Rock Snot Does Not Rock

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Nicknamed “rock snot” because of its slimy green appearance, the single-celled protist, didymo (Didymosephenia geminata) is invading rivers across the country. Didymo has been historically confined to cold, clear, low-nutrient streams in northern Europe, Russia, and northern North America. Recently, a new strain has been found in warmer climates and nutrient-rich waters.

Didymo can survive for months in fishing equipment and has been unintentionally carried by humans from location to location. Even a single microscopic cell of didymo can be enough to start a new colony. In this way, didymo has been spread to an increasing number of states in the U.S., and even all the way to New Zealand.

Didymo can form carpet-like “blooms” that wreak havoc on river systems. The blooms consist of stalks that the didymo uses to attach to rocks on the bottom of a riverbed. The didymo eventually die off but vast masses of stalks persist, choking out the native species of algae and plants, disrupting insect habitats, and affecting their fish predators. Didymo can also clog water intake pipes such as irrigation pipes or municipal water pipes.

The presence of didymo has been confirmed for the first time in the Youghiogheny River in Pennsylvania. Your task is to assume the role of one of the following members of a Pennsylvania community:

Trout Fisherman
Environmental Activist
Aquatic Biologist
Fish and Boat Commission Director
Department of Conservation and Natural Resources Official
Local Resident
Bureau of State Parks Director

You will attend a community meeting to discuss what should be done about the existence of didymo in Pennsylvania. Use the research questions below to help you prepare for the meeting. As you research and conduct your meeting, consider how the didymo issue affects you specifically in your role.


1. What should or can be done to eliminate the existing didymo populations from the Youghiogheny River?

2. What specific laws or regulations, if any, should be put into place to prevent the further spread of didymo?

3. What public information may help prevent the further spread of didymo and how should it be distributed? If signage is used, where should it be placed?

4. How are other states handling this issue?

More to Explore

Rock Snot Found in Pennsylvania
USDA Invasive Species Profile for Didymo
West Virginia Best Practices