Imagine a fungus that turns an ant into a walking delivery system or a tiny crustacean that replaces a fish’s tongue and takes over its place inside the fish’s mouth. These are not scenes from a science-fiction movie. They are real examples of parasitism, one of nature’s strangest survival strategies.
Most people think of parasites as organisms that simply live inside another animal and steal its nutrients. But some parasites do much more than that. They can influence their hosts’ behavior, interfere with development, alter reproduction and even change how other organisms interact with their environment.
Their life cycles can be so unusual that they seem almost impossible to believe. Here are ten examples that reveal just how strange the relationship between parasites and their hosts can become.
1. The fungus that turns ants into living puppets
Photo courtesy of Andreas Kay
One of the most famous examples of behavioral manipulation comes from the fungus Ophiocordyceps, often called the zombie-ant fungus. After infecting certain ants, the fungus can cause them to leave their normal colony activities, climb vegetation and bite down on a leaf or twig. The ant remains attached in a position that provides favorable conditions for the fungus to grow and release spores.
The remarkable part is that this is not simply a random movement caused by illness. The behavior is highly stereotyped and can be specific to particular fungus–ant combinations. Researchers are investigating how fungal chemicals and changes to biological rhythms may contribute to this manipulation.
Scientists still do not fully understand the exact mechanisms, which makes the phenomenon even more fascinating. A
2024 review of the mechanisms behind Ophiocordyceps infection and behavioral manipulation explores how much remains to be discovered.
2. The worm that makes crickets jump into water
Photo courtesy of Phil Myers / Animal Diversity Web.
Crickets normally have very little reason to jump into water. For certain crickets infected by hairworms, however, entering water can become part of the parasite’s life cycle. Hairworms develop inside terrestrial insects, but adult worms need an aquatic environment to reproduce. Somehow, infected insects become more likely to enter water, allowing the mature worm to emerge and continue its life cycle.
In a 2002 study, researchers compared infected and uninfected crickets and found that infected crickets were more likely to jump into water. Interestingly, the researchers did not find evidence that the worms simply made the insects detect water from far away. Instead, changes in movement may bring infected insects closer to water before another behavioral change encourages them to enter it.
3. The parasite that makes ants climb grass
Photo courtesy of Brian Lund Fredensborg / University of Copenhagen
Some parasites need more than one host to complete their life cycles. The lancet liver fluke, Dicrocoelium dendriticum, has developed a particularly unusual way of moving between them. Its life cycle involves snails, ants and grazing mammals such as sheep and cattle.
After entering an ant, some of the parasite larvae travel to the ant’s nervous system. Infected ants may climb onto blades of grass and clamp their jaws onto the vegetation, particularly under certain environmental conditions. This makes them more likely to be swallowed by grazing animals, where the parasite can continue developing.
A
2018 study used micro-CT imaging to examine the physical relationship between the parasite and the ant’s nervous tissue. Researchers found that one larva could make direct contact with a region of the ant’s brain-related nervous system. The exact mechanism behind the behavioral change is still being investigated.
4. The parasite that replaces a fish’s tongue
Photo courtesy of Vista al Mar
This one sounds especially unbelievable. Cymothoa exigua is a parasitic crustacean that enters a fish’s mouth and attaches to its tongue. As the parasite feeds on the tongue’s blood supply, the tongue gradually deteriorates. Eventually, the parasite remains attached at the base of the tongue and effectively takes its place.
Even stranger, the fish can continue using the parasite as a functional support for its mouth. The crustacean does not literally become a new tongue made of fish tissue, but it occupies the position of the original organ and can help the fish continue feeding.
This unusual relationship is considered an example of anatomical replacement by a parasite. A scientific study on the attachment structures of parasitic cymothoid isopods explains how these crustaceans attach themselves to fish and maintain their position on their hosts.
5. The barnacle that changes a crab’s reproductive biology
Photo courtesy of Boerdeman / Wikimedia Commons
Barnacles are usually associated with rocks, boats and the shells of marine animals. But Sacculina is a parasitic barnacle with a very different lifestyle. Instead of simply attaching itself to a crab’s surface, it develops an extensive network of root-like structures inside the crab’s body, absorbing nutrients from its host.
The parasite can interfere with the crab’s reproductive system. Infected crabs may experience reproductive suppression, and male crabs can develop some female-like physical characteristics. In some cases, infected crabs also display behaviors associated with caring for the parasite’s reproductive structure.
A 2023 study documented morphological feminization in male crabs infected by sacculinid parasites. The researchers examined changes in external characteristics that made infected males resemble females. The study is described in “Sacculina-Induced Morphological Feminization in the Grapsid Crab Pachygrapsus crassipes”. The parasite is not simply taking nutrients from its host; it is altering major aspects of the host’s biology.
