Guild structure of larval trematodes in the snail Helisoma anceps: patterns and processes at the individual host level

Factors that influenced the infracommunity structure of trematodes parasitizing the pulmonate snail Helisoma anceps were studied over a 15-mo period; the guild included 8 species of parasites. Infracommunities were depauperate, with double patent infections observed in only 7 of 1,485 infected snails; a total of 4,899 was examined. Halipegus occidualis-Haematoloechus longiplexus was the most common dual infection. Both species share the same definitive host and, in both cases, eggs are the infective stage for the snail. Switches and losses of infections in individual snails were observed, suggesting the occurrence of dynamic interactions within the guild. A dominance hierarchy was constructed based on field observations and experimental infections. Echinostomatids were dominant; species without rediae in their life cycles were subordinates. Halipegus occidualis (which has rediae) was intermediate in dominance. Spatial and temporal heterogeneity in the distribution and abundance of trematode infective stages indicate that not all the snails have the same probability of becoming infected. Habitat structure, behavior of the definitive host, the nature of the infective stages, and snail population dynamics (mortality, recruitment, and size structure) generated spatial and temporal heterogeneity in this system. As a consequence, predictions of the probabilities of interspecific interactions based on an analysis of observed and expected frequencies of multiple infections could be inappropriate unless the potential sources of heterogeneity are considered.     

Structure and temporal variation of trematode and gastropod communities in a freshwater ecosystem.

The infro- and component community dynamics of digenetic trematodes in a freshwater gastropod community were examined over a 33-month period. The gastropod and trematode communities were composed of 17 and 10 species respectively. A total of 9,831 snails was collected; among them, 192 belonging to 14 species were infected by larval trematodes. The size of infected snails was significantly greater than that of healthy ones, and the increase of prevalence with size/age was interpreted as related to the increased probability of ultimately becoming parasitized. The trematode community was rich in allogenic species, but the most frequent trematode (cercariaeum) was autogenic and generalist (a range of 12 snail host species). There was a significantly positive relationship between the frequency of trematode species in the community and the number of first intermediate host species. A great temporal heterogeneity occurred in the prevalence of the snails, mainly attributed to the great temporal fluctuations of snail host populations and the variability of freshwater ecological conditions. The data on the occurrence of larval trematodes in 14 host species over the 33-month study allowed indicate a significant negative correlation between the abundance of gastropods and the prevalence of trematodes.     

The component community structure of larval trematodes in the pulmonate snail Helisoma anceps

Factors that affected the component community structure of larval trematodes in the pulmonate snail Helisoma anceps in Charlie's Pond, North Carolina, were studied over a 15-mo period using a multiple mark-recapture protocol. Patent infections of 8 species were observed in 1,485 of 4,899 snails examined. Reproductive activity, population size, and survival rate of the snail population were estimated to evaluate the extent of resource availability for the parasites. Antagonistic interactions between trematode species that occurred at the infracommunity level had a neglible effect on the composition and structure of the component community. The patterns observed at this level were related to temporal heterogeneity in the abundance of infective stages (mostly miracidia), differential responses of trematode species to the diverse and constantly changing distribution of snail size and abundance, differential mortality of snails infected with certain trematode species, constant recruitment of 1 trematode species over time, and the existence of predictable disturbances such as the complete mortality of the host population and recruitment of a replacement population during a 6-8 wk period. The last factor operated as a reset mechanism for this snail-trematode system once each year. A model of patch dynamics, with snails as patch resources, best explains the organization and dynamics of this system.     

