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.     

Leaf litter transport and retention in a tropical Rift Valley stream: an experimental approach

The impact of a drought on freshwater snail and trematode communities was investigated in a lake. Before the drought, 15 gastropod species (Valvatidae, Planorbidae, Lymnaeidae, Ancylidae, Physidae) and 10 trematode species (cercariaeum, xiphidiocercariae, echinostome, furcocercariae, notocotyle, lophocercous) were recorded. The rate of parasitism was 5.13% and there were 11 host species. The 2 major consequences of desiccation were the disappearance of snails, except Valvata piscinalis and Lymnaea peregra, and the absence of trematodes infecting the surviving snails. As soon as favourable conditions were restored, the littoral area was recolonized, first by hygrophilic and amphibious species, second by aquatic species. Nine months after the drought, the gastropod community was restored. Recolonization by the trematodes was delayed compared with that of gastropods. During the study, the overall prevalence was equal to 0.36% and only 4 trematode species and 5 host species were recorded. Because of the great variability of freshwater ecosystems, long-term studies are necessary to understand the dynamics of snail and trematode populations and determine the regulatory effect of parasitism in the field.     

Biodiversity of aquatic gastropods in the Mont St-Michel basin (France) in relation to salinity and drying of habitats

Seventy-seven polder and marsh stations were sampled for aquatic and riparian gastropods in the terrestrial basin of Mont St-Michel from March 1994–March 1995 and during a drought in 1996. In addition, four ponds with riparian coppice (bocage) were investigated. Twenty-one stations, mostly in polders, lacked snails; high values of conductivity, and by implication salinity, appeared to make the habitats unfavourable. Only the euryhaline hydrobid Potamopyrgus antipodarum and the amphibious lymnaeid Lymnaea truncatula survived in areas of high conductivity. In associated lagoons, the maximum level of salinity tolerated by freshwater gastropods was 35. Of the 14 species of gastropods collected, Lymnaea peregra and Anisus leucostoma were the most abundant. Stations had 1–7 species and the average gastropod species richness was 3.32 (±1.67). The canals that were the most susceptible to drying had a community of desiccation-resistant species including L. truncatula, Aplexa hypnorum, A. leucostoma and L. peregra. It appears that gastropod assemblages in the terrestrial basin of Mont St-Michel have achieved their current diversity by surviving strong ecological constraints.     

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.     

Differential Parasitism of Native and Introduced Snails: Replacement of a Parasite Fauna

The role of parasites in a marine invasion was assessed by first examining regional patterns of trematode parasitism in the introduced Japanese mud snail, Batillaria cumingi (= B. attramentaria), in nearly all of its introduced range along the Pacific Coast of North America. Only one parasite species, which was itself a non-native species, Cercaria batillariae was recovered. Its prevalence ranged from 3 to 86%. Trematode diversity and prevalence in B. cumingi and a native sympatric mud snail, Cerithidea californica, were also compared in Bolinas Lagoon, California. Prevalence of larval trematodes infecting snails as first intermediate hosts was not significantly different (14% in B. cumingi vs 15% in C. californica). However, while the non-native snail was parasitized only by one introduced trematode species, the native snail was parasitized by 10 native trematode species. Furthermore, only the native, C. californica, was infected as a second intermediate host, by Acanthoparyphium spinulosum(78% prevalence). Given the high host specificity of trematodes for first intermediate hosts, in marshes where B. cumingi is competitively excluding C. californica, 10 or more native trematodes will also become locally extinct.     

Effect of starvation on parasite-induced mortality in a freshwater snail (Potamopyrgus antipodarum)

The level of host exploitation is expected, under theory, to be selected to maximise (subject to constraints) the lifetime reproductive success of the parasite. Here we studied the effect of two castrating trematode species on their intermediate snail host, Potamopyrgus antipodarum. One of the trematode species, Microphallus sp., encysts in the snail host and the encysted larvae “hatch” following ingestion of infected snails by birds. The other species, Notocotylus gippyensis, by contrast, releases swimming larvae; ingestion of the snail host is not required for, and does not aid, transmission to the final host. We isolated field-collected snails for 3 months in the laboratory, and followed the survival of infected and uninfected snails under two conditions: not fed and fed ad libitum. Mortality of the infected hosts was higher than mortality of the uninfected ones, but the response to starvation treatment was parasite species specific. N. gippyensis induced significantly higher mortality in starved snails than did Microphallus. Based on these results, we suggest that host exploitation by different species of trematodes may depend on the type of transmission. Encysting in the snail host may select for a reduced rate of host exploitation so as to increase the probability of transmission to the final host. Received: 29 July 1998 / Accepted: 1 February 1999     

