TREMATODE SPECIES DETECTION AND QUANTIFICATION BY ENVIRONMENTAL DNA-qPCR ASSAY IN LAKE CHANY, RUSSIA

Environmental DNA (eDNA) surveys promise to be a sensitive and powerful tool for the detection of trematodes. This can contribute to the limited studies on trematode ecology, specifically in aquatic ecosystems. Here, we developed species-specific primer and probe sets for Moliniella anceps, Opisthioglyphe ranae, and Plagiorchis multiglandularis cercariae and applied a novel eDNA qPCR assay to detect larval trematodes quantitatively. We evaluated the effectiveness of the assays using filtered lake water samples collected from different sites of Lake Fadikha and Kargat River Estuary in Lake Chany, Russia, showing high species specificity and sensitivity in all 3 assays. Further, all 3 assays had high efficiencies ranging from 94.9 to 105.8%. Moliniella anceps, O. ranae, and P. multiglandularis were detected in the environmental water samples through real-time PCR. Thus, we anticipate that our approach will be beneficial for biomonitoring, measuring, and managing ecological systems.

Parasite avoidance behaviours in aquatic environments

Parasites, including macroparasites, protists, fungi, bacteria and viruses, can impose a heavy burden upon host animals. However, hosts are not without defences. One aspect of host defence, behavioural avoidance, has been studied in the terrestrial realm for over 50 years, but was first reported from the aquatic environment approximately 20 years ago. Evidence has mounted on the importance of parasite avoidance behaviours and it is increasingly apparent that there are core similarities in the function and benefit of this defence mechanism between terrestrial and aquatic systems. However, there are also stark differences driven by the unique biotic and abiotic characteristics of terrestrial and aquatic (marine and freshwater) environments. Here, we review avoidance behaviours in a comparative framework and highlight the characteristics of each environment that drive differences in the suite of mechanisms and cues that animals use to avoid parasites. We then explore trade-offs, potential negative effects of avoidance behaviour and the influence of human activities on avoidance behaviours. We conclude that avoidance behaviours are understudied in aquatic environments but can have significant implications for disease ecology and epidemiology, especially considering the accelerating emergence and re-emergence of parasites.This article is part of the Theo Murphy meeting issue 'Evolution of pathogen and parasite avoidance behaviours'.

Parasite richness and abundance within aquatic macroinvertebrates: testing the roles of host- and habitat-level factors

The importance of parasites as both members of biological communities and as structuring agents of host communities has been increasingly emphasized. Yet parasites of aquatic macroinvertebrates and the environmental factors regulating their richness and abundance remain poorly studied. Here we quantified parasite richness and abundance within 12 genera of odonate naiads and opportunistically sampled four additional orders of aquatic macroinvertebrates from 35 freshwater ponds in the San Francisco Bay Area of California, USA. We also tested the relative contributions of host- and habitat-level factors in driving patterns of infection abundance for the most commonly encountered parasite (the trematode Haematoloechus sp.) in nymphal damselflies and dragonflies using hierarchical generalized linear mixed models. Over the course of two years, we quantified the presence and intensity of parasites from 1,612 individuals. We identified six parasite taxa: two digenetic trematodes, one larval nematode, one larval acanthocephalan, one gregarine, and a mite, for which the highest infection prevalence (39%) occurred in the damselfly genus, Ishnura sp. Based on the hierarchical analysis of Haematoloechus sp. occurrence, infection prevalence and abundance were associated predominantly with site-level factors, including definitive host (frog) presence, nymphal odonate density, water pH and conductivity. In addition, host suborder interacted with the presence of fishes, such that damselflies had higher infection rates in sites with fish relative to those without, whereas the opposite was true for dragonfly nymphs. These findings offer insights into the potential interaction between host- and site-level factors in shaping parasite populations within macroinvertebrate taxa.

