Didymosphenia geminata (Lyngb.) M. Schmidt

Common Name: Rock snot

Synonyms and Other Names:

Echinella geminata, Gomphonema geminatum, Didymosphenia geminatum [synonyms for the Great Lakes native strain]; Styllaria geminata, Dendrella geminata, Lyngbyea pulvinata var. geminata; Didymo

Identification: Cells are distinguished by their large silica cell wall (frustule) shaped like a curved bottle (MISIN 2017). Cells feature a raphe, which is a structure used for movement on surfaces. Extracellular stalks that form nuisance blooms are secreted from the raphe (Spaulding and Elwell 2007). While microscopic, Didymosphenia geminata forms dense mats of extracellular stalks that are pale yellowish brown to white in color. Nuisance mats resemble fiberglass and have the texture of wet wool, but do not feel slimy to the touch ( Pennsylvania Sea Grant 2013).

Size: cells to 140 micrometers

Native Range: Northern Europe and Asia.  Possibly northern North America.  

Map Key
This map only depicts Great Lakes introductions.

Great Lakes Nonindigenous Occurrences: While the species is cryptogenic in origin, it has been observed throughout the Great Lakes region.  Prior to ~1990, all records of this form were as short-stalked unicells which did not form mats.   

Pre-1990 records and records of short-stalked unicells are presumed to be native are not included in the map or table.  These include: Great Lakes (Stoermer 1975, Stoermer & Kreis 1978, Stoermer et al. 1999), Lake Superior (Fox et al. 1967, 1969, 1973, Nelson et al. 1973, Stoermer 1980, 1993, Stoermer et al. 1986, Moffat 1994), Mackinaw Island (Bailey 1842), Lake Michigan (Briggs 1872, Thomas & Chase 1887, Marsh 1895, Ward 1896, Chase 1902, Britton 1944), Chicago (Boyer 1927), Indiana (Palmer 1930, Lindsey et al. 1969), New York State (Hohn 1951), Buffalo (Day 1882), Niagara River (Kützing 1849, De Toni 1891), and the Cayuga Lake Basin (Burkholder 1931) as well as Canadian records for Island of Montreal (Miller 1915), Ontario (Duthie 1973), and  Quebec (Bellavance 2006, Côté 2007, MDDEP-MRNF 2007, Blais 2008).


Table 1. Great Lakes region nonindigenous occurrences, the earliest and latest observations in each state/province, and the tally and names of HUCs with observations†. Names and dates are hyperlinked to their relevant specimen records. The list of references for all nonindigenous occurrences of Didymosphenia geminata are found here.

State/ProvinceYear of earliest observationYear of last observationTotal HUCs with observations†HUCs with observations†
Michigan199420152Lake Superior; St. Marys

Table last updated 11/17/2018

† Populations may not be currently present.

Ecology: Didymosphenia geminata can be found in freshwater rivers, streams and lakes. This taxon prefers cool water of low conductivity (Patrick and Reimer 1975) and may be useful as indicators of increases in salt concentrations in the Great Lakes (E. F. Stoermer, personal communication).  The species is predominantly benthic but has been observed in planktonic samples (Blanco and Ector 2009). It prefers clear streams and rivers with shallow, moderately-flowing water with a pH of 7 or above and rocky substrate (MISIN 2017; Sea Grant Pennsylvania 2013; Spaulding and Elwell 2007). Historically the species was restricted to low-nutrient waters but has recently seen large range expansions reportedly occurring in eutrophic rivers, showing much greater tolerance for nutrient and flow conditions than previously expected. This may be attributed to a genetic variant with broader tolerances than the original species.  Nuisance benthic growths are only known to occur in flowing water.  While this species seems to be confined to cold areas, it reaches its highest biomass at higher water temperatures of ~20°C (Blanco and Ector 2009). It has also been observed at temperatures of 27°C (Spaulding and Elwell 2007).  

Didymosphenia geminata is capable of sexual reproduction, but recent nuisance blooms occurring globally have been a result of asexual vegetative cell division (Bothwell and Spaulding 2008). When the cell divides, the stalk divides as well (Spaulding and Elwell 2007).  Didymosphenia geminata typically blooms in streams with low to undetectable levels of phosphorous.  Short-term nitrogen and phosphorus additions followed by nutrient limitation have been reported to increase cell densities and division rates, possibly accelerating mat formation. The addition of nitrogen and/or phosphorus can stimulate cell division and increases the frequency of dividing Didymosphenia geminata cells (Bray et al. 2017; Jackson et al. 2016). As nutrient  concentrations (particularly dissolved phosphorus) decrease, stalk production tends to increase  (Jackson et al. 2016).  Current research suggests that stalk production allows for better access to light in addition to providing habitat for nutrient cycling and phosphorus acquisition via phosphatase production (Bray et al. 2017).

