Saprolegnia

Saprolegnia is a genus of freshwater mould. It is often called a "cotton mould" because of the characteristic white or grey fibrous patches it forms.

Habits
Saprolegnia, like most water moulds, is both a saprotroph and necrotroph. Typically feeding on waste from fish or other dead cells, they will also take advantage of creatures that have been injured or compromised eggs. When they inhabit a live animal, they exhibit as a fungal infection known as mycoses.

Saprolegnia is tolerant to a wide range of temperature, 3°C to 33°C, but is more prevalent in lower temperatures. While it is found most frequently in freshwater, it will also tolerate brackish water and even moist soil.

Saprolegnia filaments (hyphae) are long with rounded ends, containing the zoospores. Saprolegnia generally travels in colonies consisting of one or more species. They first form a mass of individual hyphae. When the mass of hyphae grows large enough in size to be seen without use of a microscope, it can be called a mycelium. Colonies are generally white in color, though they may turn grey under the precesence of bacteria or other debris which has become caught in the fibrous mass.

Reproduction
It has a diploid life cycle which includes both sexual and asexual reproduction. In the asexual phase, a spore of Saprolegnia releases zoospores. Within a few minutes, this zoospore will encyst, germinate and release another zoospore. This second zoospore has a longer cycle during which most dispersal happens; it will continue to encyst and release a new spore in a process called polyplanetism until it finds a suitable substrate. When a suitable medium, is located, the hairs surrounding the spore will lock onto the substrate so that the sexual reproduction phase can start. It is also during this stage of polyplanetism that the Saprolegnia are capable of causing infection; the most pathogenic species have tiny hooks at the end of their hairs to enhance their infectious ability.

Once firmly attached, sexual reproduction begins with the production of male and female gametangium, antheridia and oogonium respectively. These unite and fuse together via fertilization tubes. The zygote produced is named an oospore.

Characteristics of Infection
Saprolegnia is generally a secondary pathogen, though in the right circumstances, it can act as primary. It most frequently targets fish, both in the wild and in tank environments. Through cellular necrosis and other epidermal damage, Saprolegnia will spread across the surface of its host as a cotton-like film. Though it often stays in the epidermal layers, the mould does not appear to be tissue specific. A Saprolegnia infection is usually fatal, eventually causing heamodilution, though the time to death varies depending on the initial site of the infection, rate of growth and the ability of the organism to withstand the stress of the infection.

The extensive mortalities of salmon and migratory trout in the rivers of western Europe in the 1970's and 1980's in the UDN outbreak were probably almost all ultimately caused by the secondary Saprolegnia infections.

DNA studies suggest the Saprolegnia species affecting Australian freshwater fish is an introduced strain, presumably imported in the 1800s with exotic fish species.

Other sites

 * Saprolegnia - water fungus
 * Saprolegnia mixta - video
 * Genetic breakdown - Saprolegnia ferax

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