Spatial heterogeneity in recruitment of larval trematodes to snail intermediate hosts

Spatial variation in parasitism is commonly observed in intermediate host populations. However, the factors that determine the causes of this variation remain unclear. Increasing evidence has suggested that spatial heterogeneity in parasitism among intermediate hosts may result from variation in recruitment processes initiated by definitive hosts. I studied the perching and habitat use patterns of wading birds, the definitive hosts in this system, and its consequences for the recruitment of parasites in snail intermediate hosts. Populations of the mangrove snail, Cerithidea scalariformis, collected from mangrove swamps on the east coast of central Florida are parasitized by a diverse community of trematode parasites. These parasites are transmitted from wading birds, which frequently perch on dead mangrove trees. I tested the hypothesis that mangrove perches act as transmission foci for trematode infections of C. scalariformis and that the spatial variation of parasitism frequently observed in this system is likely to emanate from the distribution of wading birds. On this fine spatial scale, definitive host behaviors, responding to a habitat variable, influenced the distribution, abundance and species composition of parasite recruitment to snails. This causal chain of events is supported by regressions between perch density, bird abundance, bird dropping density and ultimately parasite prevalence in snails. Variation between prevalence of parasites in free-ranging snails versus caged snails shows that while avian definitive hosts initiate spatial patterns of parasitism in snails through their perching behaviors, these patterns may be modified by the movement of snail hosts. Snail movement could disperse their associated parasite populations within the marsh, which may potentially homogenize or further increase parasite patchiness initiated by definitive hosts.     

Rapid colonisation of Lymnaea stagnalis by larval trematodes in eutrophic ponds in central Europe

High recruitment rates of multiple species and hierarchical competition are the keys to a competitive exclusion model of community assembly in larval trematode communities in molluscs. Eutrophic environments provide conditions for accelerating trematode transmission and this would increase the strength of interspecific interactions. To test these predictions, we provide the first known assessment for a pulmonate snail host, and for highly productive aquatic environments, of the rates of colonisation and extinction at the level of individual snail host patches, of a large guild of trematode species. Using a uniquely large dataset from a relatively long-term mark-recapture study of Lymnaea stagnalis in six eutrophic fishponds in central Europe, we demonstrate extraordinarily rapid colonisation by trematodes of a snail host, thus meeting the assumptions of the competitive exclusion model. Overall annual colonisation rates ranged from 243% to 503% year⁻¹ so that the odds of trematode establishment in an individual snail in these ponds are two to five times per year. Extinction rates were substantially lower than colonisation rates and, therefore, would not result in turnover rates high enough to significantly affect prevalence patterns in the snail populations. At the species level, analyses of sample-based estimates of probabilities of colonisation revealed that shared species traits associated with transmission and competitive abilities determined the limits of colonisation abilities. Colonisation rates were exceedingly high for the species transmitted to the snails passively via eggs. There was a significant effect of species competitive abilities on colonisation rates due to subordinate species being substantially better colonisers than both strong and weak dominants, a pattern consistent with the predictions of the competition-colonisation trade-off hypothesis. Our results suggest that, with the extraordinarily high trematode colonisation potential in the area studied, the spatial and temporal patterns of intraspecific heterogeneity in recruitment may provide conditions for intensification of interspecific interactions so that complex community assembly rules may be involved.     

Evaluating the role of regional and local processes in structuring a larval trematode metacommunity of Helisoma trivolvis

Metacommunity theory has advanced our understanding of how local and regional processes affect the structure of ecological communities. While parasites have largely been omitted from metacommunity research, parasite communities can provide the large sample sizes and discrete boundaries often required for evaluating metacommunity patterns. Here, we used assemblages of flatworm parasites that infect freshwater snails (Helisoma trivolvis) to evaluate three questions: 1) what factors affect individual host infections within ponds? 2) Is the parasite metacommunity structured among ponds? And 3) what is the relative role of local versus regional processes in determining metacommunity structure and species richness among ponds? We examined 10 821 snails from 96 sites in five park complexes in the San Francisco Bay area, California, and found 953 infections from six parasite groups. At the within‐pond level, infection status of host snails correlated positively with individual snail size and pond infection prevalence for all six parasite groups. Using an ordination method to test for metacommunity structure, we found that the parasite metacommunity was organized in a non‐random pattern with species responding individually along an environmental gradient. Based on a model selection approach involving local and regional predictors, parasite species richness and metacommunity structure correlated with both local abiotic (pH and total dissolved nitrogen) and biotic (non‐host mollusk density, and H. trivolvis biomass) factors, with little support for regional predictors. Overall, this trematode metacommunity most closely followed the predictions from the species sorting or mass effects metacommunity paradigm, in which community diversity is filtered by local site characteristics.     