Host life history and the effect of parasitic castration on growth: A field study of Cerithidea californica Haldeman (Gastropoda : Prosobranchia) and its trematode parasites

The prevalence of parasitic infection by larval digenetic trematodes in natural populations of the mud snail, Cerithidea californica Haldeman, was found to increase with snail length; all snails ≥ 33 mm were infected. Distributions of infections by the seven most common larval trematodes were heterogeneous due to two species being more common than expected in the smaller size classes of snails, two being more common than expected in the larger size-classes of snails and three species being most prevalent in snails of intermediate length. The relative abundances of trematodes in different size-classes reflected these distributional patterns.A mark-recapture field study of snail growth rates failed to demonstrate that parasitic infection causes gigantism in Cerithidea. Parasitism tended to stunt the growth of juvenile snails and to a lesser degree, that of adult snails. The effects of trematodes on snail growth was shown to be species specific. This finding contrasts with those of earlier studies in which gigantic growth was observed in infected snails. This discrepancy is attributed to differences in the life histories of the host snails. It is predicted that gigantism will occur commonly in short-lived or semelparous species of snails but rarely, if ever, in long-lived iteroparous species which are predominately marine.     

Does parasitic infection compromise host survival under extreme environmental conditions? The case for Cerithidea californica (Gastropoda: Prosobranchia)

Summary This laboratory study examined the influence of parasitic infection by larval trematodes on the survival of extreme environmental conditions by the salt marsh snail, Cerithidea californica. Experimental treatments simulated the durations, combinations, and levels of potentially lethal environmental extremes to which the snail is exposed in its natural habitat, as determined from long-term field measurements. No significant difference was found in the rates of mortality suffered by infected and uninfected snails when exposed to simulated natural extremes of water temperature, water salinity, or exposure in air. Exposure to low levels of dissolved oxygen was the only treatment that caused differential mortality: infected snails died at higher rates than uninfected. This differential mortality was accentuated by high water temperature, and varied with the species of infecting parasite. The potential impact of this interaction between parasitism and anoxia on snail survival and population dynamics is discussed.     

香港淡水及海产贝类感染吸虫幼虫期的调查研究

作者于1983年4月及1986年4月二次在香港检查该地区20个村庄11种淡水螺(共11680粒),及14个海区包括红树林地带和无红树林的海滩中的22种海产贝类(共12580粒)。查获26种吸虫幼虫期,其中12种见于淡水螺(5种〕,8种寄生于红树林地带的海螺(7种),5种寄生于无红树林海区的贝类(6种),1种见于在红树林地带和无红树林海滩生存的海螺(5种)。寄生淡水螺的吸虫幼虫期分隶于Cortrematidae;Maseniidae;Schistosomatidae;Notocotylidae;Strigeidae;Paramphi-stomidae;Plagiorchidae;Philophthalmidaes;Microphallidae及Heterophyidae等科。寄生于海产贝类的吸虫幼虫期分隶于Philophthalmidae; Heterophyidae;Fellodistomidae; Cyathoco-tylidae;Echinostomatidae;Opecoelidae等科及Plagiorchioidea总科。     

洞庭湖外睾吸虫新种及其生活史

本文报告洞庭湖区鲶鱼肠道寄生的洞庭湖外睾吸虫Exorchis dongtinghuensis sp.nov(新种)及其全程生活史,其第一中间宿主为湖北钉螺Oncomelania hupensis;第二中间宿主为鲤鱼、鲫鱼和金鱼;终宿主为鲶鱼Parasilurus asotus。作者对各期宿主作了人工感染试验和现场自然感染调查。对其发育过程作了观察比较。     

Indirect effects of a parasite on a benthic community: an experiment with trematodes, snails and periphyton

