Parasite Infects Fish Eyes And Makes Them More Likely To Be Eaten By Birds

The parasite needs to be eaten by birds, but only when it has matured. Wang LiQiang/Shutterstock

Josh Davis 05 May 2017, 19:21

Like something straight out of science fiction, a flatworm burrows into a fish and makes its way into the eye of the victim, where it then takes control of the unfortunate host. But depending on the age of the parasite, it makes the fish behave in different ways.  

When the eye fluke (Diplostomum pseudospathaceum) is young, it forces the infected fish to act in a way that lessens the chance it will be eaten, making it stay near the bottom of the water and move less. As soon as the parasite has matured and is ready to move on to its next host, it reverses these behaviors, making the fish swim near the surface and move around more. This increases the chance the fishy host will be eaten by a passing bird.

But the parasite is not finished there. When it is ingested by the bird, mature flukes mate in the avian stomach and their subsequent eggs are then pooped out into the water. When these hatch, the larvae go in search of a snail to enter, where they grow and multiply. Finally, they leave their mollusc host to complete the cycle, infecting a fish and migrating to the lens in the eye to begin their control.

While the parasite is nothing new, and previous research conducted by the same group has already investigated how the parasite makes younger fish behave in a way that minimizes the chance they will be eaten, this latest work has focused on the second phase of control. Published in Behavioral Ecology and Sociobiology, the researchers have shown how the mature parasites in the fish try to increase the chance their host will be consumed by birds.

They conducted experiments in which they tested how both infected and uninfected fish behaved in tanks, and how they then responded to bird attacks, which were simulated by passing a bird-shaped shadow over the aquarium. When the bird shadow passed over, both infected and uninfected fish froze, but the infected fish began moving again much more quickly.

Many parasites are known to take over their host and control them. There are thousands of species of cordyceps that infect insects and alter their behavior, and the parasite Toxoplasma gondii is even thought to make mice more likely to get chomped on by cats. But this parasite is weirder in that at different stages, it makes the fish behave in different ways.

The results of this latest study reveal that the behavioral changes induced by the parasites play a vital role in freshwater food webs.

[H/T: New Scientist]

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