Although we have been mainly concerned with the kinds of predators that might eat a fish, it is an unfortunate fact of life for wild fishes that this tells only half the story. Parasites are everywhere in the wild, and it is a rare fish that does not harbor some parasites, perhaps even thousands, during the course of its life.
What exactly is a parasite?
A parasite is a general term for any animal that gains all or part of its nutrition from another, usually larger, animal. Some parasites, such as fish lice, live outside their hosts, making a connection only when they want to feed. Others live inside, eating their host from the inside out.
How can fish avoid parasites?
In simple terms, they cannot avoid them completely, but they do have an inbuilt resistance, more effective in some individuals than in others. For example, a healthy, well-fed adult will be less prone than a juvenile or weak fish. In addition, fish have some behavioral tricks to help them avoid parasites. Some fish are known to move between freshwater and the estuaries of their river, thereby applying a salt bath to control the parasites. Most schooling fish are able to recognize when a conspecific is parasitized and avoid it if possible.
How do parasites get onto or inside a fish in the first place?
The ways of parasites are many and devious. Some, such as bacteria and fungus that cause infections, and others, such as the protozoans responsible for white spot disease, are present in the water all the time and are able to attack when a fish is weakened, stressed, or wounded. Others use some surprising tactics. Fish lice actively hunt their victims using smell and vision and are especially attracted to anything shiny which, underwater, usually means a fish. Fluke worms, such as Gyrodoctylus, cause the fish that they infect to clamp their fins and wave their bodies, a behavior sometimes known as "shimmying.' This behavior apparently causes other fish to approach and investigate, putting them at greater risk of infection, as well as creating a
small local current of water that the parasites can use to disperse, almost like a sneeze can carry cold germs between humans. The larvae of trematode worms infect the tiny invertebrates that fish feed on, such as Cyclops and Daphnia. When a fish eats one, the parasite enters its unwitting host like the Trojan horse.
Do parasites change fishes' behavior?
Some parasites have complex life cycles that require them to move between different hosts as they mature. In many cases, the parasites move from the fish into predatory birds. For this to occur; the bird must eat an infected fish. To maximize the chances of this happening, parasites can alter the behavior of the fish, so that it is more likely to be eaten. One parasite that infects killifish damages the fish's brain and makes it swim erratically at the surface of the water. Others lodge in the lens of their host fish's eye, preventing the victim from seeing an approaching predator. The tapeworm Ligula seems capable of making its fish host completely fearless, so that instead of dashing for cover when danger threatens, it hangs around, often with fatal consequences.