Crepidula fornicata is not a first intermediate host for trematodes: who is?

Histopathological survey of putative parasites and pathogens in non-native slipper limpets Crepidula fornicata

Two populations of the invasive slipper limpet Crepidula fornicata were sampled in Swansea Bay and Milford Haven, Wales, UK, to determine the presence of putative pathogens and parasites known to affect co-located commercially important shellfish (e.g. oysters). A multi-resource screen, including molecular and histological diagnoses, was used to assess 1800 individuals over 12 mo for microparasites, notably haplosporidians, microsporidians and paramyxids. Although initial PCR-based methods suggested the presence of these microparasites, there was no evidence of infection when assessed histologically, or when all PCR amplicons (n = 294) were sequenced. Whole tissue histology of 305 individuals revealed turbellarians in the lumen of the alimentary canal, in addition to unusual cells of unknown origin in the epithelial lining. In total, 6% of C. fornicata screened histologically harboured turbellarians, and approximately 33% contained the abnormal cells-so named due to their altered cytoplasm and condensed chromatin. A small number of limpets (~1%) also had pathologies in the digestive gland including tubule necrosis, haemocytic infiltration and sloughed cells in the tubule lumen. Overall, these data suggest that C. fornicata are not susceptible to substantive infections by microparasites outside of their native range, which may contribute in part to their invasion success.

Invasive slipper limpets (Crepidula fornicata) act like a sink, rather than source, of Vibrio spp

A large knowledge gap exists regarding the disease profile and pathologic condition of the invasive, slipper limpet Crepidula fornicata. To help address this, we performed a year-long health survey at two sites in South Wales, UK – subtidal Swansea Bay and intertidal Milford Haven. In total, 1,800 limpets were screened systematically for haemolymph bacterial burdens using both general and vibrio-selective growth media (TSA + 2% NaCl and TCBS, respectively), haemolymph (blood) inspection using microscopy, a PCR-based assay targeting Vibrio spp., and multi-tissue histology. Over 99% of haemolymph samples contained cultivable bacterial colony-forming units, and 83% of limpets tested positive for the presence of vibrios via PCR (confirmed via Sanger sequencing). Vibrio presence did not vary greatly between sites, yet a strong seasonal effect was observed with significantly higher bacterial loads during the summer. Binomial logistic regression models revealed larger (older) limpets were more likely to harbour vibrios, and the growth of bacteria on TCBS was a key predictor for PCR-based vibrio detection. Histological assessment of > 340 animals revealed little evidence of inflammation, sepsis or immune reactivity despite the gross bacterial numbers. We contend that slipper limpets are not highly susceptible to bacteriosis at either site, and do not harbour vibrios known to be pathogenic to humans. The lack of susceptibility to local pathogenic bacteria may explain, in part, the invasion success of C. fornicata across this region.

Invasive slipper limpets Crepidula fornicata are hosts for sterilizing digenean parasites

Invasion and spread of alien species can drive ecosystem changes, such as, the dynamics of infectious diseases. The non-native, marine gastropod Crepidula fornicata has become established across European coastlines over the last century, but there remains little insight into its disease carrying capacity and potential role as a source/sink of parasites. To address this knowledge gap, we surveyed limpets from two sites in South Wales, UK for signatures of disease/pathology using polymerase chain reaction-based methods (haemolymph) and histology (solid tissue). We encountered trematode-like parasites in ~1% individuals (5 out of 462). Three limpets displayed gross damage in the gonad, i.e. castration, and encysted metacercariae were found in the muscle of two other individuals. On the basis of 28S rDNA and internal transcribed spacer 2 genomic targets, we identified the gonad-infecting trematodes as members of the family Microphallidae – putative novel species related to the genus Longiductotrema. Earlier reports suggest that C. fornicata is not a host for trematode parasites in either its native or alien range but may act as a sink due to its filter feeding lifestyle. We provide clear evidence that C. fornicata is parasitized by at least one trematode species at two sites in Wales, UK, and likely act as a spillback or accidental host among native littorinids.

Three Strigeid cercariae from Littorina littorea snail, Qarun Lake, Fayoum, Egypt

Aim:

The present study aims to focus on the role of common marine snails (Littorina littorea) as a vector for some trematode parasites.

Materials and Methods:

A total of 327 marine water L. littorea snails were collected during the summer of 2016 from a Qarun lake in the EL-Fayoum Governorate, Egypt. The snails were investigated for infection by trematode parthenitae through induction of cercarial shedding by exposure to light and crushing the snails. The species were stored in Search Laboratory of Zoology Department, Faculty of Science, Fayoum University.

