A bizarre parasite that lived in the eyeballs of fish protect their hosts while they are young, but alter their behavior in an attempt to intentionally get them eaten by birds, according to research published online earlier this week by the journal Behavioral Ecology and Sociobiology.
While it sounds like the kind of mind control that you’d normally see in a science fiction film, the behavior is very real, and according to New Scientist, it’s part of the parasite’s plan to ensure its own survival, as it reproduces in the digestive tract of the bird that devours its former host.
The parasite in question is called the eye fluke or Diplostomum pseudospathaceum, and during its life cycle, it invades three different types of host, the website explained. After it mates within the bird’s digestive tract, its eggs are eliminated along with the bird’s feces. Those eggs hatch in the water, then the eye fluke’s larvae hunt down freshwater snails and infect them.
Once they infect the snails, the eye fluke grow and multiply, then are released into the water and look for a fish to infect. Once they find their target, they penetrate the skin of that fish and travel to its eye, where they grow and control the fish until it is time to get eaten and repeat the cycle.
Cycle is likely a key part of the food web, say study authors
In 2015, a team of scientists led by Mikhail Gopko from the Severtsov Institute of Ecology and Evolution in Moscow found that fish that had been infected with immature fluke larvae tended to be less-active swimmers than their uninfected counterparts, making them harder for predators to spot and less likely to be caught by a net, according to New Scientist.
Now, Gopko and his colleagues conducted a series of tests in which they infected rainbow trout with mature eye flukes and found that these infected fish were more active swimmers and stayed closer to the surface than uninfected fish. Furthermore, when the researchers simulated an attack by a predatory bird, the infected fish were found to recover more quickly than uninfected ones.
“Our results suggest that the eye fluke changes its host’s behavior in order to make it more vulnerable to the final host,” the authors wrote in their study. “Most importantly, the observed behavioral changes arose, when the infection intensity was similar to rates found in natural conditions. This implies that, in natural conditions, eye flukes can substantially alter host anti-predatory defenses and affect predator–prey interactions.”
However, as Gopko told New Scientist, the manner in which the eye flukes will influence their hosts’ behavior depends on how old they are. Younger parasites “are too young and innocent to infect a next host,” he said, so they must protect the fish they have infected until they are mature. Once they are ready to reproduce, however, they do everything they can to get inside a bird, and that means doing everything in its power to ensure that its host fish gets eaten.
“Our results suggest that the D. pseudospathaceum metacercariae can change rainbow trout’s behavior predisposing them to avian predation,” he and his co-authors wrote. “Since eye flukes are common freshwater fish parasites, the resulting behavioral changes caused by these parasites likely play an important role in freshwater food webs.”
Image credit: Dr. Andrew Lee/Solent News