Infection success of Echinoparyphium aconiatum (Trematoda) in its snail host under high temperature: role of host resistance

Each coin has 2 sides: a positive role of alien Potamopyrgus antipodarum (Grey, 1843) snails in reducing the infection of native lymnaeids with trematodes

The change in the distribution of organisms in freshwater ecosystems due to natural or manmade processes raises the question of the impact of alien species on local communities. Although most studies indicate a negative effect, the positive one is more difficult to discern, especially in multispecies systems, including hosts and parasites. The purpose of the study was to check whether the presence of an alien host, Potamopyrgus antipodarum, reduces the intensity of Echinoparyphium aconiatum metacercariae in a native host, Radix spp. We additionally tested the impact of water temperature and the biomass of the alien host on the dilution effect. We experimentally studied (1) the lifespan of echinostome cercariae in different temperatures, (2) the infectivity of cercariae toward the alien host and native host, and (3) the impact of different biomass of the alien host on the intensity of metacercariae in the native host. We found that cercarial survival and infectivity were temperature dependent. However, cercarial survival decreased with increasing temperature, contrary to cercarial infectivity. Echinostome cercariae entered the renal cavity of both the native host and alien host, and successfully transformed into metacercariae. The number of metacercariae in the native host decreased with the increasing biomass of the alien host. Our results indicate that lymnaeids may benefit from the co-occurrence with P. antipodarum, as the presence of additional hosts of different origins may reduce the prevalence of parasites in native communities. However, the scale of the dilution effect depends not only on the increased spectrum of susceptible hosts but also on the other variables of the environment, including water temperature and host density.

Environmental parasitology: stressor effects on aquatic parasites

Anthropogenic stressors are causing fundamental changes in aquatic habitats and to the organisms inhabiting these ecosystems. Yet, we are still far from understanding the diverse responses of parasites and their hosts to these environmental stressors and predicting how these stressors will affect host-parasite communities. Here, we provide an overview of the impacts of major stressors affecting aquatic ecosystems in the Anthropocene (habitat alteration, global warming, and pollution) and highlight their consequences for aquatic parasites at multiple levels of organisation, from the individual to the community level. We provide directions and ideas for future research to better understand responses to stressors in aquatic host-parasite systems.

Immunological Resistance of Pseudosuccinea columella Snails From Cuba to Fasciola hepatica (Trematoda) Infection: What We Know and Where We Go on Comparative Molecular and Mechanistic Immunobiology, Ecology and Evolution

One of the most interesting biological models is that of snail–trematode interactions, many of which ultimately result in the transmission of several important diseases, particularly in the tropics. Herein, we review the scientific advances on a trematode–snail system in which certain populations of Pseudosuccinea columella (a common host species for trematodes) have been demonstrated naturally-resistant to Fasciola hepatica, in association with an effective encapsulation of the parasite by innate immune cells of the host, the hemocytes. Emphasis is made on the molecular and immunological features characterizing each P. columella phenotype in relation to their anti-parasitic competence, their distinctive ecological patterns and the existence of a significant cost of resistance. An integrative overview of the resistance to F. hepatica through comparative immunobiology, genetics and ecology is presented to hypothesize on the possible origins and evolution of this phenomenon and to postulate significant roles for parasite mediated-selection and environmental factors in shaping and maintaining the resistant phenotype in the field. Lastly, clues into future experimental perspectives to deeply characterize the interplay between P. columella and F. hepatica and the immunobiology of the resistance are also included. The advances revised in the present paper are only beginning to unravel mechanisms of anti-parasite innate defense responses and their evolutionary bases, and can facilitate the development of prospective approaches towards practical applications of P. columella resistance.

Diurnal variation around an optimum and near-critically high temperature does not alter the performance of an ectothermic aquatic grazer

The growing threat of global climate change has led to a profusion of studies examining the effects of warming on biota. Despite the potential importance of natural variability such as diurnal temperature fluctuations, most experimental studies on warming are conducted under stable temperatures. Here, we investigated whether the responses of an aquatic invertebrate grazer (Lymnaea stagnalis) to an increased average temperature differ when the thermal regime is either constant or fluctuates diurnally. Using thermal response curves for several life-history and immune defense traits, we first identified the optimum and near-critically high temperatures that Lymnaea potentially experience during summer heat waves. We then exposed individuals that originated from three different populations to these two temperatures under constant or fluctuating thermal conditions. After 7 days, we assessed growth, reproduction, and two immune parameters (phenoloxidase-like activity and antibacterial activity of hemolymph) from each individual. Exposure to the near-critically high temperature led to increased growth rates and decreased antibacterial activity of hemolymph compared to the optimum temperature, whilst temperature fluctuations had no effect on these traits. The results indicate that the temperature level per se, rather than the variability in temperature was the main driver altering trait responses in our study species. Forecasting responses in temperature-related responses remains challenging, due to system-specific properties that can include intraspecific variation. However, our study indicates that experiments examining the effects of warming using constant temperatures can give similar predictions as studies with fluctuating thermal dynamics, and may thus be useful indicators of responses in nature.

