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The following article is acknowledged with thanks from R.E. Carlson of

Saprolegnia - Water Fungus

Like all fresh water environments, koi and goldfish ponds are living ecosystems. They contain plants, algae, invertebrates, protozoa, bacteria and molds or fungi. Many of the life forms in our ponds are dependent upon each other. This is known as symbiosis (living together). An extreme form of symbiosis is parasitism, where one life form lives at the expense of another. One of the more common forms of parasitism in our ponds is saprolegnia or water fungus. For the average ponder, great zoological detail about saprolegnia is not required to understand what it is, what it does and how to treat the pond and fish if it becomes a problem, therefore not much detail on the zoology of saprolegnia is offered here: just good-to-know information on how to identify and treat it as well as some tips to help prevent it from affecting your fish.

What It Is

Saprolegnia (or sap) is a freshwater fungus which simply means it lives in fresh water environments and needs water to grow and reproduce. Sap can also be found in brackish water and moist soil. It is often referred to as a “cold water” fungus as it flourishes in colder water, but it lives well in a wide range of water temperatures extending from 37 deg F to 91 deg F (3 to 31 deg C). While it has been described also as a “mold”, sap is a genus of fungus, with the difference being a “mold” is a mass of fungi.

SAPUnder the microscope, sap is composed of filaments that tend to have spherical ends. It is these spherical ends that house the zoospores, or the “seeds” of sap, that allow it to proliferate and spread. The filaments are called hyphae and give sap its cotton-like appearance. It is the hyphae, or more exactly, the hooked, foot-end of the hyphae, which penetrate the tissue of the fish as they seek nutrients. In the lower-power micrograph below, you can see the hyphae with the spherical ends. With a 400x microscope, the structure will look basically the same.

SAPIn the water, sap looks like fluffy cotton; however, out of the water it appears to be a matted mess of slime. Sap starts out either white or grey in color. The grey appearance also may indicate the presence of bacteria growing with sap’s structure. Over a short period of time, sap may turn brown or green as organic particles in the water (such as algae) adhere to the filaments. Note the picture below. This is a classic presentation of sap that has invaded a bacterial infection of the dorsal fin area of a koi. The green color is actually embedded algae particles within the sap’s filaments.

SAPBy appearance, sap can be easily confused with epistylus, a freshwater parasite that presents as a white cottony substance growing on the skin of the fish. Epistylus does not gather organic particles and will remain white. Confirmation of epistylus versus sap should be done with a microscope. The following is a microscope shot of epistylus at 400x: The favorite food of sap is dead organic tissue. We can usually see evidence of sap on dead and dying fish, live and dead fish eggs and even food left in the water so long that it has begun to rot. Typically we see infertile koi eggs being infected first with the fungus then it spreads to kill live, fertile eggs. Infected eggs have a typical fluffy cotton wool-like covering. Sap also likes to feast on exposed and decaying tissue caused by bacterial infections, such as ulcers. It is most prevalent on the head and fins of the fish as these are the areas that offer the least amount of natural resistance provided by the mucous coat.

water fungusPrimary and Secondary Invader

Typically we think of sap as a “secondary” invader. This means that something else has violated the integrity of the fish’s skin allowing bacteria to enter and provide direct tissue access for the sap hyphae to embed themselves. In treating sap, it is imperative to treat BOTH the sap and the underlying primary cause. This will be discussed in more detail in the treatment section below.

water fungusHowever, sap can also be a primary invader under the right conditions and this is where it gets the reputation as a “cold water” fungus. We all know that cold water conditions, including dramatic temperature changes, cause great stress on the fish and suppress the fish’s natural immune system. As stated above, sap flourishes in colder water by producing and releasing increased zoospore counts into the water. The combination of stress, suppressed immune systems, and increased spore counts give sap the ability to cause major problems in colder water. Sap infections are commonly the cause of “winter kill.”

The following is a list of conditions that support the spread of sap: Obviously the underlying theme in the above list is “stress” and this is the one thing that we need to guard against first and foremost to keep our fish healthy. In healthy conditions, our fish have some natural protection against sap with the mucous layer being the most effective first line of defense. The mucous layer provides the ability to reject a sap attack by sloughing off a layer of mucous and sending the sap with it. The mucous coat also provides a natural fungicide at the cell-level. So you can see that improper handling or any activity that reduces the mucous coat offers an increased opportunity for sap to take hold.

Death by SAP

An increased morbidity of fish caused by sap can be traced to three major things: We know that sap attacks fish weakened by any of the above and the suppressed nature of the immune system under cold water or high stress conditions leaves the fish mostly defenseless. However, what kills the fish in a sap attack is a condition known as hemodilution. By definition, hemodilution is “a decreased concentration (as after hemorrhage) of cells and solids in the blood resulting from gain of fluid from the tissues.” … this causes the blood to loose electrolytes (blood salts) and make it less than life-supporting. Then as the sap hyphae penetrate the tissue layers of the skin, water begins to enter the fish thereby further diluting the fish’s blood salts. This explains why fish grossly affected by sap appear lethargic and often lose their equilibrium.

Once it has (literally!) taken root, sap can spread rapidly over the surface tissue of the fish. While it is rare that sap will penetrate deep into tissue layers, even superficial damage to the fish’s initial tissue layers (and particularly the gills) can be deadly. Obviously, the more wide-spread the sap infection becomes, the higher the rate of hemodilution and greater chance that the fish will not recover. Therefore, managing the sap infection quickly becomes the key to saving the fish.