6. The parasite that makes snails look like they have moving eyeballs
Photo courtesy of JoostP / Shutterstock
If you see a snail with brightly colored, pulsating structures inside its eyestalks, you might assume you have discovered an extremely unusual species. In reality, you may be looking at a snail infected by Leucochloridium, a parasitic flatworm.
The parasite develops inside the snail and forms colorful structures called broodsacs. Their patterns and movements can make the snail’s eyestalks resemble caterpillars. This is important because birds are the parasite’s final hosts. When a bird eats an infected snail, the parasite can continue its life cycle inside the bird.
The parasite therefore uses one animal as a temporary home while making it more noticeable to another animal that it needs to reach. Researchers have studied the genetic diversity and identification of these unusual flukes, including a study of Leucochloridium species.
7. The parasite that can cause frogs to develop extra limbs
Photo courtesy of Pieter Johnson / University of Colorado via AP
Not all parasites manipulate behavior. Some can interfere with how an animal develops. Ribeiroia ondatrae is a parasitic flatworm whose life cycle involves aquatic snails, amphibians and birds. During its development, the parasite can infect tadpoles near the areas where their limbs are forming.
Research has shown that infection can disrupt normal limb development, resulting in abnormalities such as extra limbs, missing limbs, or unusual limb structures. In a 1999 experiment published in Science, researchers found that exposure to the parasite could induce severe limb abnormalities in Pacific treefrogs. Later research also linked the parasite to amphibian malformations across parts of the western United States.
The findings reveal that a parasite can influence not only an animal’s behavior or health, but also the physical development of its body. The original research is available through the U.S. Geological Survey.
8. The parasite that makes fish more likely to be eaten
Photo courtesy of Kelly Weinersmith / Phys.org
For many animals, avoiding predators is essential for survival. But for a parasite that needs to reach a predator to complete its life cycle, getting its host eaten can be exactly what it needs.
The trematode Euhaplorchis californiensis infects California killifish and forms cysts in their brains. Infected fish can display unusual swimming behaviors that make them more visible to birds. This is not a particularly helpful development for the fish, but birds are among the parasite’s final hosts.
Research has found that infected killifish can be significantly more vulnerable to bird predation than uninfected fish. One study reported that heavily infected fish could contain thousands of parasite cysts in their braincases while still growing and reproducing normally. The parasite’s life cycle depends on moving from fish to bird, making the fish’s increased visibility an important part of its survival strategy. Read more in this U.S. Geological Survey research summary.
9. The parasite that may change a mouse’s fear of cats
Photo courtesy of ViroGen / Toxoplasma gondii MIC 3 Recombinant Antigen
Toxoplasma gondii is a single-celled parasite that can infect many warm-blooded animals. Cats are its definitive hosts, meaning that sexual reproduction of the parasite takes place in felines. Other animals, including rodents, can serve as intermediate hosts.
One of the most studied aspects of T. gondii infection is its possible influence on rodent behavior. Some experiments have found that infected rodents show reduced avoidance of cat-related odors. Since cats can become infected by eating rodents, this behavioral change could potentially help the parasite reach its definitive host.
However, scientists are still investigating exactly how consistent these effects are and whether they represent deliberate adaptive manipulation by the parasite or indirect consequences of infection. The evidence is more complicated than the popular claim that the parasite simply turns mice fearless. A detailed review, “Behavioral biology of Toxoplasma gondii infection,” examines the proposed mechanisms and the remaining scientific questions.
10. The wasp that uses a virus to disarm a caterpillar’s immune system
Photo courtesy of Richard Crook / Flickr
Some parasitic wasps have an unusual biological partnership with viruses. When certain female wasps lay eggs inside caterpillars, they also introduce virus particles known as polydnaviruses. These viruses carry genetic material that changes how the caterpillar’s body responds to the developing wasp larvae.
Normally, a caterpillar’s immune system may recognize foreign eggs and surround them with defensive cells, preventing them from developing. But proteins produced from the viral genes can interfere with these immune responses. The wasp larvae are then able to develop inside the caterpillar while avoiding some of its normal defenses.
What makes this relationship particularly fascinating is that the viruses are not simply independent parasites infecting the caterpillar. They are essential partners in the wasps’ reproductive strategy. The virus helps create conditions in which the wasp’s offspring can survive. Researchers have examined this unusual relationship in “Parasitoid polydnaviruses and immune interaction with secondary hosts”.
Parasites are often described as organisms that take something from their hosts. But these ten examples show that the relationship can be far more complicated. Some parasites influence where an animal moves, others interfere with reproduction or development, and some even use viruses as biological partners. Their strategies are not necessarily the result of conscious planning; they are products of evolution, shaped over countless generations by the demands of survival and transmission.
Perhaps the most surprising lesson is that an animal’s body and behavior are not always shaped by its own genes alone. Other organisms can become deeply involved in how it develops, moves, reproduces and interacts with the world. In nature, even the smallest passenger can sometimes have an extraordinary influence on the host carrying it.
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