Host diversity begets parasite diversity: bird final hosts and trematodes in snail intermediate hosts

An unappreciated facet of biodiversity is that rich communities and high abundance may foster parasitism. For parasites that sequentially use different host species throughout complex life cycles, parasite diversity and abundance in 'downstream' hosts should logically increase with the diversity and abundance of 'upstream' hosts (which carry the preceding stages of parasites). Surprisingly, this logical assumption has little empirical support, especially regarding metazoan parasites. Few studies have attempted direct tests of this idea and most have lacked the appropriate scale of investigation. In two different studies, we used time-lapse videography to quantify birds at fine spatial scales, and then related bird communities to larval trematode communities in snail populations sampled at the same small spatial scales. Species richness, species heterogeneity and abundance of final host birds were positively correlated with species richness, species heterogeneity and abundance of trematodes in host snails. Such community-level interactions have rarely been demonstrated and have implications for community theory, epidemiological theory and ecosystem management.     

Lowland boreal forests characterization in Algonquin Provincial Park relative to beaver (Castor canadensis) foraging and edaphic factors

Beaver (Caster canadensis) foraging and edaphic conditions can modify the vegetational characteristics of woody plant community in lowland boreal forests. Effective management of these areas requires an understanding of the relative contribution of these factors in shaping the woody plant community structure. Our objective was to quantify the effects of herbivory by beavers and edaphic conditions on woody plant community organization of lowland boreal forests surrounding beaver ponds. Woody vegetation and soils were sampled at 15 ponds occupied by beavers and one other pond abandoned by them in southern Algonquin Park, Ontario. We measured spatial variation in plant diversity, foraging rates and sapling recruitment of trees and shrubs along gradients of beaver foraging intensity and soil moisture, P, K, Mg, and pH. Beavers fed preferentially on a small number of deciduous species and the number of cut stems declined sharply with increasing distance from ponds. Conifers increased in relative dominance to deciduous species in the presence of beavers. Plant species richness and stem and basal area diversity peaked at intermediate distances (about 25 m) from ponds. Sapling recruitment by non-preferred species was positively related to foraging intensity. Total stem abundance and basal area and sapling recruitment by four preferred species (Populus tremuloides, Acer rubrum, Acer saccharum and Corylus cornuta) were negatively related to foraging intensity. However, by including Alnus rugosa and Salix bebbiana (also preferred by beavers) these patterns changed, becoming positively related to foraging intensity. There was also a pronounced gradient in soil moisture, which also decreased with distance from ponds. The other measured edaphic variables did not vary consistently with distance from ponds. Sapling recruitment in mesic versus xeric species varied consistently with hydrid conditions along the moisture gradient, such that variation in moisture also could produce the observed pattern of plant diversity. Diversity patterns changed three years after beaver abandonment of a pond, though sapling recruitment patterns in preferred and non-preferred species around the abandoned pond were similar to the occupied ponds. These observations suggest spatial variation in woody plant richness and diversity could be determined by combined effects of both herbivory (disturbance by beavers) and variable responses of different species to edaphic conditions.     

Whether larval amphibians school does not affect the parasite aggregation rule: testing the effects of host spatial heterogeneity in field and experimental studies

