A classic example of parasitism in the freshwater biome is the relationship between the parasitic flatworm *Schistocephalus solidus* and the three-spined stickleback fish.

Here's how it works:

* The parasite: *Schistocephalus solidus* is a flatworm that has a complex life cycle involving two hosts.

* The primary host: The parasite's primary host is a copepod, a tiny crustacean found in freshwater. The flatworm reproduces within the copepod, producing larvae.

* The secondary host: The stickleback fish becomes infected when it eats the infected copepod. The flatworm larvae grow and mature inside the fish's body cavity, consuming its resources.

* The impact: The flatworm's presence weakens the stickleback, making it more vulnerable to predators and less likely to reproduce successfully. The parasite benefits by completing its life cycle, while the fish suffers negative consequences.

This is just one example; many other parasitic relationships exist in freshwater ecosystems.