A common parasite that lives inside a fish eyeballs apears to be a driver behind the fish’s behaviour, pulling the strings from inside its eyes
The parasite, while it's still young, helps its host(fish) to stay safe from predators. But once the parasite gets matured, it does everything it can to make sure that the fish gets eaten by a bird and so continue its life cycle.
The parasite which is called the eye fluke Diplostomum pseudospathaceum has a unique life cycle that occurs in three different types of animal. First, the parasites mate in a bird’s digestive tract, after which, they lay their eggs in its faeces. The eggs are then hatched in the water into larvae that search for freshwater snails to infect. Then they grow and multiply while inside the snails before getting released into the water, ready to track down their next host, which is a fish. The parasites then find its way into the skin of fish, and travel to the lens of the eye to hide out and grow. Afterwards, the fish gets eaten by a bird and the cycle starts all over again.
There are a lot of parasite that changes the behaviour of an animal to fit their own needs. The Mice for instance, that gets infected with the parasite Toxoplasma gondii, always end up losing their fear of cats which is the animal that the parasite needs to reproduce inside.
In a study from 2015, Mikhail Gopko and his colleagues at the Severtsov Institute of Ecology and Evolution in Moscow showed that fish infected with juvenile fluke larvae swam less actively than they normaly do, (which makes thems less visible to predators) and were difficult to catch with a net than uninfected controls. Now, the same team tested rainbow trout harbouring mature eye flukes(parasites ready to reproduce inside their bird hosts). The team discovered that these trout swam more actively than uninfected controls and stayed closer to the surface of the water. Both traits should make fish more apparent to birds. When the team of researchers faked a bird attack by making a shadow swoop over the tank, the fish froze, but infected fish resumed swimming sooner than the ones that weren't infected.
Mikhail Gopko says both studies shows that the way eye flukes controls their host’s behaviour depends on how old they are. Juvenile parasites “are too young and innocent to infect a next host”, he says, so their aim is to safeguard the fish they are living in. The Mature parasites, on the other hand, are ready to reproduce, and to do so they need to get inside a bird’s gut.
Some studies done earlier, suggested that fish infected with fluke act differently because of deteriorated vision. But the authors of this earlier study say vision problems wouldn’t explain changes to unfreezing time, or the opposite effects of mature and immature parasites. The researchers also tested how long it took fish to unfreeze after attack when they were infected with both mature and immature parasites at once. Their behaviour matched that of fish carrying only mature parasites. When the parasites’ goals conflict, Gopko says, “mature guys are clear winners”.
This fits a pattern of young parasites decreasing their host’s likelihood of being preyed on, while older parasites increase it, says Nina Hafer, a parasitologist at the Max Planck Institute for Evolutionary Biology in Plön, Germany. Few studies have pitted mature and immature parasites against each other in one host, she says.
“It contributes to showing how many traits and species can be affected by host manipulation, which should make it an important factor in how parasites alter the ecological interactions of their hosts,” she says.