ANNAPOLIS, MD (May 4, 2008)
Oh No! Not Rock Snot! What do you know about Didymo?
Didymosphenia geminata (known as Didymo or rock snot) has recently been found in a Maryland stream.
This diatomaceous alga forms heavy mats during its growing period and can coat stream bottoms. It can impact
fishing, boating, and water intake structures by fouling equipment with this tough mat material. It can disrupt
the habitat of insect larvae and other organisms that live on stream or lake bottoms, but research has shown no
major impacts on fish populations. The only threats to humans could be that it can mess up water supplies
and is abrasive if rubbed against sensitive skin.
Didymo is a diatom that lives at the end of a robust stalk that it builds. Many of these stalks can weave
together to form thick dense mats. The diatoms have a shell made of silica and the stalks of mucopolysacharide;
both are very tough. The mats trail in the water and can be light brown to white in color. Some people say it
looks like toilet paper flapping in the water. It looks slimy, thus spawning the nickname “rock snot.” In
reality it is thick and rough to the touch - like wet cotton. If you grab a Didymo mass, it is very tough to
pull apart and hard to detach from rocks. There are some other algae that can feel coarse and tough, but they
have some green color to them. Tough, dense, abrasive material in the water in white or tan shades usually
means you’ve got rock snot. The mats can persist for up to two months after the diatoms die.
Until recently, Didymo was a rare alga distributed across the United States. It had always been outcompeted
by other algal species or excluded by environmental conditions. In the past, it flourished in waters that were
cold, low nutrient waters. It originally was described in waters with these characteristics in the late 1800’s
and early 1900’s in Scotland, Finland, Sweden and northern China. It formed heavy mats in these, but was never
seen doing this elsewhere. In the early 1990’s, worldwide changes began to be seen in D. geminata populations.
More and more areas began showing larger, more extensive blooms. Major blooms have been observed in New Zealand
where the government has mounted serious gear restrictions and disinfection policies on infested rivers.
The diatom seems to prefer regulated rivers, particularly below dams, but this may be changing as well. It
now tolerates warmer temperatures and higher nutrient levels. Didymo has a low tolerance for silt or sediment
and tends to prefer clearer water and rocks for substrate.
Didymo is not a true exotic in the United States because it can be found in historic records. In its
history in this country, Didymo did not exhibit invasive tendencies. However, researchers are afraid that
what is being seen in rivers now is a different substrain or altered version of the diatom. It is spread
primarily by man on materials that can stay wet and are transported from one area to another. It is possible
that animals can do this as well, but humans have the most impact. Porous materials, such as felt bottom waders
and clothing have been implicated, since many of the waters that have developed Didymo problems in the last
five years are major, premier trout fishing areas. The problem has existed on the West Coast of the US for
a while, but has been discovered on the East Coast in the last five years. Research is ongoing and biologists
are trying to determine why the diatom is becoming more of a problem. With these changes and the unknown
factors with this and other invasive species, management agencies are making strong efforts to educate the
public and encourage checking, cleaning and drying all items that come in contact with open water before
moving to another area. The major impacts of the blooms other than being an eyesore and nuisance are that
the growths alter macroinvertebrate and algal community structure and may impact water quality. To date,
no studies have shown impacts on fish or other aquatic vertebrate species.
Prevent the spread of Didymo by being fastidious with your equipment. When you leave a stream, clean all
dirt off fishing gear, canoes, inner tubes, etc. Clean the gear; many things kill Didymo including drying,
scrubbing with hot soapy water, wiping down with hand disinfectant and freezing. The one item that is not as
effective and is discouraged in Maryland is chlorine (bleach). Chlorine can be very toxic, but is also unstable
and may not maintain disinfecting levels because of its volatility. It has also been shown that chlorine does
not penetrate dense materials well unless it has soap added. Other prevention methods suggest moving away from
using porous materials, including boot felts, around water, or having two or more sets of gear, so one set can
be drying for at least five days at any time.
All state agencies experiencing Didymo outbreaks are using similar approaches – educate the public to
prevent the introduction or spread of this and other agents from water to water, identify characteristics
that may spur blooms and learn to manage around the problem. Once this has gotten into open waters it cannot
For more information about other Invasive Species of Concern,
visit www.mdinvasivesp.org or call the Maryland Department of Agriculture at 410-841-5920.
For more information on the Internet:
MAF Biosecurity New Zealand, www.biosecurity.govt.nz/didymo
Didymo In The Gunpowder River Video, http://www.mefeedia.com/entry/didymo-in-the-gunpowder-river/8926531/