The fluke Euhaplorchis californiensis begins its life in an ocean-dwelling horn snail, where it produces larvae that then seek their next host, a killifish.
Once it finds a fish, the parasite latches on to its gills and makes its way to the brain. But this isn't its final stop.
The fluke needs to get inside the gut of a water bird in order to reproduce. So inside the killifish's brain, the fluke releases chemicals that cause the fish to shimmy, jerk, and jump.
Jenny Shaw, then at the University of California, Santa Barbara, and colleagues found that the parasite decreases serotonin and increases dopamine levels in the fish's brain. The switch in this brain chemistry stimulates the fish to swim and behave more aggressively.
These moves attract the attention of birds, which may eat the fish—and the flukes. The flukes mate, and their eggs are released back into the water in the bird's droppings to be eaten by horn snails and start the cycle anew.