What is Saprolegnia?
Saprolegnia (pronounced: "Sap-ro-leg-ni-ah") is a fungal-like organism that infects freshwater fish and their eggs, amphibians and their eggs, and crustaceans. There are several different species of Saprolegnia, but Saprolegnia parasitica (S. parasitca) is the main species that infects fish. Saprolegnia is part of the natural ecology of freshwater environments, therefore it is not unusual to see occasional Saprolegnia infections in wild fish.
S. parasitica causes the disease saprolegniasis. Typical signs of infection include patches of white cotton-like fluff on the skin, fins and gills of infected fish. This fluffy "fungus" destroys the surface tissues of the fish before penetrating into the muscle layers and blood vessels.
As the infection progresses, the regulation of water and mineral salts in the fish’s blood (a process known as osmoregulation) becomes impaired, eventually resulting in death.
Saprolegnia species are tolerant to a wide range of temperatures (3°C - 30°C) but more prevalent at lower temperatures. They are considered both saprotrophic (feed on dead or decaying matter) and necrotrophic (destroy living tissue and then feed on the dead or decaying matter).
Saprolegnia filaments (hyphae) are long with rounded ends. During "fungus" growth, a mass of hyphae is formed. When the mass of hyphae is large enough to be seen without use of a microscope, it is referred to as mycelium.
Life cycle of Saprolegnia
Saprolegnia has a diploid life cycle, meaning it includes both sexual and asexual reproductive stages.
The Asexual Phase
The asexual stage begins with the production of primary zoospores, these form within the hyphal tips of the Saprolegnia mycelia, known as sporangia. A decrease in available nutrients or sudden drop in environmental temperature are two factors known to trigger primary zoospore formation. The sporangia containing the primary zoospores swell and eventually burst. Upon release, the primary zoospores are able swim for a short period before they encyst, offering a minimal level of independent dispersal.
Primary cysts can then release secondary zoospores, which can swim for longer periods than their primary precursors and are considered the main infective unit of Saprolegnia species. For many Saprolegnia species, including S. parasitica, secondary zoospores can undergo repeated cycles of zoospore encystment and release, during a process known as “polyplanetism” or “repeated zoospore emergence” (RZE). This cycle allows secondary zoospores several attempts to locate a host and establish an infection. Secondary cysts of S. parasitica also possess a set of hairs or boathooks which are thought to aid in the attachment to passing fish. These hairs are thought to enhance the infective ability of S. parasitica.
The Sexual Stage
Upon locating a host, the sexual stage in the life cycle commences. The first step of which is the production of the male and female reproductive structures, the antheridium and oogonium respectively. Fertilisation occurs upon fusion of these sexual structures and the sexual spore produced is called an oospore.
Impacts of Saprolegnia infections
Economic impacts: Aquaculture is the world's fastest growing food industry. S. parasitica infections have a huge impact on this industry, with saprolegniasis-induced deaths reported in 1 in 10 farm-raised salmon. The Scottish salmon farming industry alone suffers an annual £5 million loss due to this fluffy fiend!
Environmental impacts: Declines in wild fish populations, particularly salmon, have also been associated with S. parasitica infections. It’s been suggested that infections in aquaculture may be transferred to wild fish stocks. However, the reverse may indeed be true. There is currently very little scientific evidence to resolve this issue.