Results:

Three species of Strigeid littorina cercaria were identified from the infected snails. They are described here and they identified in relation to close-up morphological features and linked to its snail hosts. They give the following names: Cercaria strigeid littorina type 1, C. strigeid littorina type 2, and C. strigeid littorina type 3. The incidence of infection by these cercariae was 33%, 25.7%, and 2.4%, respectively.

Conclusion:

This study is clarifying the importance of this marine snail as intermediate hosts for new trematode species.

The ecology, evolution, impacts and management of host-parasite interactions of marine molluscs

Molluscs are economically and ecologically important components of aquatic ecosystems. In addition to supporting valuable aquaculture and wild-harvest industries, their populations determine the structure of benthic communities, cycling of nutrients, serve as prey resources for higher trophic levels and, in some instances, stabilize shorelines and maintain water quality. This paper reviews existing knowledge of the ecology of host-parasite interactions involving marine molluscs, with a focus on gastropods and bivalves. It considers the ecological and evolutionary impacts of molluscan parasites on their hosts and vice versa, and on the communities and ecosystems in which they are a part, as well as disease management and its ecological impacts. An increasing number of case studies show that disease can have important effects on marine molluscs, their ecological interactions and ecosystem services, at spatial scales from centimeters to thousands of kilometers and timescales ranging from hours to years. In some instances the cascading indirect effects arising from parasitic infection of molluscs extend well beyond the temporal and spatial scales at which molluscs are affected by disease. In addition to the direct effects of molluscan disease, there can be large indirect impacts on marine environments resulting from strategies, such as introduction of non-native species and selective breeding for disease resistance, put in place to manage disease. Much of our understanding of impacts of molluscan diseases on the marine environment has been derived from just a handful of intensively studied marine parasite-host systems, namely gastropod-trematode, cockle-trematode, and oyster-protistan interactions. Understanding molluscan host-parasite dynamics is of growing importance because: (1) expanding aquaculture; (2) current and future climate change; (3) movement of non-native species; and (4) coastal development are modifying molluscan disease dynamics, ultimately leading to complex relationships between diseases and cultivated and natural molluscan populations. Further, in some instances the enhancement or restoration of valued ecosystem services may be contingent on management of molluscan disease. The application of newly emerging molecular tools and remote sensing techniques to the study of molluscan disease will be important in identifying how changes at varying spatial and temporal scales with global change are modifying host-parasite systems.

A survey of larval digeneans in the commonest intertidal snails from Northern Patagonian coast, Argentina

Over a two-years period, a survey was carried out in order to increase the knowledge of digeneans parasitising the commonest intertidal gastropods on the Patagonian coast, Southwestern Atlantic Ocean. A total of 4,725 gastropods were examined. Six species of digenean parasitising four snail species were found; four of them were registered for first time: Maritrema sp. 1 (Microphallidae) in Crepidula dilatata (Calyptraeidae), Parorchis sp. (Philophtalmidae) and sporocyst of Renicolidae in Trophon geversianus (Muricidae), and Diphterostomum sp. (Zoogonidae) in Buccinanops globulosus (Nassariidae). Two other species were found in Siphonaria lessoni (Siphonariidae): Maritrema sp. 2 and Hemiuridae. One snail species, Tegula patagonica (Trochidae) was not parasitised. These gastropods act as first intermediate host, and C. dilatata, S. lessoni and B. globulosus also frequently host metacercariae within the sporocyst. Overall prevalences varied from 0.16% of Diphterostomum in the intertidal population of B. globulosus to 33.45% of Maritrema sp. 1 in C. dilatata.