Abiotic versus biotic hierarchies in the assembly of parasite populations

The presence or absence of parasites within host populations is the result of a complex of factors, both biotic and abiotic. This study uses a non-parametric classification tree approach to evaluate the relative importance of key abiotic and biotic drivers controlling the presence/absence of parasites with complex life cycles in a sentinel, the common killifish Fundulus heteroclitus. Parasite communities were classified from 480 individuals representing 15 fish from 4 distinct marsh sites in each of 4 consecutive seasons between 2006 and 2007. Abiotic parameters were recorded at continuous water monitoring stations located at each of the 4 sites. Classification trees identified the presence of benthic invertebrate species (Gammarus sp. and Littorina sp.) as the most important variables in determining parasite presence: secondary splitters were dominated by abiotic variables including conductance, pH and temperature. Seventy percent of hosts were successfully classified into the correct category (infected/uninfected) based on only these criteria. The presence of competent definitive hosts was not considered to be an important explanatory variable. These data suggest that the most important determinant of the presence of these parasite populations in the common killifish is the availability of diverse communities of benthic invertebrates.

The dynamics of intestinal helminth communities in eels Anguilla anguilla in a small stream: long-term changes in richness and structure

The prediction that species richness and diversity of intestinal helminth communities in eels would change over time in response to habitat changes was tested over a period of 13 years in a small stream subjected to extensive human management. Nearly all measures of helminth community structure adopted indicated a decline in richness and diversity over the first 6 years followed by a recovery over the last few years to levels unexpectedly close to those at the start of the investigation. Changes in total number of species suggested that the component community was far richer (from 3–9 species) at the end of the period. By contrast, changes in diversity and dominance measures revealed less variation than expected and suggested that there was an underlying stability of community structure characterized by high dominance by a single species, although the identity of this changed, low diversity and a large proportion of the eel population harbouring 0 or only 1 species. A similar pattern of changes was recorded in the infra-communities, where values of species richness and diversity were very similar at the commencement and termination of the study. It appeared that those helminths that colonized in the recovery period contributed to community richness but had little impact on community structure. The helminth communities clearly did change in response to habitat changes, and the evidence for a fixed number of niches and an underlying constancy in helminth community structure in eels is evaluated.

Temporal variation in prevalence and abundance of metacercariae in the pulmonate snail Lymnaea stagnalis in Chany Lake, West Siberia, Russia: long-term patterns and environmental covariates

Infrapopulations of trematode metacercariae were monitored in the snail Lymnaea stagnalis over 17 yr (1982-1999) at Chany Lake, Novosibirskaya Oblast', Russia. Eighteen trematode species were recorded. Patterns of occurrence varied from 4 species (Echinoparyphium aconiatum, Echinoparyphium recurvatum, Moliniella anceps, and Cotylurus cornutus) that persisted at relatively high prevalence (> 60% of samples) across sites, seasons, and years, to species that were very rare and sporadic in occurrence. The stability of the 4 common species was probably because of their occurrence either in a wide range of definitive hosts or in a host adapted to the extreme abiotic changes that occurred from year to year in these wetlands. The prevalence and mean abundance of C. cornutus were negatively correlated with water level in the wetlands; its prevalence was also correlated with water temperature. The mean abundance of M. anceps was positively correlated with water level. The most probable explanation for the cyclic dynamics of infections of the common species is change in population sizes and densities of definitive and intermediate hosts, which mediated cyclic alterations in water levels.

Parasites of the superorganism: Are they indicators of ecosystem health?

The concept of ecosystem health is derived from analogies with human health, which subsequently leads to the implication that the ecosystem has organismal properties, a 'superorganism' in the Clementsian sense. Its application and usefulness has been the subject of a contentious debate; yet, the term 'ecosystem health' has captured the public's imagination and woven its way into the current lexicon, even incorporated into public policy. However, the application of parasites as bioindicators of ecosystem health poses a curious conundrum. Perceptions of parasites range from mild distaste to sheer disgust among the general public, the media, environmental managers and non-parasitologists in the scientific community. Nevertheless, the biological nature of parasitism incorporates natural characteristics that are informative and useful for environmental management. The helminths in particular have evolved elegant means to ensure their transmission, often relying on complex life cycle interactions that include a variety of invertebrate and vertebrate hosts. The assemblage of these diverse parasites within a host organism potentially reflect that host's trophic position within the food web as well as the presence in the ecosystem of any other organisms that participate in the various parasite life cycles. Perturbations in ecosystem structure and function that affect food web topology will also impact upon parasite transmission, thus affecting parasite species abundance and composition. As such, parasite populations and communities are useful indicators of environmental stress, food web structure and biodiversity. In addition, there may be useful other means to utilise parasitic organisms based on their biology and life histories such as suites or guilds that may be effective bioindicators of particular forms of environmental degradation. The challenge for parasitology is to convince resource managers and fellow scientists that parasites are a natural part of all ecosystems, each species being a potentially useful information unit, and that healthy ecosystems have healthy parasites.

