Bucephalus polymorphus
...is a species of digenean trematode that is known to parasitize some freshwater molluscs and fish. B.polymorphus starts its life-cycle inside a Zebra Mussel (Dreissena polymorpha) where the larvae will move from its miracidium stage to the free-swimming cercariae stage. From there it will seek out a fish and attach to it and transform to its metacercariae stage. The third and final stage occurs when a predatory fish consumes the infected fish and the trematode will reproduce and the cycle will continue.
Bucephalus polymorphus is known to show some influence on its hosts. Specifically on the zebra mussel, where it can castrate it via heavy infection. This is good for the trematode because it allows the mussel to live longer and allow the trematode to produce more larvae.
Phylogeny
Animalia-Platyhelminthes-Trematoda-Digenea-Strigeidida-Bucephaloidea-Bucephalidae-Bucephalus-polymorphus
Image: Ernst Haeckel