Climate change, parasitism and the structure of intertidal ecosystems

Evidence is accumulating rapidly showing that temperature and other climatic variables are driving many ecological processes. At the same time, recent research has highlighted the role of parasitism in the dynamics of animal populations and the structure of animal communities. Here, the likely interactions between climate change and parasitism are discussed in the context of intertidal ecosystems. Firstly, using the soft-sediment intertidal communities of Otago Harbour, New Zealand, as a case study, parasites are shown to be ubiquitous components of intertidal communities, found in practically all major animal species in the system. With the help of specific examples from Otago Harbour, it is demonstrated that parasites can regulate host population density, influence the diversity of the entire benthic community, and affect the structure of the intertidal food web. Secondly, we document the extreme sensitivity of cercarial production in parasitic trematodes to increases in temperature, and discuss how global warming could lead to enhanced trematode infections. Thirdly, the results of a simulation model are used to argue that parasite-mediated local extinctions of intertidal animals are a likely outcome of global warming. Specifically, the model predicts that following a temperature increase of less than 4°C, populations of the amphipodCorophium volutator, a hugely abundant tube-building amphipod on the mudflats of the Danish Wadden Sea, are likely to crash repeatedly due to mortality induced by microphallid trematodes. The available evidence indicates that climate-mediated changes in local parasite abundance will have significant repercussions for intertidal ecosystems. On the bright side, the marked effects of even slight increases in temperature on cercarial production in trematodes could form the basis for monitoring programmes, with these sensitive parasites providing early warning signals of the environmental impacts of global warming.

Parasites alter host phenotype and may create a new ecological niche for snail hosts

By modifying the behaviour and morphology of hosts, parasites may strongly impact host individuals, populations and communities. We examined the effects of a common trematode parasite on its snail host, Batillaria cumingi (Batillariidae). This widespread snail is usually the most abundant invertebrate in salt marshes and mudflats of the northeastern coast of Asia. More than half (52.6%, n=1360) of the snails in our study were infected. We found that snails living in the lower intertidal zone were markedly larger and exhibited different shell morphology than those in the upper intertidal zone. The large morphotypes in the lower tidal zone were all infected by the trematode, Cercaria batillariae (Heterophyidae). We used a transplant experiment, a mark-and-recapture experiment and stable carbon isotope ratios to reveal that snails infected by the trematode move to the lower intertidal zone, resume growth after maturation and consume different resources. By simultaneously changing the morphology and behaviour of individual hosts, this parasite alters the demographics and potentially modifies resource use of the snail population. Since trematodes are common and often abundant in marine and freshwater habitats throughout the world, their effects potentially alter food webs in many systems.

Parasitism shaping host life-history evolution: adaptive responses in a marine gastropod to infection by trematodes

1. Variation in life-history strategies among conspecific populations indicates the action of local selective pressures; recently, parasitism has been suggested as one of these local forces. 2. Effects of trematode infections on reproductive effort, juvenile growth, size at maturity and susceptibility were investigated among different natural populations of the marine gastropod Zeacumantus subcarinatus, Sowerby 1855. 3. Reproductive effort was not higher in uninfected snails from populations experiencing a high trematode prevalence, but females from high prevalence populations produced significantly larger offspring compared with their conspecifics from other populations. 4. Juvenile growth rate was significantly higher in laboratory-raised snails originating from females in a high prevalence population compared with other populations. 5. Size at maturity, determined by the appearance of functional gonads, was significantly and negatively related to trematode prevalence, and positively related to mean snail size, across 10 populations in the study area. 6. There was no evidence of different host resistance against trematodes in sentinel snails from high and low prevalence populations exposed to the same infection pressure in the field. 7. Our results strongly indicate that Z. subcarinatus adapt to trematodes by reaching maturity early, thereby maximizing their chance of reproducing in populations experiencing a high prevalence of infection by castrating trematodes.

Biology of the schistosome genus Trichobilharzia

Trichobilharzia is the largest genus within the family Schistosomatidae, covering over 40 species of avian parasites. To clarify the existing confusion in the systematics of the genus, we recommend combining knowledge of life cycles and developmental stages, snail/bird hosts, cytogenetical and molecular data together with morphological criteria for the characterization of particular species. The high specificity of Trichobilharzia for the intermediate host is a likely reflection of the ability to avoid the internal defence of specific snails. The spectrum of final hosts (birds) seems to be much wider. The infection of birds--trichobilharziasis--may lead to considerable tissue injuries, caused by eggs of the parasite or migration of immature/mature worms through the body. Most Trichobilharzia (visceral species) migrate through the viscera of the host, but nasal species display a neurotropic mode of migration. Due to a low specificity of penetrating cercariae, mammals (including humans) can be attacked. This leads to cercarial dermatitis, predominantly in sensitized hosts. Experimental infections indicate that Trichobilharzia never mature in an incompatible (mammalian) host. However, not all cercariae and schistosomula are necessarily trapped and eliminated in the skin, and parasites may migrate throughout the viscera and the nervous system of mammals. These findings suggest that the pathogenicity of Trichobilharzia may have been underestimated in the past and health risks associated with trichobilharziasis need to be studied further.