Common water mold
Saprolegnia parasitica
The common water mold is an oomycete that typically appears as white, cotton-like tufts on fish and their eggs. It primarily lives as a saprophyte on decaying organic matter but acts as an opportunistic parasite causing saprolegniasis in aquatic animals such as fish and amphibians. Reproduction occurs both sexually via oospores and asexually through motile zoospores that can actively navigate toward potential hosts. In aquaculture, it is recognized as one of the most significant pathogens leading to substantial economic losses.
Details
Identification
Cotton-like, white to dingy grey growths on skin, gills, or eggs; microscopically visible, sparsely branched hyphae without septa.
Diet
Feeds heterotrophically as a saprophyte on organic decay products or as a parasite on living tissue (proteins and lipids of host cells).
Hunting strategy
Chemical detection of host factors by motile zoospores (chemotaxis).
Overwintering
Survival as resistant oospores within the sediment.
Ecology
Ecological role
Important decomposer in the breakdown of organic matter; acts as a natural regulator of fish populations by selecting weakened individuals.
Natural predators
Microorganisms, protozoa, and small benthic invertebrates.
Competitor species
Other aquatic fungi and bacteria colonizing the same substrates.
Ecosystem service
Acceleration of nutrient recycling through decomposition of dead biomass.
Threats
None; the species is widespread and often benefits from human activities (eutrophication, aquaculture).
Scientific profile
Profile
Habitat
Freshwater ecosystems worldwide, particularly in rivers, lakes, and aquaculture facilities (salmonid farming).
Reproduction
Asexual via zoospores in zoosporangia; sexual via oogonia and antheridia forming thick-walled oospores.
Ecological role
Significant primary and secondary pathogen of fish (Saprolegniasis) and decomposer of organic matter.