Didymosphenia geminata is a food source for macroinvertebrates (Spaulding and Elwell 2007). Mats formed in blooms create microenvironments that can be colonized by diatoms and other organisms (Blanco and Ector 2009). Recent studies on blooms have revealed supersaturated concentrations of dissolved oxygen in D. geminata mats, suggesting that other photosynthetic organisms exist within these mats, allowing Didymosphenia geminata to compete with other algae for both nutrients and light (Blanco and Ector 2009; Spaulding and Elwell 2007).

Means of Introduction: Didymosphenia geminata has been shown to survive outside of the stream environment. Cells are able to survive and remain viable for 40 days in cool, dark, damp conditions. Angling equipment, boot tops, neoprene waders, and felt-soles provide a particularly suitable environment for cells to remain viable. Cells can hitchhike on this equipment and other recreational equipment into new bodies of water (Spaulding and Elwell 2007). Freshwater diatoms are dispersed through the flow of water and transport by other organisms, primarily waterfowl (Kristiansen 1996).

Status: Expanding range in the Great Lakes.  Didymosphenia geminata is considered native to the Great Lakes (Stoermer 1965 collection).  Lack of evidence for the native status has led to the idea that recent blooms of D. geminata are due to introductions or range expansions, or possibly a separate strain of the species that is not native (Taylor and Bothwell 2009). In recent history, Didymosphenia geminata has seen dramatic range expansions globally, including in North America. Taylor and Bothwell (2009) argue that observational and experimental evidence shows that changing environmental conditions, specifically low phosphorus concentrations, are causing nuisance blooms which are responsible for drawing attention to the species.

Great Lakes Impacts: Didymosphenia geminata has a moderate environmental impact in the Great Lakes.

Most impacts are associated with large blooms, which have so far been rare in the Great Lakes.  Blooms of Didymosphenia geminata form mats which can be over 20 cm thick. Extracellular stalks trap fine sediment, changing the nature of substrate and have potential long lasting effects due to the apparent resistance of stalks to degradation by bacteria and fungi (Spaulding and Elwell 2007).  D. geminata blooms are associated with diurnal dissolved oxygen fluctuations (Blanco and Ector 2009). The decomposition of mats depletes dissolved oxygen in the water. However, dissolved oxygen in studied rivers remained high at concentrations of 8.1–10.7 mg l-1 (Bhatt et al. 2008) and recent studies on blooms have revealed that the mats contain concentrations of dissolved oxygen which are supersaturated with respect to the atmosphere. This suggests that the mats contain other photosynthetic organisms which actively produce oxygen, allowing D. geminata to compete with other algae for both nutrients and light (Blanco and Ector 2009; Spaulding and Elwell 2007).  In the Restigouche River basin, D. geminata have altered the structure and diversity of diatom assemblages (Gillis and Lavoie 2014).

These mats are capable of engulfing the stream bottom, smothering native species of plants, insects, mollusks, and algae, and reducing habitat for insects for aquatic insects and fish (Pennsylvania Sea Grant 2013). Macroinvertebrates that require exposed sediment are expected to be negatively impacted by this loss of habitat (Spaulding and Elwell 2007).  Streams outside the Great Lakes region harshly impacted by these mats have seen invertebrate populations decrease, macrophyte elimination, and absence of fish (Blanco and Ector 2009).  Didymosphenia geminata is a food source for macroinvertebrates, but of low nutritional value (Spaulding and Elwell 2007). With increasing D. geminata biomass invertebrate communities were seen to become more homogenous spatially, however no decline to taxon diversity or richness were seen (Kilroy et al. 2009).  In New Zealand rivers, D. geminata blooms have been observed in conjunction with changed community composition. Observed changes include increased periphyton biomass, increased invertebrate, Chironomidae, Cladocera, and Nematode densities. Communities were also observed to transition to Oligochaeta dominated (Kilroy et al.  2009). 

Fish reproduction may be directly impacted by reduction of suitable spawning areas (Blanco and Ector 2009).  Additionally, Didymosphenia geminata mats may provide habitat for Tubifex tubifex, which is a host to whirling disease (Myxobolus cerebralis) affecting salmonids; whirling disease prevalence is 3x higher in streams with blooms of D. geminata (Byle 2014).

It has been hypothesized that a new strain of Didymosphenia geminata is now dominant (Bothwell et al., 2006), and is responsible for the invasive behaviour. However, the presence of a new genetic strain has not been established.

