens.

Disease ecology of Hematodinium perezi in a high salinity estuary: investigating seasonal trends in environmental detection

The blue crab Callinectes sapidus has seen a general decline in population levels. One factor influencing mortality is infections by Hematodinium perezi, a dinoflagellate parasite. A 2 yr study was conducted in 2014 and 2015 to monitor H. perezi DNA within the Maryland (USA) coastal bays, comparing seasonal cycles in the abundance of parasite DNA in environmental samples to parasite presence in host blue crabs. A late summer to early fall peak in H. perezi infections in blue crabs was observed, consistent with previous work. Infection intensities matched this trend, showing a slow progression of low intensity infections early in the year, with a peak in moderate and heavy infections occurring between July and September, for both years. It was hypothesized that the peak in water column occurrence would coincide with those months when infection intensities were highest in blue crabs. As the peaks in water column occurrence were in July 2014 and August-September 2015, this is consistent with sporulation being the primary contributor to environmental detection in summer months. An additional peak in environmental detection occurred in both years during the early spring months, the cause of which is currently unknown but may be related to infections in overwintering crabs or alternate hosts. Several new crustacean hosts were identified within this estuary, including grass shrimp Palaemonetes spp. and the sand shrimp Crangon septemspinosa, as well as the mud crab Dyspanopeus sayi. Improved knowledge of this disease system will allow for better management of this important fishery.

Molecular detection of the parasitic dinoflagellate Hematodinium perezi from blue crabs Callinectes sapidus in Louisiana, USA

The dinoflagellate Hematodinium perezi is a prolific pathogen of the blue crab Callinectes sapidus along the Atlantic and Gulf of Mexico coasts of North America. High prevalence, sometimes approaching 100%, and outbreaks with high mortality are associated with higher salinities. H. perezi has not been reported previously in blue crabs from Louisiana, USA, where salinities in coastal habitats are generally below the parasite's favorable range. However, the possibility that H. perezi infects blue crabs in higher salinity habitats offshore has not been investigated. A PCR-based test for H. perezi was used to screen blue crabs collected from both high and low salinity areas. These included juvenile and adult crabs from inshore marshes where salinities are relatively low and from higher salinity offshore shoals that are spawning sites for females. H. perezi was detected in blue crabs from offshore shoals (prevalence=5.6%) but not in juvenile or adult crabs from inshore habitats. Megalopae (post-larvae) were also collected from inshore locations. Although megalopae settle inshore where salinities are relatively low, the megalopal stage is preceded by a planktonic phase in higher salinity offshore waters. We detected H. perezi in 11.2% of settling megalopae tested. Although the prevalence of H. perezi was relatively low within our samples, if spawning females and megalopae are especially vulnerable, the impact on the population could be compounded. This is the first report of H. perezi from blue crabs in Louisiana and demonstrates the importance of examining all life stages in determining the prevalence of a harmful parasite.

Territoriality and Conflict Avoidance Explain Asociality (Solitariness) of the Endosymbiotic Pea Crab Tunicotheres moseri

Host monopolization theory predicts symbiotic organisms inhabiting morphologically simple, relatively small and scarce hosts to live solitarily as a result of territorial behaviors. We tested this prediction with Tunicotheres moseri, an endosymbiotic crab dwelling in the atrial chamber of the morphologically simple, small, and relatively scarce ascidian Styela plicata. As predicted, natural populations of T. moseri inhabit ascidian hosts solitarily with greater frequency than expected by chance alone. Furthermore, laboratory experiments demonstrated that intruder crabs take significantly longer to colonize previously infected compared to uninfected hosts, indicating as expected, that resident crabs exhibit monopolization behaviors. While territoriality does occur, agonistic behaviors employed by T. moseri do not mirror the overt behaviors commonly reported for other territorial crustaceans. Documented double and triple cohabitations in the field coupled with laboratory observations demonstrating the almost invariable success of intruder crabs colonizing occupied hosts, suggest that territoriality is ineffective in completely explaining the solitary social habit of this species. Additional experiments showed that T. moseri juveniles and adults, when searching for ascidians use chemical cues to avoid hosts occupied by conspecifics. This conspecific avoidance behavior reported herein is a novel strategy most likely employed to preemptively resolve costly territorial conflicts. In general, this study supports predictions central to host monopolization theory, but also implies that alternative behavioral strategies (i.e., conflict avoidance) may be more important than originally thought in explaining the host use pattern of symbiotic organisms.