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Golden Alga (Prymnesium parvum)
Photo credit:  Proyecto Agua.


(Didymosphenia geminata)

Report this Species!

If you believe you have found this species anywhere in Pennsylvania, please report your findings to iMapInvasives by submitting an observation record.

Species at a Glance

Didymo, also called “rock snot,” is a single-celled diatom alga found in the cool waters of northern Europe and North America. Since the mid-1980s, it has begun to take on the characteristics of an invasive species, releasing stalk material when conditions are right that form massive blooms that can blanket stream and river bottoms, threatening a variety of aquatic systems.


Beginning as small circular brown blotches on rocks and other substrates, didymo can grow in two stalked forms – short-stalked and long-stalked. The short form generally appears as a coating on hard substrates, while the long-stalked form can take on the appearance of wet fiberglass or toilet paper. Didymo has also been mistaken for raw sewage. The external cell wall is made of silica and is capable of producing the extracellular stalk material that forms thick nuisance mats that can be over 20 cm (8 in) thick. While it appears slimy, it is actually rough to the touch, feeling like wet wool or cotton and is very difficult to pull apart and detach from rocks.

Similar Species

To determine whether a suspect specimen is didymo, squeeze out as much water as possible and rub it between your fingers. Unlike other species of algae, didymo does not break apart when rubbed between your fingers, nor does it feel slimy to the touch. Only microscopic examination can confirm the identity of didymo over other stalk-forming diatoms such as Gomphoneis and Cymbella.


Didymo is both epilithic (attaching to stones) and epiphytic (attaching to plants) and can thrive in a wide range of physical and chemical conditions within lakes and rivers. It prefers relatively shallow, clear, moderately-flowing and nutrient-poor waters with rocky substrates and plenty of sunshine. Nuisance blooms are only known to occur in flowing water or where there is plenty of wave action.


Anglers, kayakers, canoeists, and boaters can accidentally spread this microscopic hitchhiker when it clings to boats, fishing gear, waders, and boots. Felt-soled waders are especially good at transporting didymo because they can stay wet for longer periods of time, and didymo can survive outside of a stream in a cool, damp environment for at least 40 days. Only one cell is needed for didymo to spread.


Historically, didymo was found in cooler waters in the northern hemisphere and was considered a rare alga in the United States. However, in recent years it has exhibited a much greater tolerance for different water quality conditions and has expanded to diverse areas including parts of Canada, New Zealand, and scattered areas in the United States, including New England, Lake Superior, the western United States, and the Mid-Atlantic region in Maryland, New York, North Carolina, Pennsylvania, Virginia, and West Virginia.


Note: Distribution data for this species may have changed since the publication of the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016), the source of information for this description.

Environmental Impacts

Didymo cells can create large amounts of stalk material that form thick mats capable of engulfing a stream bottom, covering substrates, smothering aquatic organisms, and ultimately reducing fish habitat and food. Didymo does not appear to affect the safety of drinking water, does not produce an odor, and while aesthetically unappealing, does not appear to be a threat to human health.


Information for this species profile comes from the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016).

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