Puppetmaster parasite steers fish – from inside their eyeballs.

New Scientist has the creepy story of a parasitic worm that controls fish behavior by controlling how they see the world:

When the parasite is young, it helps its host stay safe from predators. But once the parasite matures, it does everything it can to get that fish eaten by a bird and so continue its life cycle.

The eye fluke Diplostomum pseudospathaceum has a life cycle that takes place in three different types of animal. First, parasites mate in a bird’s digestive tract, shedding their eggs in its faeces. The eggs hatch in the water into larvae that seek out freshwater snails to infect. They grow and multiply inside the snails before being released into the water, ready to track down their next host, fish. The parasites then penetrate the skin of fish, and travel to the lens of the eye to hide out and grow. The fish then get eaten by a bird – and the cycle starts again.

In a 2015 study, Mikhail Gopko at the Severtsov Institute of Ecology and Evolution in Moscow and his colleagues showed that fish infected with immature fluke larvae swam less actively than usual – making themselves less visible to predators – and were harder to catch with a net than uninfected controls.

Now, the same team has tested rainbow trout harbouring mature eye flukes – parasites ready to reproduce inside their bird hosts. The team found that these trout swam more actively than uninfected controls and stayed closer to the water’s surface.

Both traits should make fish more conspicuous to birds. When the researchers simulated a bird attack by making a shadow swoop over the tank, the fish froze – but infected fish resumed swimming sooner than uninfected ones.

Gopko says both studies show that how eye flukes manipulate their host’s behaviour depends on their age. Immature parasites “are too young and innocent to infect a next host”, he says, so their goal is to protect the fish they are living in. Mature parasites, however, are ready to reproduce – and to do so they need to get inside a bird’s gut.

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