Almost all macroparasites show over‐dispersed infections within natural host populations such that most parasites are distributed among a few heavily‐infected individuals. Despite the importance of parasite aggregation for understanding system stability, the potential for population regulation, and super‐spreading events, many questions persist about its underlying drivers. Theoretically, aggregation results from heterogeneity in host exposure, resistance, and tolerance. However, few studies have examined how host spatial arrangement – which likely affects both parasite encounter and density‐dependent interactions – influences infection and dispersion, representing a critical gap in our current knowledge regarding the possible drivers of parasite aggregation. Using field data from over 165 ponds and 8000 hosts, we evaluated how the spatial clustering of amphibian larvae within ponds 1) varied among different amphibian species, and 2), affected the distribution of parasites within the host population using Taylor's power law. A complementary mesocosm experiment used field‐guided manipulations of the spatial arrangement of larval amphibians to create a gradient in host clustering while controlling host density, thereby testing for spatial effects on both infection success and aggregation by three different trematode species. Our field data indicated that larval amphibians exhibited significant spatial clustering that was well captured by Taylor's power law (R² 0.92 to 0.97 for different host species), but the residual variation only weakly correlated with observed patterns of trematode parasite over‐dispersion. Correspondingly, experimental manipulation of host clustering had no effects on parasite infection success or the degree of parasite aggregation among cages or mesocosms. Given the importance of parasite over‐dispersion for host populations and disease dynamics, we advocate for further investigations of host and parasite spatial aggregation, particularly studies that incorporate and/or control for heterogeneity in exposure and susceptibility.     

Relationship between Snail Population Density and Infection Status of Snails and Fish with Zoonotic Trematodes in Vietnamese Carp Nurseries

Background

Fish-borne zoonotic trematodes (FZT) are a food safety and health concern in Vietnam. Humans and other final hosts acquire these parasites from eating raw or under-cooked fish with FZT metacercariae. Fish raised in ponds are exposed to cercariae shed by snail hosts that are common in fish farm ponds. Previous risk assessment on FZT transmission in the Red River Delta of Vietnam identified carp nursery ponds as major sites of transmission. In this study, we analyzed the association between snail population density and heterophyid trematode infection in snails with the rate of FZT transmission to juvenile fish raised in carp nurseries.

Methodology/Principal Findings

Snail population density and prevalence of trematode (Heterophyidae) infections were determined in 48 carp nurseries producing Rohu juveniles, (Labeo rohita) in the Red River Delta area. Fish samples were examined at 3, 6 and 9 weeks after the juvenile fish were introduced into the ponds. There was a significant positive correlation between prevalence of FZT metacercariae in juvenile fish and density of infected snails. Thus, the odds of infection in juvenile fish were 4.36 and 11.32 times higher for ponds with medium and high density of snails, respectively, compared to ponds where no infected snails were found. Further, the intensity of fish FZT infections increased with the density of infected snails. Interestingly, however, some ponds with no or few infected snails were collected also had high prevalence and intensity of FZT in juvenile fish. This may be due to immigration of cercariae into the pond from external water sources.

Conclusions/Significance

The total number and density of potential host snails and density of host snails infected with heterophyid trematodes in the aquaculture pond is a useful predictor for infections in juvenile fish, although infection levels in juvenile fish can occur despite low density or absence infected snails. This suggests that intervention programs to control FZT infection of fish should include not only intra-pond snail control, but also include water sources of allochthonous cercariae, i.e. canals supplying water to ponds as well as snail habitats outside the pond such as rice fields and surrounding ponds.     

How large is the hand in the puppet? Ecological and evolutionary factors affecting body mass of 15 trematode parasitic castrators in their snail host

Parasitic castration is an adaptive strategy where the parasite usurps its host’s phenotype, most notably the host’s reproductive effort. Though castrators are loosely known to be large relative to their hosts (compared to typical parasites), their mass has rarely been quantified and little is known about size variation, even if such variation exists. By cross-sectioning snails, we examined intra- and inter-specific variation in the parasite/host mass of 15 trematode species that castrate the California horn snail, Cerithidea californica. Trematode species occupied 14–39% (mean = 20.3%) of an infected snail’s soft tissue mass. Intraspecific variation in castrator mass fluctuated with variables that covary with energy available for host reproduction. Specifically, trematode mass was 24% higher in summer than in winter, 15% greater in snails from intertidal flats than from tidal channels, and increased with host mass to the 1.37 power (a finding contrary to that previously documented for other types of parasites). Relative body mass differed across trematode species, varying interspecifically with: (1) taxonomic family, (2) host tissue use (larger species used more types of host-tissue), (3) position in the trematode interspecific competitive dominance hierarchy (the two most subordinate species were the largest, otherwise size tended to increase with dominance), and (4) type of host used by offspring (species whose offspring infect relatively predictably occurring benthic invertebrates were larger than those infecting transient vertebrates). Our findings suggest that ecological constraints and life history trade-offs between reproduction and survival influence the mass of these very large parasites.     

