First record of an aquatic oligochaete infesting fish.

This case represents the first documented report of aquatic oligochaetes () infesting fish. Oligochaetes are common and ubiquitous in aquatic environments, but parasitic (and symbiotic) species are extremely rare with a few species occurring in frogs. During a disease surveillance project, live oligochaetes were observed in fresh preparations of gills of the Australian freshwater Murray cod () that had been reared in a recirculating aquaculture system. Large numbers of oligochaetes were also found in detritus from the biofiltration system of the tanks. In autopsied fish, patches of filaments showed marked and diffuse hyperplasia, goblet cell metaplasia and mainly mononuclear infiltrate. This infestation may have caused sufficient damage to compromise the health of the fish and even death, considering that heightened mortality had occurred prior to their discovery. This infestation was considered not a case of parasitism but rather an opportunistic colonisation event triggered by a number of factors including the presence of a large population of within the recirculating aquaculture system and unhealthy (or stressed) fish that could not ward off infestation.