Fish Eye Flukes
Diplostomum pseudospathaceum is parasitic fluke that infects snails, fish, and birds. Adult flukes mate in a bird’s digestive tract, shedding their eggs in its feces. The eggs hatch into larvae in the water, then seek out freshwater snails to infect. Fluke larvae grow and asexually multiply inside snails, eating their hosts’ bodily reserves before being released into the water, ready to track down their next victim: fish.
Upon finding a fish, flukes make their way into the fish’s eye lenses, which lack blood vessels, and so are protected from attack by the fish’s immune system. From this remote post Diplostomum energetically alters the fish’s behavior. While the larvae in the eye mature, the host fish is less active than usual, making itself less visible to predators, and so more likely to survive. Immature flukes are too young to infect their next (avian) host, so they protect the fish they are in.
Once the fluke larvae are fully grown, they have their host swim more actively near the surface, making the fish more conspicuous to piscivorous birds. The flukes repress the normal evasive responses the fish would otherwise have when feeling threatened (such as a shadow over it). By this time the fish is likely mostly blind, its sight having suffered the ravages of hosting growing flukes. The fluke completes its life cycle once the fish has been caught and eaten by a bird.