Host quality and spatial patterning in infections of the Eastern mudsnail (Ilyanassa obsoleta) by two trematodes (Himasthla quissetensis and Zoogonus rubellus)

Several studies have suggested that the fitness of a parasite can be directly impacted by the quality of its host. In such cases, selective pressures could act to funnel parasites towards the highest-quality hosts in a population. The results of this study demonstrate that snail host quality is strongly correlated with spatial patterning in trematode infections and that habitat type is the underlying driver for both of these variables. Two trematodes (Himasthla quissetensis and Zoogonus rubellus) with very different life cycles assume the same spatial infection pattern in populations of the first intermediate host (Ilyanassa obsoleta) in coastal marsh habitats. Infected snails are disproportionately recovered from intertidal panne habitats, which offer more hospitable environs for snails than do adjacent habitats (intertidal creeks, coastal flats, and subtidal creeks), in terms of protection from turbulence and wave action, as well as the availability of food stuffs. Snails in intertidal panne habitats are of higher quality when assessed in terms of average size-specific mass, growth rate, and fecundity. In mark-recapture experiments, snails frequently dispersed into intertidal pannes but were never observed leaving them. In addition, field experiments demonstrate that snails confined to intertidal panne habitats are disproportionately infected by both trematode species, relative to conspecifics confined to adjacent habitats. Laboratory experiments show that infected snails suffer significant energetic losses and consume more than uninfected conspecifics, suggesting that infected snails in intertidal pannes may survive better than in adjacent habitats. We speculate that 1 possible mechanism for the observed patterns is that the life cycles of both trematode species allows them to contact the highest-quality snails in this marsh ecosystem.     

Larval trematode infections and spatial distributions of snails

Abstract. The hypothesis that infecting trematodes influence the spatial distribution of the estuarine snail Ilyanassa obsoleta was tested. This work was conducted in the Savages Ditch habitat, Rehoboth Bay, DE, USA, which has an essentially flat, sandy-mud bottom bordered by saltmarsh shorelines and many infected snails. In 1996, two groups of snails were individually marked and released from one location after being screened for trematode infections. One group, transplanted from sites where snails tended not to be infected, consisted of snails that tested as uninfected. The other group consisted of snails native to Savages Ditch. Species of trematode carried by each snail was recorded. Marked snails were found and their positions were recorded until 2001. Snails were in five infection categories: (1) not infected, and infected with (2) Himasthla quissetensis , or (3) Lepocreadium setiferoides or (4) Zoogonus rubellus , or (5) with both H. quissetensis and Z. rubellus . The results show that the spatial distributions of snails depended on whether or not they were infected and, if infected, on which trematode species they carried. To complete life cycles, these parasites must accomplish transmission from the first (the snail) to the second intermediate hosts by short-lived, swimming cercariae. These data do not allow resolution of why snails distributed as they did, but sighting distributions of infected snails can be related to distributions of second hosts and it is proposed that parasites engender host snail distributions that improve chances of transmission.     

Disturbance effects on plant community diversity: spatial scales and dominance hierarchies

It is proposed that evaluations of disturbance effects upon community diversity will be influenced by two factors currently overlooked in models addressing disturbance-diversity relationships: (1) the spatial scale of inquiry, and (2) the level of the species abundance (dominance) hierarchy at which the search for diversity is done. We analyzed how two disturbance types—cattle grazing and large flooding—affected community diversity at two spatial scales (stand and patch) and three levels of species dominance in a grassland of the Flooding Pampa, Argentina. The effect of disturbance interaction was also examined. Species diversity at the stand scale was reduced by either grazing or flooding. Both disturbances decreased community spatial heterogeneity. At the patch scale, diversity declined with flooding but was enhanced by grazing. Flooding increased diversity under grazing conditions. However, stand diversity was highest in the undisturbed grassland; pattern diversity was the salient feature in this condition. The combination of disturbances yielded the highest patch-scale diversity; grazing increased richness whilst flooding enhanced evenness. Comparisons among grassland conditions appeared scale-dependent. Moreover the extent of disturbance effects varied with the level of dominance hierarchy considered. We point out the relevance of site history and initial conditions, encompassing the possibility of disturbances interaction, to the patters produced by disturbance events. Effects perceived at different spatial scales, or in species positioned at separate dominance levels, may parallel meaningful changes in the relative importance of factors controlling species coexistence and community organization.     