Didymosphenia geminata has a moderate socioeconomic  impact in the Great Lakes.

Most impacts are associated with large blooms, which have so far been rare in the Great Lakes. 

Didymosphenia geminata is frequently reported by homeowners, recreationalists and land managers to be unsightly; growths are frequently mistaken for raw sewage (Spaulding and Elwell 2007). Diminished aesthetic value can impact tourism revenue generated by outdoor recreation and thick mats making fly-fishing difficult (Spaulding and Elwell 2007).

Didymosphenia geminata expansion is correlated with a decline in fishery industries in South Dakota (Blanco and Ector 2009).  In Pennsylvania D. geminata have the potential to impact trout fisheries by altering the aquatic invertebrate food base (Pennsylvania Sea Grant 2013) by reduction of suitable spawning areas (Blanco and Ector 2009), and by providing habitat which increases prevalence of whirling disease (Byle 2014).

Nuisance algae including Didymosphenia geminata thrive in and clog canal systems which are used for water transport for hydropower, agriculture, and human consumption (Spaulding and Elwell 2007).

There is little or no evidence to support that Didymosphenia geminata has significant beneficial impact in the Great Lakes.

Management: Regulations

It is prohibited to transport, possess, or introduce Didymosphenia geminata in Wisconsin, with the exception of within Lake Superior (Invasive Species Identification, Classification and Control  Chapter NR 40)

Pennsylvania considers Didymosphenia geminata an aquatic invasive species. Management goals include placing the species on the invasive species list of 58 Pa. Code§71.6 and §73.1, public education, further research, and rapid response including the application of algaecides (Pennsylvania FIsh & Boat Commission 2011). The New York Department of Natural Conservation provides public information on the impacts of Didymo how its spread can be limited

Note: Check federal, state and local regulations for the most up-to-date information.



There are no known biological control methods for this species.

Managers of canal systems implement regular removals of Didymosphenia geminata by scraping growths from the concrete surfaces (Spaulding and Elwell 2007).  Flushing flows like those used for channel maintenance have been shown to reduce D. geminata benthic mats in the Rocky Mountains, Canada (Cullis et al., 2015).

The spread of Didymosphenia geminata can be reduced by cleaning boots, waders, boats, float tubes, and angling gear before traveling between bodies of water. Decontamination of gear has been shown to effectively destroy cells of Didymosphenia geminata. Heating for prolonged periods or freezing for several days can sterilize gear. Gear can also be soaked in hot water (140°F) for one minute to sanitize. Education and outreach related to this procedure could be implemented to minimize the spread of the species (Hoddle 2017; Sea Grant Pennsylvania 2013; Spaulding and Elwell 2007).

Absorbent items require longer soak times to properly sanitize. Items should be soaked for at least 40 minutes in hot water above 140°F for 30 minutes if the hot water is kept above 115°F is mixed with 5% dishwashing detergent (Sea Grant Pennsylvania 2013).

Investigation of ten potential control agents (algaecides/biocides) was initiated through experimental trials (Jellyman et al., 2006). The control agents were rated based on their effectiveness on causing cell mortality and degradation of biomass. Trials in artificial channels showed that a chelated copper compound was the most effective in killing D. geminata cells and minimizing effects on non-target species (Clearwater et al., 2007a).

Low phosphate levels are associated with bloom formation, enrichment with slow- and quick-release fertilizers was tested in a river in South Dakota, USA, and reduced D. geminata biomass locally (James et al., 2015).

Recreational gear can be sanitized by soaking gear in a 2% bleach solution, 5% salt solution, antiseptic hand cleaner, or dishwashing detergent for one minute (Sea Grant Pennsylvania 2013).

Remarks: Records from the Great Lakes region prior to 1990 or that are unicells with short stalks rather than mat-forming colonies are not included in the mapping data as they are presumed to be native.  

References: (click for full references)

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Other Resources:
Author: Sturtevant, R. and K. Hopper

Contributing Agencies:

Revision Date: 8/14/2018

Citation for this information:
Sturtevant, R. and K. Hopper, 2019, Didymosphenia geminata (Lyngb.) M. Schmidt: U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, FL, and NOAA Great Lakes Aquatic Nonindigenous Species Information System, Ann Arbor, MI, https://nas.er.usgs.gov/queries/greatLakes/FactSheet.aspx?SpeciesID=2856&Potential=N&Type=0&HUCNumber=DGreatLakes, Revision Date: 8/14/2018, Access Date: 1/21/2019

This information is preliminary or provisional and is subject to revision. It is being provided to meet the need for timely best science. The information has not received final approval by the U.S. Geological Survey (USGS) and is provided on the condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from the authorized or unauthorized use of the information.