--Structure and function in living systems
--Populations and ecosystems
--Diversity and adaptations of organisms
--Behavior of organisms
Students learn how unusual parasites survive in competitive ecosystems. These horrifying yet compelling creatures help students understand adaptive behaviors and key concepts of evolution. The image above, of a fish whose mouth houses the Cymothoa exigua parasite, is courtesy of Marco Vinci.
Don’t miss a related EducationWorld resource: Creepy Crawly: A Life Science Lesson Plan.
Parasites, ecosystem, adaptation, adaptive behavior, evolution, survival
1. Loxothylacus panopaei
This video below explains how the Loxothylacus parasite is threatening the survival of mud crabs.
Teacher Note: The video uses the term “castrating” to describe how the parasite takes over the crab’s reproductive system.
Viewing questions: What does “salinity” mean? Under what salinity conditions does Loxothylacus not survive? How does the biologists’ experiment demonstrate the scientific method?
Loxothylacus panopaei is a barnacle that takes over a mud crab’s reproductive system and impregnates it with parasitic offspring. This parasite was discovered in the Chesapeake Bay, and it is found in the Gulf of Mexico going all the way to Florida as well as the Caribbean.
After birth, the Loxothylacus is a free-swimming larva. The female infects a recently molted crab by burying itself in the crab’s shell. The Loxothylacus then starts to control the crab’s functions, mainly the molting and reproductive processes.
The Loxothylacus becomes full-grown inside the crab. Eventually the female’s additional sacs grow through the crab’s abdomen, typically followed by a fresh molting. Then, a free-swimming male Loxothylacus comes and fertilizes them. The sacs are filled with thousands of larvae and birthed into the sea to find new hosts. Afterwards, the crab can no longer reproduce, and it will care for the parasite’s developing offspring as if they were its own.
2. Cymothoa exigua
The video below explains how this “tongue louse” takes over its fish host.
Viewing questions: Explain how Cymothoa functions as a hermaphrodite. What kind of fish do Cymothoa prefer as hosts?
Several male Cymothoa enter a fish through its gills. Then one of them becomes a female and latches onto the fish’s tongue. After consuming the tongue, the female Cymothoa takes its place, becoming the tongue. Then it continues to live within the host’s mouth, eating the fish’s blood and mucus. The males stay behind in the fish’s gills and can even mate with the female.
3. Leucochloridium paradoxum (image at right courtesy of giphy)
The video below shows a parasite with an unusually creative life cycle.
Viewing questions: Explain how the Leucochloridium uses two hosts in order to survive. Why do you think the video is titled “Zombie Snails”?
The Leucochloridium paradoxum flatworm takes control of its snail host when the snail consumes bird droppings that contain the parasite’s eggs. The Leucochloridium goes through several life cycles within the snail, eventually storing embryos (offspring) in brood sacs within the snail’s eye stalks. The parasites transform the snail’s antennae into colorful, pulsating tentacles. In addition, the parasites take over the snail’s brain, convincing it to come out in the daylight (normally snails prefer darkness).
Once the snail is out in the light, birds are attracted to its bright tentacles (which look like wriggling caterpillars or maggots) and consume them. Little do the birds know that they also have consumed parasites that are just waiting to start the cycle again.
4. Toxoplasma gondii
The video below explains another complex parasite that can even infect humans.
Viewing questions: How does the behavior of infected and uninfected rats differ? What did the Stanford experiment (discussion starts about 4 minutes into the video) show, and how does the experiment demonstrate the scientific method?
Teacher Note: The video uses the word “promiscuous” as it very briefly lists one of the behavioral changes that can occur in infected humans.
The tiny Toxoplasma parasite is similar to the Leucochloridium, in the sense that it needs multiple hosts in order to survive. Toxoplasma gondii spends part of its life in a cat’s digestive system, then spreads to rodents via cat feces. It invades the rodents’ brains and causes them to behave fearlessly towards cats and get eaten. At that point, the parasite returns to the cat’s gut to complete its life cycle. Other animal species can be infected with Toxoplasma, but cats are the only animal in which the parasite can complete its life cycle.
So how does the parasite achieve rodent mind control? It appears that the parasite hijacks the animal’s immune system, causing its white blood cells to secrete a chemical neurotransmitter called GABA, which is normally found in the brain. The white blood cells respond by becoming “hypermobile” and quickly spreading the parasites around the rodent’s body. When the parasite-infested, GABA-crazed cells reach the brain, they reduce fear and anxiety to the point where they cause the rodent to behave recklessly.
Interestingly, humans also can be infected by the Toxoplasma parasite (an estimated 30-40% of the human population is in fact infected). Most of the time the parasite will not cause any major problems in people, but in cases of more severe infection (due to a weakened immune system), they may experience behavioral changes similar to what happens in rodents. Methods of infection for humans include contact with cat feces or contaminated soil, consuming undercooked meat (for example, cows are often infected with Toxoplasma) and pregnant women passing the parasite to their babies.
Evaluate students in terms of the following:
Lesson Plan Source
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