Parasites Part Two: Size Doesn’t Matter When You Can Control Minds

Author: Brittany Maule

The second part of the promised parasite post is here. I’m sure you were itching to see it. Maybe in more ways than one after reading last week's post. Last week I focused on the general background of parasites: what parasitism is, ways they move around, and some examples of ways they obtain food. Today’s post will focus on the role parasites play at an ecosystem-level, and why that role is more important than you might think. From the ecosystem-level parasites can affect a lot of processes that shape the way an ecosystem is structured, both directly and indirectly. To help explain this topic, I’ve interviewed my colleague Nicole Chodkowski, a Ph.D. Candidate in Environmental Studies at Ball State University. Nicole studies how trematodes (a class of parasites) can alter the way a host metabolizes food, and as a result how the parasite might change the way nutrients are recycling in an ecosystem. She also has a bearded dragon that she is very fond of named Sebastian.

Nicole's bearded dragon,Sebastian, feeling festive

How do parasites affect ecosystem function?

As we learned last week, parasites need to a host to survive, and often require more than one host to complete a life cycle. Because of this, parasites can serve several important roles in an ecosystem. “Depending on the parasite, they can be environmental regulators or ecosystem engineers,” says Chodkowski.

Parasites can serve to:

·       Strengthen predator-prey interactions: Parasites can increase the likelihood of an organism being eaten by a predator. Predator-prey interactions can determine how stable an ecosystem is, or essentially how susceptible it is to disturbance.

·       Decrease reproductive ability: Some parasites gain nutrients by eating the reproductive organs of their host. When this happens, the number of individuals in a population can decline because they are not able to reproduce.

·       Alter rates of nutrient cycling: Parasites can affect how many nutrients move through an organism. For example, if a snail needs to obtain a certain amount of nitrogen to survive, a parasite might absorb some of that nitrogen and cause the snail to need to eat more.

 In this way, parasites can have cascading effects on ecosystems in which their actions influence how their host might act, and thus how the predator might react, and so on. In some examples parasites are even capable of things similar to mind control where they lead unwilling hosts to their deaths so they can survive or reproduce.

Survival of the weirdest:

Chodkowski explains how “Parasitism is one of the most common consumer strategies.” As a result, there are many different species and classes of parasites to explore. However, as parasites have some “fascinating adaptations” for making sure they survive and move to the next host, I want to highlight a few of the crazier ones, including a favorite of Chodkowski.

·       Nematomorphs: More commonly known as “horse-hair” worms are one of the examples where it seems like parasites might be performing mind-control on their hosts. These horse-hair worms infect organisms like crickets and grasshoppers. When infected, a cricket for example will seek out and jump into water so the parasite can emerge as an aquatic adult¹ When this happens the cricket is now much more susceptible to predation. Also, the nutrients it would have kept in the terrestrial system, have now moved to the aquatic one, i.e. parasites manipulating the host have affected nutrient cycling within those ecosystems².

·       The Zombie Snail: One of Chodkowski’s favorite parasites: Leucochloridium paradoxum. This parasite is affectionately named for creating zombie snails that change their behavior so birds will attack them to eat the parasites in their eyestalks. Don’t worry, it’s even more fascinating than it sounds. This parasite causes the snail’s normally thin eyestalks to bulge out, and pulsate while the parasite exhibits bright colors. The overall effect is eyes that look like appetizing caterpillars for predatory birds. The parasite needs these birds to complete its life cycle. Thus, snails are even manipulated to move towards tree tops during daylight (something they wouldn’t do otherwise) under the influence of this parasite. National Geographic has a video you can check out with some great dramatic music: Zombie Snails

·       Acanthocephala: My personal favorite, because it is one of the weirdest things I have ever heard. Parasites of the phylum Acanthocephala or “spiny-headed worms” have a crazy mode of reproduction to make sure the males successfully mate with a female and carry on their genes. Male worms who have mated with a female will secrete cement, yes cement, and block the female’s ability to mate with another male by covering her with the cement³. This ensures the male will be the only one she mates with, and the male’s genes will survive into the next generation.

A nematomorph parasite. Photo Credit: https://goo.gl/SbN4C7

What are the next directions for understanding parasites?

As we’ve seen, the importance of considering parasites at an ecosystem-level could aid in understanding overall ecosystem function. If anything, it may explain why snails or grasshoppers are voluntarily offering themselves up for dinner. In terms of the future, Chodkowski explains how a lot of research emphasis is in the medical field and understanding the effect of parasitism with respect to human disease: “I think a lot of research is still going into helping human populations suffering from painful or lethal parasite infections.” However, she also notes studying parasites at the ecosystem level is still filled with unanswered questions such as “how parasites fit into bigger ecological theories such as metabolic theory of ecology, food web theory, and ecological stoichiometry.”

With these future directions, you may start to see these little being incorporated into more ecological concepts. The topic of parasites does seem to be very infectious (pun intended) as Chodkowski puts it, “the more I learn about them, the more interested I become!”

Additional Resources:

1.       Thomas, F., A. Schmidt-Rhaesa, G. Martin, C. Manu, P. Durand, and F. Renaud. 2002. Do hairworms (Nematomorpha) manipulate the water seeking behavior of their terrestrial hosts? Journal of Evolutionary Biology 153: 356 361.

2.       Takuya, S., W. Katsutoshi, M. Kanaiwa, Y. Niizuma, Y. Harada, and K. D. Lafferty. 2011. Nematomorph parasites drive energy flow through a riparian ecosystem. Ecology 92(1): 201-207.

3.       Parker, G. A. 1970. Sperm competition and its evolutionary consequences in the insects. Biological Reviews 45: 525-567.