1. Few studies have directly addressed the role played by parasites in the structure and function of ecosystems. Parasites influence the behaviour, reproduction and overall fitness of their hosts, but have been usually overlooked in community and ecosystem‐level studies. We investigated the effects of trematode parasites on snail–periphyton interactions. 2. Physa  acuta (Gastropoda: Pulmonata) snails infected with the trematode Posthodiplostomum minimum (often >30% of within‐shell biomass) grazed more rapidly than uninfected snails. Trematode effects on snail grazing indirectly affected the standing stock and community structure of periphyton. Populations of snails with 50% infected individuals reduced algal biomass by 20% more than populations with lesser (10% or 0%) infection rates. 3. The alga Cladophora glomerata dominated periphyton communities grazed by snail populations with 50% infection rates, whereas diatoms and blue–green algal taxa dominated when grazed by snail populations with lower infection rates. 4. Thus, trematodes indirectly affected periphyton communities by altering host snail behaviour, a trait‐mediated indirect effect. These results indicate that trematodes can indirectly influence benthic community structure beyond simple population fitness, with possible related effects on ecosystem function.     

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.     

Interspecific Interactions Among Larval Trematode Parasites of Freshwater and Marine Snails

Freshwater and marine snails serve as intermediate hosts fornumerous species of larval trematodes. Any particular populationof snails may be infected by several species. It is commonlyobserved that mixed species infections are less frequent thanexpected by change in collections of host snails from naturalpopulations. While several mechanisms might generate such negativeassociations, laboratory studies of freshwater snail-trematodeassociations have demonstrated the presence of strong antagonisticinteractions between intramolluscan larval stages (rediae andsporocysts) of species that infect the same host individual.Both predatory and non-predatory antagonism has been observed,the former taking the form of predation by large, dominant redialforms on the sporocysts and rediae of subordinate species. Theseinteractions are largely hierarchical, although in some systemspriority effects have been observed, and in one case a sporocystspecies replaced a redial species by strong non-predatory antagonism.Several instances of positive association between larval trematodespecies have also been observed. In such cases, interferencewith host defense mechanisms by the first parasite appears toenhance superinfection by the second. My own study of the larvaltrematode guild that infects the salt marsh snail, Cerithideacalifornica, has revealed patterns of association and interactionthat are very similar to those demonstrated by laboratory studiesof freshwater systems. Ultimately, the frequency of interactionsamong larval trematodes depends on the availability, relativeto the numbers of susceptible snails, of infective eggs andmiracidial larvae transmitted from definitive hosts.     

Parasites and salinity: costly tradeoffs in a threatened species

Parasites and environmental conditions can have direct and indirect effects on individuals. We explore the relationship between salinity and parasites in an endemic New Mexico State threatened fish, the White Sands pupfish (Cyprinodon tularosa). Spatial variation in salinity limits the distribution of the endemic springsnail (Juturnia tularosae) within Salt Creek, a small desert stream. The springsnail is the presumed intermediate host for trematodes that infect the White Sands pupfish, and trematode prevalence and intensity in pupfish are positively associated with the springsnail. Salinity and parasites both have negative impacts on pupfish, but in areas of high salinity, pupfish can effectively escape parasites. Pupfish trematodes were absent from sites lacking snails. At the upstream site, the absence of parasites and lower variance in salinity were correlated with larger pupfish that were in better condition than pupfish at either the middle or lower sites. Springsnails were present in the middle section, an area with moderate salinity, and all pupfish had trematodes (median abundance 847 trematodes/fish). Lipid levels and condition were lowest in fish from the middle site. Additionally, fewer older fish indicated an increased mortality rate. At the lower site, springsnails were absent due to high salinity; pupfish trematode abundance was much lower (six trematodes/fish), and fish condition was intermediate. An additional experiment revealed that snail activity and survival were significantly reduced at high salinities commonly present at the lower site. Although both high salinity and parasites significantly affect pupfish, parasites might be more detrimental.     

