Gyrodactylus salaris

Gyrodactylus salaris, commonly known as salmon fluke, salmon killer, or the Norwegian salmon killer is a tiny monogenean ectoparasite which lives on the body surface of freshwater fish. This leech-like parasite has been implicated in the reduction of Atlantic salmon populations in the Norwegian fjords. It also parasitises other species, including rainbow trout. G. salaris requires fresh water, but can survive in brackish water for up to 18 hours.

The parasite is 0.5 mm (0.02 in) long, and cannot be seen with the naked eye, but it can be seen with a magnifying glass. On its posterior end is a haptor, a specialized organ for attaching to the host fish, which has sixteen hooks around its edge. The parasite is viviparous, that is, it produces live offspring. The parasites give birth to live young nearly as big as themselves and at this time, a further generation is already growing inside the neonates.