Geographic variation in sterilizing parasite species and the Red Queen

The Red Queen hypothesis predicts that sexual reproduction should be favoured in locations where the risk of infection by virulent parasites is consistently high. When hosts are exposed to multiple parasites over their geographic range, the coevolving parasite species may vary among host populations. We surveyed 26 streams on the South Island of New Zealand to determine whether the frequency of snails ( Potamopyrgus antipodarum ) infected by various sterilizing trematode parasite species was correlated with the frequency of sexual individuals. We compared the results with a survey conducted over 20 years ago to determine whether the associations were consistent. We also evaluated different measures of parasite-mediated selection among populations, including prevalence of the most common local parasite (MCLP) species and parasite diversity to assess the best predictor of sexual reproduction among stream populations. The results showed that the relationship between male frequency and parasite infection is more geographically widespread than previously recorded. Additionally, we found that the prevalence of the MCLP was the best predictor of sex in habitats where hosts populations are infected with multiple parasites (approximately 15 trematode species). This study provides evidence that sexual snails occur more often in environments with high infection levels, and that the pattern of parasite-imposed selection is geographically variable. Support for the Red Queen may be strengthened by focussing on the MCLP, which may vary among host populations.     

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

Trematode larvae must generally invade a molluscan intermediate host, usually a gastropod, before they can reach reproductive maturity in another definitive host. The research literature to date has focused almost exclusively on the documented specificity between particular trematode species and particular molluscan hosts; little attention has been paid to gastropod species that do not appear to serve as hosts. We sampled Rhode Island and Massachusetts populations of the marine gastropod Crepidula fornicata to determine whether this widespread species serves as a first intermediate host for trematodes. We also sampled from the same habitat populations of Littorina littorea and Ilyanassa obsoleta, gastropods known to serve as first intermediate hosts for several trematode species. All individuals were examined by dissection for the presence of sporocysts, rediae, or developing cercariae. Although 4-28% of L. littorea (N=112) and I. obsoleta (N=84) were infected by larvae of at least one trematode species, no individuals of C. fornicata sampled from the same locations were so infected (N=136). A survey of the Biological Abstracts computer database indicates that snails in only about 10% of marine gastropod families are known to serve as first intermediate hosts for trematodes. We suggest that more attention be paid to marine gastropods that appear not to be infected by trematode miracidia. Such species may productively serve as new models for understanding trematode host specificity and gastropod resistance to infection.     

Environmental factors influencing community composition of gastropods and their trematode parasites in southern Ontario

Agricultural activity and landscape features have previously been associated with diversity and prevalence of trematode species in amphibian second intermediate hosts. In this study, the density, diversity, and size of snail first intermediate hosts, and the diversity and prevalence of their trematode species, were assessed in 2 types of ponds, i.e., those adjacent to cornfields and those from the same region in southwestern Ontario that were adjacent to nonagricultural settings. Species of trematodes included, but were not restricted to, those that are known parasites of larval and adult frogs. We also assessed landscape factors likely to influence use by definitive hosts. Presence of the herbicide atrazine in ponds was measured to check that ponds adjacent to agriculture had potential to be affected by agricultural runoff. Both snail size and the proportion of snails releasing cercariae were greater in nonagricultural ponds, contrasting with a previous finding of lower trematode infection in tadpoles from nonagricultural ponds. Percentage of forest cover was associated with prevalence of certain trematode species, but not with estimates of combined prevalence. Absence of relations of trematode prevalence to measures of road density also contrasted with previous studies. We interpret our results in light of how agricultural activity might influence trematode viability, snail growth, and use by wildlife definitive hosts, independently of landscape factors.     