Introduction,distribution, spread,and impacts of exotic freshwater gastropods in Texas

We examined the patterns of distribution, vectors of introduction, and potential ecological impacts of freshwater exotic species in Texas over the last 45 years. Currently, five species of exotic gastropods are established: channeled-type applesnail (Pomacea insularum), red-rim melania (Melanoides tuberculatus), quilted melania (Tarebia granifera), giant rams-horn snail (Marisa cornuarietis), and Chinese mysterysnail (Cipangopaludina chinensis). In contrast to the northern part of the US, where shipping appears to be the most important vector for the introduction of aquatic invasive species, aquarium and ornamental trade dominated among unintentional vectors of introduction of all freshwater exotics in Texas, resulting in different patterns of distribution, spread, and ecological impacts. The rate of spread of exotic gastropods in Texas varied from 39 waterbodies colonized over 18 years for P. insularum to only three waterbodies during last 45 years for C. chinensis. Four of five exotic gastropods were found in highly vulnerable aquifer-fed springs and rivers, which contain numerous endemic and endangered species. The fifth species, Pomacea insularum, is an agricultural pest. Potential negative ecological effects of exotic gastropods include impacts on wetlands and wetland restoration, competitive exclusion of native snails, and the introduction of exotic parasites, trematodes, which could infect fish and waterfowl, including federally protected species. Aquifer springs with stable temperature regimes are refuges for both cold and warm intolerant species. Handling editor: D. Dudgeon     

Biogeographical patterns of marine larval trematode parasites in two intermediate snail hosts in Europe

Aim We used published inventories of trematodes in Littorina littorea (L.) and Hydrobia ulvae (Pennant) in European seas to search for two basic biogeographical patterns in the spatial occurrence of various trematode species: (1) do parasite distribution and richness patterns in the two host snails overlap with known ecoregions of free‐living organisms; and (2) does trematode species richness in the snails follow latitudinal or longitudinal gradients? Location North East Atlantic. Methods We used multidimensional scaling (MDS), analysis of similarity (ANOSIM) and analysis of variance (ANOVA) to test whether there were overlaps of parasite distribution and richness with known ecoregions of free‐living organisms. In addition, we used linear regression analyses to test whether trematode richness in snails (corrected for sampling effort) was correlated with the latitude or longitude of the sampling sites. Results When corrected for sampling effort, mean trematode species richness per site did not differ among the different ecoregions in L. littorea. In contrast, in H. ulvae, mean species richness was much lower for sites from the Celtic Sea compared with sites from the Baltic Sea and the North Sea. Based on the results of MDS analyses, trematode species composition was distinct among ecoregions; in particular, communities from the Baltic Sea differed markedly from communities in the Celtic Sea, for both snail species. Latitude and longitude were not significantly correlated with parasite species richness in either snail species. Most trematode species had restricted distributions, and only three species in L. littorea and five species in H. ulvae occurred at more than 50% of the sites. Main conclusions There is more structure in the large‐scale distribution of trematodes in gastropods than one would expect from the large‐scale dispersal capabilities of their bird and fish final hosts. We propose mechanisms based both on limited dispersal via fish and bird final hosts and on gradients in environmental factors to explain the observed patterns.     

Can parasites be indicators of free-living diversity? Relationships between species richness and the abundance of larval trematodes and of local benthos and fishes

Measuring biodiversity is difficult. This has led to efforts to seek taxa whose species richness correlates with the species richness of other taxa. Such indicator taxa could then reduce the time and cost of assessing the biodiversity of the more extensive community. The search for species richness correlations has yielded mixed results, however. This may be primarily because of the lack of functional relationships between the taxa studied. Trematode parasites are highly promising bioindicators. Diverse assemblages of larval trematode parasites are easily sampled in intermediate host snails. Through their life cycles these parasites are functionally coupled with the surrounding free-living diversity of vertebrate and invertebrate animals. It has been shown that larval trematodes in snails correlate positively with bird diversity and abundance. Here, we explore whether trematodes also correlate with standard measures of fishes, and large and small benthos, for 32 sites in three wetlands. We found associations between trematodes and benthic communities that were not consistent across wetlands. The associations were, however, consistently positive for large benthic species richness and density. Some of the contrasting associations between trematode and benthos may be explained by negative associations between large and small benthos. We found no associations with fish communities (probably because of the inadequacy of standard “snapshot” sampling methods for highly mobile fishes). The results support further exploration of trematodes as bioindicators of diversity and abundance of animal communities. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Long-term dynamics and community structure of freshwater gastropods exposed to parasitism and other environmental stressors

