An increase in temperature in water bodies can lead to a noticeable change in the life cycle of parasites, according to the research of scientists from the A.N. Severtsov Institute of Ecology and Evolution RAS. Due to global warming, it is possible to change the “parasite-host” chains, which will lead to a restructuring of the entire appearance of freshwater ecosystems.
In recent years, there has been growing interest among scientists in how temperature affects the risk of transmission of infections in ecosystems, or, in other words, the interaction of parasites and their hosts. To find out, scientists from the IEE RAS studied how temperature affects the separation of the dispersal stages (larvae/cercariae) of two closely related species of parasitic flatworms (trematodes): Rhipidocotyle campanula and R. fennica. The first intermediate host in these species of trematodes is the mollusk - common toothless (Anodonta anatina), the second roach. And only the final owners - predatory fish - differ.
To conduct the experiment, scientists collected mollusks from rivers and distributed them in flow tanks with three temperature regimes. The average temperature in these tanks was 18°C (range 7-24°C), 15°C (7-20°C) and 13°C (6-18°C). The design of the experiment did not assume a constant temperature of the molluscs. On the contrary, it had to fluctuate in accordance with seasonal changes, as in natural ecosystems, to emulate the real world in terms of global warming.
The experiment lasted five months. Every two weeks, the scientists checked whether the molluscs secreted parasitic larvae (cercariae) and, if so, how many.
“We were interested in how temperature affects the duration of the emission of cercariae by molluscs, the time of its start and end, the total number of isolated cercariae and other indicators. It turned out that these closely related species of trematodes are very different in their temperature preferences. Cercariae of R. campanula began to be released at 9-10°C, and a further increase in temperature only modestly increased their production by the mollusk. At the same time, the proportion of mollusks secreting cercariae was approximately the same in all temperature regimes. R. fennica turned out to be much more thermophilic. The temperature optimum for the isolation of cercariae for this trematode was above 15°C, and the dependence of the number of produced cercariae on temperature was much stronger. Moreover, the proportion of mollusks producing cercariae of this parasite increased with temperature”, says Mikhail Gopko, one of the authors of the study.
The scientists came to the conclusion that at typical northern latitude summer temperatures, many individuals of R. fennica do not have time to accumulate enough degree-days to continue their life cycle. In this case, it turns out that an increase in average annual temperatures can bring much more benefit to R. fennica than to R. campanula. At high temperatures, the beginning and end of the production period of cercariae is shifted closer to the beginning of the season. At similar temperatures, mollusks produced fewer cercariae in autumn than in winter.
“In other words, temperature is temperature, and “fatigue” of the host organism imposes natural restrictions on the ability of the parasite to force its reproduction,” adds Ekaterina Mironova, co-author of the article.
Scientists came to the conclusion that due to global warming, the balance in the R. fennica - roach - R. campanula system may shift towards the predominance of a more heat-loving parasite species. Thus, an increase in temperature in water bodies can lead to a noticeable change in the periodization of the life cycle of parasites. Ultimately, given the incredible abundance of host-parasite systems, this has the potential to change the entire face of freshwater ecosystems.
In their next study, the scientists plan to explore how these changes will be synchronized with changes in the life cycle of the parasite's next host, the fish.
What will happen with changes in the "parasite-host" systems? Answered by Mikhail Gopko
There are at least two realistic scenarios for the development of relationships between parasites and their hosts under global warming. According to the first of them, with an increase in temperature, parasites will gain an advantage over their hosts. Parasites have a faster rate of evolution, which is important in changing conditions, and they also have higher productivity at moderately elevated ambient temperatures. For aquatic organisms, such as fish, the situation is further complicated by the fact that at high temperatures their immune system begins to fail, making it easier for infections to enter the host organism.
It is possible that all the benefits that parasites get from warming will be only temporary - most experiments on the effects of warming on parasites are short-lived. In addition, parasites may even benefit from the desynchronization of the life cycles of the parasite and the host. Let's say that warming will lead to the fact that the parasites will be ready to infect the host earlier in the season, but no one guarantees that the host will be already “ready” for this, say, not yet migrating to a suitable part of the reservoir.
The second scenario can lead to very strong changes in ecosystems. The fact is that parasites play an unusually important and specific role in communities. This is a kind of glue that unites various trophic levels. When a cheetah rushes across the savannah, chasing a Thompson's gazelle, or a gluttonous gull tries to grab a gaping fry from the surface of the water, the false impression can be created that everything that happens is solely the work of a predator and its prey. In fact, the parasites that weaken each side sometimes play an even more significant role. The desynchronization of the life cycles of parasites and their hosts can lead to a "domino effect", disrupting trophic interactions between predators and their prey.
A new study by scientists supports the second point of view. However, the applicability of its results to all "parasite-host" systems is out of the question. In addition, there is no information about how quickly changes in ecosystems will occur and whether nature will be able to adapt to them. As Mikhail Gopko notes, the impact of global warming on host- parasite systems and ecosystems will be generally stronger as we approach the poles and the equator, but in temperate latitudes, everything will likely be more moderate.
The article “Contrasting temperature responses in seasonal timing of cercariae shedding by Rhipidocotyle trematodes” was published in the journal Parasitology.
Dependence of the average (for all mollusks that produced cercariae on the day of
observation) number of cercariae produced on temperature for R. fennica (A, B) and R.
campanula (C, D). On the right, partial regression plots (added variable plot) taking into
account seasonality. Note the outliers in the "underbelly" of graph A. This is the production of
cercariae during the autumn months, when the emission volume was lower at the same
temperatures. Adding a seasonality factor (the number of days since the start of the
experiment) “corrects” the dependence, and the points fit very neatly on the regression line
(plot C). For R. campanula (bottom row), the situation is similar.
The work was published in the journal Parasitology
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