Long-term dynamics of Ligula intestinalis and roach Rutilus rutilus: a study of three epizootic cycles over thirty-one years

Data are presented on 2 full epizootic cycles and the start of a third of Ligula intestinalis in roach Rutilus rutilus in a small lake, and the relationships of these cycles to the densities of rudd, Scardinius erythrophthalmus, and Great Crested Grebes, Podiceps cristatus, over 31 years. The parasite was introduced to the lake by P. cristatus in 1973 at a time when the roach population had increased in response to eutrophication to a level at which individual fish growth was stunted and the hithero dominant rudd population had declined in numbers as a consequence of inter-specific competition with roach. Ligula prevalence peaked at 28% in only 2 years: thereafter parasite-induced host mortality caused a decline in the roach population, releasing fish from stunting and allowing the rudd population to recover. The consequent improved growth of roach individuals and their short life-span reduced Ligula transmission rates and prevalence levels declined to approximately 1% although Ligula nevertheless persisted for a further 10 years. Following a massive winter-kill of the fish populations in 1984-1985, fish and Ligula numbers declined to barely detectable levels and the parasite disappeared from samples. Rudd recovered first, then roach and interspecific competition again led to a decline in rudd numbers. This increase in roach numbers led to a decrease in roach growth rates, which coincided with the re-colonization of the lake by Ligula. This second epizootic of Ligula peaked within 2 years in 1991-1992, when up to 78% of roach were infected with a maximum abundance of 2.2 parasites and intensity of 21 parasites. Heavy parasite-induced mortality of roach led to a decline in numbers, an improvement in individual growth rate and a reduction of Ligula transmission rates such that the epizootic died out in 1996. Similar conditions of roach numbers and growth prevailed at the start of a third cycle in 1998. The course of events over the second cycle was so similar to that of the first that it confirms the interpretations of that cycle. Comparison with other localities shows that epizootics of Ligula always coincide with rapid increases in roach numbers, for whatever cause, and stunted growth, which together attract piscivorous birds. At the start of a cycle Ligula is a major determinant of the population dynamics of the roach, but at the end of the cycle the fish population dynamics determine those of the parasite. The cycles are not regulated and the roach-Ligula system is inherently unstable.

Implications of climate change for parasitism of animals in the aquatic environment

Climate change can occur over evolutionary and ecological time scales as a result of natural and anthropogenic causes. Considerable attention has been focused in recent years on the biological consequences of global warming. However, aside from studies on those deleterious parasites that cause disease in man, little effort has been dedicated to understanding the potential changes in the parasite fauna of animal populations, especially those in aquatic systems. Predictions using General Circulation Models, among others, are examined in terms of their consequences for parasite populations in freshwater and marine ecosystems, concentrating on the temperate and boreal regions of eastern North America. Biological effects due to global warming are not predictable simply in terms of temperature response. It is also essential to explore the effects on aquatic parasites of alterations in host distribution, water levels, eutrophication, stratification, ice cover, acidification, oceanic currents, ultraviolet-light penetration, weather extremes, and human interference. Evaluation of the potential response of parasites of aquatic organisms to climate change illustrates the complexity of host–parasite systems and the difficulty of making accurate predictions for biological systems. Parasites in aquatic systems will respond directly to changes in temperature but also indirectly to changes in other abiotic parameters that are mediated indirectly through changes in the distribution and abundance of their hosts. Local extirpations and introductions may be expected as a result. In the long term, climatic change may influence selection of different life-history traits, affecting parasite transmission and, potentially, virulence.