By Nathan Smith
As a general rule, the fluffier a pet is the better. Fluffy things are cuter, more cuddly, and funnier when they move; it’s a win-all situation. But like all general rules, there are always exceptions. In this case the exception is fish.
Fish, as many will have already noticed, are not usually fluffy. Indeed it would be reasonable to put forward the hypothesis that fish and fluffiness are mutually exclusive and, for healthy fish, this is certainly the case. Unfortunately fish cannot always be healthy and sometimes unhealthy fish go fluffy.
The cause of such fluffiness is an oomycete, or water mould; a type of organism which looks like a fungus but is closely related to kelp. Specifically, it is caused by the oomycete Saprolegnia parasitica, which infects freshwater fish. S. parasitica causes grey/white cotton wool like patches on the skin and gills of infected fish and can coat up to 80% of the host’s body. These patches are external signs of destruction of the host’s skin and underlying tissue and this result in lethargy of the host, making it more susceptible to predation. If the host avoids predation, symptoms of late stage infection are impaired osmoregulation, which is caused by increased haemodilution due to the large scale surface wounds. This is followed by respiratory failure, caused by the extensive infection of the gills. Organ failure is the final result. A closely related species, S. diclina, also infects fish eggs.
Despite the bizarre symptoms, infection by Saprolegnia is no niche occurrence. Whilst it is generally not an issue for wild fish, generally only infecting wounded or otherwise immunocompromised individuals, it is a considerable problem in aquaculture hatcheries and farms, due in part to the overly high densities in which fish are kept. In these environments, losses of more than 10 % due to Saprolegnia are commonplace and as high as 50% in some more extreme cases. Furthermore, it has a significant economic impact, with conservative estimates putting the losses due to Saprolegnia infection at five million pounds per year in Scotland alone.
That Saprolegnia infection is so endemic in aquaculture indicates generally low health in the fish population and highlights issues within the industry. Fluffy fish may not be cute but they can’t be ignored.