Spatial and temporal structure of the trematode component community in Valvata macrostoma (Gastropoda, Prosobranchia)

We conducted the first comprehensive study on the spatiotemporal structure of trematode communities in the large-mouthed valve snail, Valvata macrostoma. A total of 1103 snails were examined monthly between May and October 2007 from Lake Konnevesi, Central Finland, from a shallow (1-2 m deep) and an offshore site (5-6 m deep), located ca. 50-70 m apart. Snails were infected by 10 trematode species. The species composition and prevalence were strikingly different between the sites with high species diversity in the shallow site (all 10 species; total prevalence of sporocysts/rediae 12.1%, metacercariae 55.4%) compared to the deeper site (3 species; prevalence 15.0% and 1.9%, respectively). This difference persisted throughout our study and is probably related to the spatial distribution of bird definitive hosts, whereas the seasonal parasite dynamics are likely to be affected by changes in the age-structure of the snail population. The probability of sporocyst infections increased with snail size, but no such trend was observed in redial or metacercarial infections which decreased with host size. Our results show that generally well-described spatiotemporal differences in trematode infection of molluscs can emerge in very narrow spatial and temporal scales, which emphasizes the importance of these factors in community studies.     

Temporal variations in the infection of a population of Cerithidea cingulata by larval trematodes in Kuwait Bay

The prosobranch gastropod Cerithidea cingulata (Gastropoda: Potamididae) in Kuwait Bay was examined for larval trematode infections over a 17-month period. A total of 2537 snails were examined and 1265 (49.9%) found to be infected with one or more species of trematodes. The component community in the snail comprised 12 species representing the families Cyathocotylidae (2), Echinostomatidae (2), Haplosplanchnidae (1), Heterophyidae (2), Microphallidae (1), Philophthalmidae (2), Plagiorchiidae (1) and Schistosomatidae (1). Cyathocotylid II (41.6%) was by far the most prevalent species followed by the microphallid (3.9%), the two species comprised 90% of the total trematode fauna. The prevalence of infection increased with shell size and was significantly higher in male (47%) than female (33%) snails. Multiple infections were observed in only 15 (1.2%) of the infected snails; cyathocotylid I and cyathocotylid II combination occurred 14 times and heterophyid I and the microphallid occurred once. Trematode species were more diverse and prevalent in winter, and cercarial shedding peaked in summer. Behaviour of the definitive host and snail population dynamics were probably the major contributors to the detected temporal pattern in the infections.     

Spatial heterogeneity in size and parasitism: How it arises in an estuarine snail population

Populations of Ilyanassa obsoleta, a snail abundant in estuaries along the Atlantic coast of North America, exhibit much unobvious spatial heterogeneity. Lifetime dispersal of snails stands to be great, as they live for decades and can make daily moves of tens of meters. They could distribute uniformly, but in many populations there are characteristic spatial distributions of differently sized and trematode-infected snails. This is the case where this work was done in the Savages Ditch habitat of Rehoboth Bay, Delaware, U.S.A.. The study was designed to explain how the spatial heterogeneity in that habitat arises as a consequence of snail movements. Individually marked snails were tested for trematode infections and released from five well-separated positions. Snails released from each position were in three groups: (1) natives, collected from the position area, and (2) large and (3) small “outlanders”, collected from a site in the habitat that was separated from all release positions, had enhanced tidal currents, and where both size classes were available. Locations of found, released snails were noted for up to four years. Depending on collection site, many native snails were parasitized. In any case, they always tended to disperse in the vicinity where initially collected. Large outlanders (∼ 21% infected) dispersed variably, moving back toward their source area from certain release positions, but not from others. Small outlanders (∼ 4% infected), almost regardless of where released, tended to disperse back toward their source area. Dispersals of these snail groups from the five release positions explains much about how the pattern of spatial heterogeneity seen in the habitat arises.