Inbreeding does not alter the response to an experimental heat wave in a freshwater snail

Global climate change affects natural populations of many species by increasing the average temperature and the frequency of extreme weather events (e.g. summer heat waves). The ability of organisms to cope with these environmental changes can, however, depend on their genetic properties. For instance, genetic load owing to inbreeding could alter organisms' responses to climate change-mediated environmental changes but such effects are often overlooked. We investigated the effects of an experimental heat wave (25°C versus 15°C) on life history (reproduction, size) and constitutive immune defence traits (phenoloxidase-like and antibacterial activity of haemolymph) in relation to inbreeding by manipulating the mating type (outcrossing, self-fertilization) in two populations of a hermaphroditic freshwater snail, Lymnaea stagnalis. High temperature increased reproduction and size of snails but impaired their immune function. In one of the two study populations, inbreeding reduced reproductive output of snails indicating inbreeding depression. Furthermore, this effect did not depend on the temperature snails were exposed to. Our results suggest that L. stagnalis snails can be negatively affected by inbreeding but it may not alter their responses to heat waves.

Potential for adaptation to climate change: family-level variation in fitness-related traits and their responses to heat waves in a snail population

BACKGROUND

On-going global climate change poses a serious threat for natural populations unless they are able to evolutionarily adapt to changing environmental conditions (e.g. increasing average temperatures, occurrence of extreme weather events). A prerequisite for evolutionary change is within-population heritable genetic variation in traits subject to selection. In relation to climate change, mainly phenological traits as well as heat and desiccation resistance have been examined for such variation. Therefore, it is important to investigate adaptive potential under climate change conditions across a broader range of traits. This is especially true for life-history traits and defences against natural enemies (e.g. parasites) since they influence organisms' fitness both directly and through species interactions. We examined the adaptive potential of fitness-related traits and their responses to heat waves in a population of a freshwater snail, Lymnaea stagnalis. We estimated family-level variation and covariation in life history (size, reproduction) and constitutive immune defence traits [haemocyte concentration, phenoloxidase (PO)-like activity, antibacterial activity of haemolymph] in snails experimentally exposed to typical (15 °C) and heat wave (25 °C) temperatures. We also assessed variation in the reaction norms of these traits between the treatments.

RESULTS

We found that at the heat wave temperature, snails were larger and reproduced more, while their immune defence was reduced. Snails showed high family-level variation in all examined traits within both temperature treatments. The only negative genetic correlation (between reproduction and antibacterial activity) appeared at the high temperature. However, we found no family-level variation in the responses of most examined traits to the experimental heat wave (i.e. largely parallel reaction norms between the treatments). Only the reduction of PO-like activity when exposed to the high temperature showed family-level variation, suggesting that the cost of heat waves may be lower for some families and could evolve under selection.

CONCLUSION

Our results suggest that there is genetic potential for adaptation within both thermal environments and that trait evolution may not be strongly affected by trade-offs between them. However, rare differences in thermal reaction norms across families indicate limited evolutionary potential in the responses of snails to changing temperatures during extreme weather events.

Environmental temperature variation influences fitness trade-offs and tolerance in a fish-tapeworm association

Background

Increasing temperatures are predicted to strongly impact host-parasite interactions, but empirical tests are rare. Host species that are naturally exposed to a broad temperature spectrum offer the possibility to investigate the effects of elevated temperatures on hosts and parasites. Using three-spined sticklebacks, Gasterosteus aculeatus L., and tapeworms, Schistocephalus solidus (Müller, 1776), originating from a cold and a warm water site of a volcanic lake, we subjected sympatric and allopatric host-parasite combinations to cold and warm conditions in a fully crossed design. We predicted that warm temperatures would promote the development of the parasites, while the hosts might benefit from cooler temperatures. We further expected adaptations to the local temperature and mutual adaptations of local host-parasite pairs.

Results

Overall, S. solidus parasites grew faster at warm temperatures and stickleback hosts at cold temperatures. On a finer scale, we observed that parasites were able to exploit their hosts more efficiently at the parasite’s temperature of origin. In contrast, host tolerance towards parasite infection was higher when sticklebacks were infected with parasites at the parasite’s ‘foreign’ temperature. Cold-origin sticklebacks tended to grow faster and parasite infection induced a stronger immune response.

Conclusions

Our results suggest that increasing environmental temperatures promote the parasite rather than the host and that host tolerance is dependent on the interaction between parasite infection and temperature. Sticklebacks might use tolerance mechanisms towards parasite infection in combination with their high plasticity towards temperature changes to cope with increasing parasite infection pressures and rising temperatures.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2192-7) contains supplementary material, which is available to authorized users.

Thermal Change and the Dynamics of Multi-Host Parasite Life Cycles in Aquatic Ecosystems

Altered thermal regimes associated with climate change are impacting significantly on the physical, chemical, and biological characteristics of the Earth's natural ecosystems, with important implications for the biology of aquatic organisms. As well as impacting the biology of individual species, changing thermal regimes have the capacity to mediate ecological interactions between species, and the potential for climate change to impact host-parasite interactions in aquatic ecosystems is now well recognized. Predicting what will happen to the prevalence and intensity of infection of parasites with multiple hosts in their life cycles is especially challenging because the addition of each additional host dramatically increases the potential permutations of response. In this short review, we provide an overview of the diverse routes by which altered thermal regimes can impact the dynamics of multi-host parasite life cycles in aquatic ecosystems. In addition, we examine how experimentally amenable host-parasite systems are being used to determine the consequences of changing environmental temperatures for these different types of mechanism. Our overarching aim is to examine the potential of changing thermal regimes to alter not only the biology of hosts and parasites, but also the biology of interactions between hosts and parasites. We also hope to illustrate the complexity that is likely to be involved in making predictions about the dynamics of infection by multi-host parasites in thermally challenged aquatic ecosystems.