Something alive wakes up in you. Slowly, insidiously, it grows. It wraps around your organs, interferes in your fabrics. Suddenly, your muscles are tense, your gestures become erratic. First, discreet, and increasingly insistent, this organization guides your movements to the nearest river and even pushes you to jump there. Then, the shock: the water infiltrates and your lungs contract in a silent panic. At this moment, the manager finally emerges from your body: a long and viscous parasitic monstrosity.
This item – Ideas of ideas -is written by Charles Gagnon, master’s student in environmental sciences (profile with memory) of the University of Quebec in Trois-Rivières (UQTR).
Charles Gagnon, master’s degree in environmental sciences (profile with memory) of UQTR.
For a large amount of insects around the world, this horror scenario is not fictitious. This monster does exist, but rest assured, it is a parasite of arthropods. So you are not one of the potential hosts. His name? “Horse hair”, a nematomorphic worm (Phylum: Nematomorpha) capable of manipulating the behavior of his host until pushing him to drowning. At first glance, this organization might seem to be only a symbol of disgust and biological cruelty. And yet … Think again! Behind its sinister appearance hides a being with unexpected ecological functions which pushes to rethink the idea that rivers and forests work independently, when they are actually closely connected.
Several nematomorphs (Chododes fomosanus) Extracting from an Asian giant mantis (Hierodula patellifera). (Hung, 2018)
Several nematomorphs (Chododes fomosanus) Extracting from an Asian giant mantis (Hierodula patellifera). (Hung, 2018)
A complex life cycle
Often invisible and neglected, parasites play an essential role in the functioning of ecosystems, far beyond what one can imagine. Nematomorphic parasites are a good example of organisms exercising an unsuspected ecological influence – an influence made possible partly by the complexity of their life cycle. This life cycle alternates between a parasitic larval stage and a free adult stage. To reproduce, adults must imperatively join an aquatic environment, where they find a sexual partner. They succeed by manipulating the behavior of their terrestrial host and forcing him to throw himself into the water via mechanisms still little known. Once immersed, the parasite is spectacularly getting out of the exoskeleton of its host and the parasitized insect sometimes attracts the attention of aquatic predators as well as fish while struggling on the surface.
This biomass transfer between the two (aquatic and terrestrial) environments orchestrated by the parasite therefore represents a source of energy for aquatic communities. Indeed, terrestrial insects manipulated by nematomorphs can constitute up to 60 % of the annual energy supply of salmonids in certain rivers. This phenomenon, where biomass produced in one type of ecosystem is consumed in another under the effect of parasitic manipulation, illustrates a flow of energy mediated by parasitic manipulation. Nématomorphs are therefore not content to exploit their host to survive: they redirect the energy flow of land ecosystems to aquatic environments. Since they are widespread in the world and their very abundant terrestrial hosts, it is likely that this inter-ecosystem transfer is a still underestimated phenomenon.
Ecological domino effect
The consequences of this flow of matter are much greater; They are not limited to constituting the diet of fish. Indeed, this can cause what is called a trophic cascade with an descending effect (“top-down”). In an ecosystem, each level of the food chain influences the following and a change in the population of predators can cause repercussions on the organisms below in the chain. In this context, the influx of terrestrial insects makes it possible to saturate the feeding of the fish, releasing the predation pressure on the benthic invertebrates (organisms living at the bottom of the rivers). Thus, the latter can prosper, feed, grow and exert stronger pressure on their own resources. Among them, some are herbivores, feeding on algae, while others, called detritivores, decompose dead leaves and organic matter. An increase in the abundance of these two food groups can respectively lead to a reduction in the quantity of algae and an acceleration of the decomposition rate of organic matter (see diagram). Consequently, a simple change at the top of the food chain has repercussions to the basis of the ecosystem-this is the principle of the “top-down” effect. A cascade of ecological effects triggered, in the last instance, by a complex parasitic life cycle which requires a vector of transport to reach its place of reproduction. This phenomenon ultimately highlights the interaction between ecosystems and invites us to rethink them as interdependent environments rather than isolated entities.
Small essential organizations
Parasites pass most of their existence in close contact with their hosts and are often invisible to the eyes of the rest of the world. It would then be easy to assume that, because they are not very visible, their ecological impacts are secondary compared to that of free organisms. However, research shows that parasites can play an equally decisive, even sometimes superior, role in the structuring of communities. Parasitism is one of the most widespread life strategies in the living world. Recognizing it as an ecological force in its own right is not a theoretical exercise: it is an essential condition for understanding the functioning of ecosystems. It is in this perspective that Canada’s research chair on exchanges between ecosystems, led by Professor Éric Harvey (Department of Environmental Sciences, UQTR), will try to integrate parasitic dimensions into its research axes. New projects are under development within the laboratory in order to explore the role of parasites in the structuring of communities and energy flows at the scale of landscapes.
Summary diagram of the comparison of food interactions between the presence and absence of nematomorphic. Black arrows indicate direct consumption relationships; The thickness of the arrows represents the intensity of each interaction. (Sato et al.2011)
Bibliography
De Meeûs, T. & Renaud, F. (2002). Parasites within the new phylogeny of eukaryotes. Trends in Parasitology, 18(6), 247-251.
Hanelt, B., Thomas, F. & Schmidt-Rhaesa, A. (2005). Biology of the phylum Nematomorpha, Advances in Parasitology, 59243–305.
Hung, K.-C. (2018). Observation iNaturalist de Chordodes formosanus In Miaoli County, Taiwan. https://www.inaturalist.org/observations/14567767
Poinar, G. (2008). Global diversity of hairworms (Nematomorpha: Gordiaceae) in freshwater, Hydrobiologia, 59579–83.
Price, P. W., Westoby, M., Rice, B., Atsatt, P. R., Fritz, R. S., Thompson, J. N. & Mobley, K. (1986). Parasite Mediation in Ecological Interactions, Annual Review of Ecology and Systematics, 17487-505 .
Sato, T., Egusa, T., Fukushima K., Oda, T., Ohte, N., Tokushi, N., Watanabe, K., Kanaiwa, M., Murakami, I. & Lafferty, K. (2012). Nematomorph parasites indirectly alter food web and ecosystem function of streams through behavioural manipulation of their cricket hosts, Ecology Letters, 15(8), 786-793.
Sato, T., Watanabe, K., Kanaiwa, M., Niizuma, Y., Harada, Y. & Lafferty, K. (2011). Nematomorph parasites drive energy flow through a riparian ecosystem, Ecology, 92(1), 201–207.
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