/ Articles / Science on Tap looks at parasites in freshwater ecosystems

Science on Tap looks at parasites in freshwater ecosystems

February 28, 2020 by Beckie Gaskill

Dr. Dan Preston, a faculty member of the forest and wildlife ecology program at the University of Wisconsin-Madison, recently spoke with those attending and watching the Science on Tap presentation entitled Parasites in Freshwater Ecosystems. This is one of his major research interests, he said.

While most people, upon hearing the word “parasite,” think negative thoughts, he said, such as ticks and Lyme disease, there are parasites that may be actually desirable in some instances. 

“When I say the word parasite,” he said, “I’m thinking about organisms that have a symbiotic relationship, so they are living in very close association, and one organism is negatively affecting the other.” The relationship, he said, is also intimate and durable, meaning it lasts a very long time. For the sake of the presentation, he spoke about things larger than viruses and bacteria — things such as worms, ectoparasites, ticks and things along those lines.

There is a great diversity of parasites, Preston said, that many do not think about. There is data, he said, regarding particular parasites that use particular hosts, but because some parasites are undescribed, it is difficult to know how many parasites exist in the world. He said the best estimate is that there are four to five times more parasites than free living animals based on which animals can be affected by which parasites. He spoke about a whole hidden community of parasites, which is very diverse, inside of the animals we see when we step outside. 

Parasites are often very well adapted to do “unique and weird things,” Preston said. Some parasites, he said, have life cycles that require them to infect multiple different hosts. 

“They need to evolve mechanisms to move between these hosts,” he said. “And they also need to evolve mechanisms that allow them to do completely different things in these different hosts.” Many of those parasites, he said, are more like three to five different organisms molded into one.

Trematode worms are the type of parasite that cause things like swimmers’ itch, and Preston spoke about those particular parasites. These parasites typically infect between two and six completely different host species. These parasites start life in a freshwater mollusk such as a snail, usually an aquatic snail. The parasite actually castrates the snail and displaces all of the sexual organs of the snail, he said. Research has found, in cracking open some infected snails, that up to 50 percent of tissue that was originally snail, is actually parasite tissue.

“This is maybe not bolstering my argument that parasites aren’t so bad,” he said after describing how the snails are changed. Parasites are reproducing asexually in the snail at this point, but the snail is unable to reproduce. While it still looks like a snail, it is actually just a parasite, he said. 

Parasites can also cause giantism in snails. The snail can grow bigger and live longer than it would normally. The parasites, at this point, have a cast system, he said. Each portion of parasites has a specific job. There are trematodes, he said, who have the specific job of attacking other trematodes who are trying to infect the snail. 

While in the snail, the parasites are reproducing at a high rate. Not only are they reproducing parasites that will stay within the snail, they are reproducing parasites that will leave the snail. Preston said those parasites look nothing like those they will leave behind in the host. While the parasites that stay in the snail look like a worm, the ones that leave have a body and a tail and they swim through the water. 

These parasites then go looking for a new host. This may be an invertebrate or mammal — there are a wide range of hosts to choose from at this life stage. The effects the parasites have are also wide ranging. Ultimately, he said, the parasite will end up in a predator, which eats the middle host they find after leaving the snail. 

“For instance, a heron might eat a frog and that frog is infected with a trematode,” he said. “Then the heron gets infected. That process is called trophic transmission.” At that point, the trematodes reproduce, with the offspring being passed out of the host in feces. The offspring then go back into the waterbody, effectively completing their life cycle. 

When the trematode is in a host that is eaten by another host, Preston said, they often do a range of things referred to as parasite manipulation. This means they change something about the middle host that makes them more likely to get eaten by the eventual predator host. 

He gave an example of frogs eaten by predatory animals. In his work, Preston has seen the trematodes cause changes in the frog such as having extra legs and deformed legs. This was first observed in Minnesota, he said. While this was originally thought to be due to pollutants, it was found to be trematode parasites that are disrupting the leg development of the frog. The thought, he said, is that is actually adapted for the parasite, with a 10-legged frog with deformed legs being more apt to be eaten by a predator, thus allowing the parasite to complete its life cycle.

He gave another example of how trematodes can affect fish. The parasites get into the brain of the fish, he said, causing strange behavior. The fish come to the surface of the water and perform what he described as a “flashing” behavior not seen in unaffected fish. This flash is more apt to attract predatory birds, again allowing the parasite to complete its life cycle inside of a predator host. 

He then turned the discussion to things that are positive or useful about parasites. Parasites, he said, are doing more than simply causing disease and mortality. Because parasites infect multiple different hosts, he said, the diversity in parasites is often correlated with the diversity of host species. While the populations hosts may sometimes be difficult to measure, measuring the populations and diversity of parasites is often easier. 

“If one environment has forty different parasite species, we know that the host community is really rich,” he said. “So there has actually been ideas and this concept that an ecosystem that is actually healthy, one that is in good shape with a really rich community, tends to have lots and lots of parasites. So we can actually think of parasites in some case as indicators of the overall health or structure of that community in terms of the number of free living species.” A degraded system, he said, does not necessarily have a large number of parasites. For this reason, the number of parasites may be indicative of a healthy, and diverse ecosystem of free living species. 

Another parasite the nematomorph worm, sometimes called a horse hair worm, which can get very large, infect terrestrial insects such as crickets. Their life cycle requires them to enter a waterbody to reproduce. He backed up to say the worm starts out in an aquatic insect such as a mayfly, which is then eaten by the cricket or some other terrestrial insect. The parasite then affects the cricket by effectively committing suicide by jumping into a waterbody, he said. From there, the worm escapes the host, reproduces in the waterbody and completes its life cycle. 

“Whats really crazy about this is that in certain streams in Japan, is these crickets that are jumping into the water make up about 60 percent of the energetic intake on a yearly basis of a particular fish called a Japanese Char, which is essentially a trout.” He said. This species is critically endangered in Japan. So, he said, without these crickets not being infected by the parasitic worm and jumping into the water, the food source for the fish would be drastically reduced. This, he said, is a great example of a parasite’s role being beneficial in some aspect.

Those sorts of roles, he said, are not completely uncommon. He said some parasites change the flow of food and energy through an ecosystem and food web, actually improving the level of energy available in the food web. 

Also, in some cases, can help maintain biodiversity of living hosts. He pointed to studies where, when species in environments compete, parasitic involvement can hold populations of one species to the point that other species can coexist in that same environment without being overthrown by the other species that may be competitively dominant. 

Certain parasites, too, he said can be beneficial for humans. Their presence, he said can increase immunity and upregulate immune systems. This can be beneficial when fighting autoimmune diseases, he said.

While most people do not think about parasites on the same level as Preston, it would be difficult to imagine most who viewed the presentation not coming out of the talk without some new knowledge. The replay of Preston’s presentation has been uploaded on the Science on Tap website scienceontapminocqua.org. All archived presentations in the series can be viewed from the website. Upcoming presentations and topics are also listed on the website. Each presentation is slated for the first Wednesday of the month at 6:30 p.m. It can be viewed online as it is being recorded or seen live in Minocqua. Details in how and where to attend can be found on the website. The next presentation of Science on Tap will be on March 4. Patrick Goggin will present “Native Plant Gardening for Pollinators,” a hot topic for many.

Beckie Gaskill may be reached via email at [email protected].


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