1. Freshwater communities are subject to various abrupt environmental disturbances and increasing pollution levels in their habitat. According to the nature and the periodicity of disturbances (i.e. punctuated, recurrent or continuous), one may expect different kinds of response of the target gastropod communities. 2. The gastropod community of a French lake has been investigated for 10 years to study its responses to three main environmental disturbances: supra‐seasonal droughts, recurrent proliferations of cyanobacteria and parasitism by trematodes. The relationship between this latter factor, considered as a continuous stressor, and the gastropod community was a particular focus. 3. A total of 13 280 gastropods belonging to 17 species (mostly Pulmonata with Planorbidae as the dominant family) have been sampled in the lake over the 10‐year time series. Species composition of the gastropod community varied strongly during the study period as did species richness (0–14, mean of 6.5 ± 0.4 species) and abundance of gastropods (0–4456, mean of 192 ± 72 individuals). Trematode larvae belonging to 11 morphotypes of cercariae were detected in 15 of the 17 species of gastropods and had a total prevalence of 2.9%. 4. Droughts (punctuated stressor) were responsible for the disappearance of nearly all gastropod species, and thus represent a severe environmental stressor for them. Nevertheless, the extent of this stressor was limited since the gastropod community was highly resilient. 5. Recurrent proliferations of toxic cyanobacteria (recurrent stressor) coincided with the large decline of the gastropod community, corresponding to a strong decrease in abundance and species richness. Intoxication of gastropods by toxic cyanobacteria was demonstrated by the bioaccumulation of microcystins in all gastropod tissues. 6. Trematode parasitism (continuous stressor) did not affect the gastropod community despite the possibly strong impact of parasites on some host species (those with monthly prevalence as high as 100% at some times). Indeed, abundance and species richness of gastropods were scarcely influenced by trematode prevalence or species richness.     

Experimental exposure of juvenile snails (Potamopyrgus antipodarum ) to infection by trematode larvae (Microphallus sp.): infectivity, fecundity compensation and growth

Host-parasite interactions that result in host castration are evolutionarily similar to predator-prey interactions because both interactions terminate reproduction for the host or prey. Yet, host-parasite interactions differ from predator-prey interactions in that infected hosts remain alive and potentially can make adjustments to their life-history strategy before castration is complete. Here we exposed juvenile snails (Potamopyrgus antipodarum) to infection by a digenetic trematode (Microphallus sp.) in order to determine whether: (1) pre-reproductive individuals could be infected, (2) individuals that were exposed to infection shifted resources to early reproduction (fecundity compensation), and (3) infected individuals exhibit altered growth rates relative to uninfected individuals. We found that juveniles are susceptible to infection; hence P. antipodarum could be selected for earlier maturation in populations where the risk of infection is high. We also found that fecundity compensation does not occur in this snail. Finally, we found that Microphallus-infected snails exhibit altered growth rates; individuals infected as juveniles have lower growth rates and are smaller than uninfected snails. These results suggest that growth is altered by infection of a trematode parasite but reproduction in uninfected snails is not induced by exposure to trematode eggs. Received: 11 January 1998 / Accepted: 19 May 1998     

Different thermal conditions of lakes affect host–parasite systems: A case study of Viviparus contectus (Millet, 1813) and digenean trematodes

1. Thermal disturbance of aquatic ecosystems directly and/or indirectly affects interspecific interactions, including parasitism. Both hosts and parasites respond differently to environmental changes, thus, predicting how host–parasite systems behave under the influence of disturbance remains a challenge. The aim of the study was to check how the differences in thermal conditions of lakes affect life-history traits of hosts and the level of parasitism, using a Viviparus contectus–digenean trematodes model.
2. Overall, we examined 480 individuals of V. contectus collected from a thermally polluted lake (TPL) and a natural lake (NL). Host features, including body size and fecundity, as well as the prevalence and species richness of digenean trematodes in snail populations were investigated.
3. We found that V. contectus from the TPL were significantly larger, heavier, and females were more fertile than snails collected from the NL. A total of 20.4% of the collected snails were infected with digenean larvae. The species richness of parasites was twice as high in the NL compared to the TPL (six and three species, respectively). A significant difference in the percentage of snails infected with parasites was identified between both types of lakes, with a higher prevalence of V. contectus in the NL (31.3%) compared to the TPL (7.3%).
4. These results indicate that host–parasite systems follow the environmental changes in lakes due to thermal pollution by increasing fertility and metabolism rate of viviparid hosts and by decreasing the prevalence and diversity of digenean trematodes.