Please help to do your part to stop the spreading of this highly invasive algae that has already gained a foothold in the Connecticut River and highly suspect in the White River...

Didymo Frequently Asked Questions

Q: What is Didymo?
Didymosphenia geminata also known as “Rock Snot” or more commonly “Didymo” is a single-celled, freshwater alga or diatom. While the individual cells are invisible to the naked eye, Didymo can form large colonies or mats that attach via a stalk to rocks and plants smothering the bottoms of streams and rivers with a thick yellow/brown layer of growth. The massive blooms can form distinctive flowing ‘rats tails’ that turn white at their ends and look similar to tissue paper. Since the 1980s, this once rare species has been expanding its geographic range and forming excessive growths covering miles of stream reaches


Q: Why is Didymo a problem?
These massive nuisance growths may impact freshwater fish, aquatic plants, and important aquatic insects. Didymo blooms can reduce the amount and availability of suitable habitats (Biosecurity NZ, 2005). When large mats are present impacts can include: reduced size and density of some larval aquatic insects (caddisflies, stoneflies, and mayflies), increased populations of invertebrates typically associated with degraded stream habitat (midge larvae, oligochaete worms), a reduction of clean substrate for fish nesting and egg laying, and a decline in large, adult fish (EPA, 2006)

Economic impacts include fouling of water intakes and, in heavily infested areas, Didymo may be linked to a decline in freshwater angling and related tourism. Impacts to aesthetics are common, with reports of unsightly masses that appear like strands of toilet paper leading to concerns about sanitation and sewer malfunctions. The heavy, slippery brownish mats degrade swimming areas and also foul fishing gear.

Direct human impacts may be limited to swimmers complaining of eye irritation after swimming in affected areas (Kilroy, 2004).


Q: Is Didymo an invasive species?
Although considered to be native to the northern boreal regions of the Northern Hemisphere, its biological status in the United States remains uncertain (ISSG, 2005). The recent range expansion and appearance of excessive Didymo growths in North America (defined by the EPA as being those that cover more than 1 mile of stream reach and persist for more than a few weeks) suggest a precautionary management approach is advisable. In order to increase awareness of the problems posed by Didymo and to reduce the risk of an unwanted introduction the Oregon Invasive Species Council added Didymo to the state’s 2006 100 Least Wanted Species list.

Scientists do not know what has changed to allow Didymo to take on the characteristics of an invasive species. It has been postulated that the emerging invasive populations are the result of a new genetic variant or strain of Didymo that has broader environmental tolerances than the original. It has also been suggested that increased exposure to ultraviolet (UV) radiation may favor Didymo growth by either reducing the grazer population or limiting populations of other algal species that might outcompete or repress Didymo growth (Kilroy, 2004).


Q: How does Didymo spread?
Human-assisted movement of Didymo is the most likely culprit in many introductions, including long-distance dispersal such as the recent discovery of Didymo in New Zealand. Clumps of Didymo as well as single microscopic cells can be moved around by boaters, anglers, hunters and other aquatic recreationists. Didymo, like other aquatic invasders of concern such as the New Zealand mudsnail and whirling disease, can be transported on wet felted soles and boots, damp wading gear, and in any standing water associated with boats or other types of gear.

It is possible that animals could spread Didymo locally e.g., on the damp feet or feathers/fur of birds or other organisms. Short-distance wind aided dispersal has also been postulated (Kilroy 2004).


Q: What habitats are at risk?
Didymosphenia geminata tends to form excessive mats in (EPA 2006):
* Low, stable flow
* Water 3 inches to 6 feet deep
* Moderate to high current
* Below outlets of lakes and reservoirs
* Sunny, open streams


Q: Can Didymo be eradicated?
No, that’s why it is so important to prevent the spread of Didymo through clean boating, fishing and other aquatic recreation practices. Eradicating any microscopic organism from a natural environment is difficult, especially in an aquatic environment. To date there are no known systematic attempts to eradicate invasive blooms of Didymo.


Q: How can I tell if an algal growth is actually Didymo?
Mats of Didymo are much more likely to be found in a stream or river rather than a reservoir or lake and should not be confused with blue/green algal blooms. Didymo has a distinctive appearance but tufts and mats can be thick white, light gray, pale yellow-brown, or beige (not green) masses. An excessive infestation may cover over 90 percent of the river bottom in many reaches and these masses may form ratty or fringed tails that turn white and closely resemble streaming toilet paper (EPA 2006). Other times, these mats may resemble a brownish/yellow shag carpet. Although Didymo looks slimy, it’s actually spongy and sort of scratchy rather like wet cotton wool (Kilroy 2004).


Q: If Didymo can be spread by birds and animals, will it really make a difference if I clean my gear?
Yes it will. Not only can you help slow the spread of Didymo to locations where animal assisted dispersal could be unlikely but also clean gear habits can prevent the spread of other invasive species of concern such as mud snails, aquatic weeds, zebra mussels and whirling disease, as well as prevent the introduction of some invaders we might not yet be aware of.