Fact Check: No, mussels don't shut off Warsaw's water supply if they sense toxins — but they help monitor it

Yahoo

10-03-2025

Claim:
Warsaw, Poland, uses eight mussels to detect contaminants and automatically turn off city's water supply when it's too toxic.
Rating:
What's True:
Warsaw Waterworks indeed uses eight mussels to monitor water quality and trigger an alarm if they detect potential contamination.
What's False:
It's not true that the mussels automatically shut off the city's water supply. When the mussels' shell movements trigger an alarm, human specialists conduct further testing before any action is taken.
Context:
Mussels are just one part of Warsaw's water monitoring system and have never triggered an alarm. The alarm activates only if six out of eight mussels stay closed for over four minutes with an average closure above 25%.
For years, a rumor has circulated online claiming that Warsaw, the capital of Poland, uses mussels to automatically shut off the city's water supply if contamination levels become too high.
"The city of Warsaw, Poland, uses eight mussels with sensors hot-glued to their shells to monitor and automatically shut off the city water supply if the shellfish so chooses" one Reddit post on the topic read.
Similar claims have spread across social media platforms like Threads, Instagram, Facebook, Bluesky, and X, with some posts alleging that the system shuts down if four out of eight mussels close their shells.
In short, while it is true that Warsaw Waterworks uses mussels to monitor water quality, the claim that the mussels can automatically shut off the city's water supply is false. The system's alarm is triggered only if six out of eight mussels remain closed for more than four minutes and the average amount of shell closure exceeds 25%. Even then, the mussels do not directly shut off the water supply, but rather trigger an alert for further testing.
Since the biomonitoring system was introduced in 2009, the mussels have never activated an alarm. Moreover, mussels are just one part of Warsaw's broader water monitoring system, which also includes freshwater fish, laboratory testing, and electronic sensors that provide continuous water quality assessment.
Some social media posts have incorrectly referred to the mussels as "clams." Warsaw Waterworks uses swollen river mussels (Unio tumidus), not clams, for biomonitoring.
According to Warsaw Waterworks spokesperson Jolanta Maliszewska, these mussels have been part of the city's water quality monitoring system since 2009. Biomonitoring relies on bioindicators, organisms that are highly sensitive to pollutants, to detect environmental contamination. The Unio tumidus mussels are highly sensitive to water pollution and help monitor water quality in the Vistula River and Lake Zegrzyński.
After a two-week acclimatization, mussels are calibrated by measuring their natural shell opening for accurate monitoring. Equipped with sensors, the mussels are placed in a flow-through tank, where their shell movements are continuously monitored. While they can live for decades, those used at the Warsaw Waterworks are returned to their native Wielkopolska lakes after three months.
Maliszewska told us an alarm is triggered if six out of eight mussels close their shells for more than four minutes and the average shell opening falls below 25%, signaling potential water contamination. In such cases, both a visual alert on the monitor and an audible alarm would notify staff to take action.
She also emphasized that only a sudden, collective closure of the mussels can be considered a stress response. If an abrupt change in water quality occurs, the mussels quickly shut their shells, triggering the system's alarm. However, the mussels do not directly shut off the water supply, but are rather a "kind of early warning system."
"Mussels are a support. Mussels-based biomonitoring, however, is not an automatic system for cutting off water intake. It's also worth remembering that before water reaches consumers, it undergoes multistage, high-performance treatment. Not every deterioration in the quality of intake water will force the cessation of water intake," Maliszewska emphasized. Any decision to halt water intake relies on additional steps, including field assessments, laboratory tests at various treatment stages, and analysis of the results.
In addition to mussels, certain freshwater fish species help monitor water quality at Warsaw Waterworks — but water monitoring isn't limited to organisms. A team of experienced laboratory technicians using advanced analytical equipment also continuously tests Warsaw's water quality. Online electronic devices track key parameters in real time at every treatment stage as well.
"We use mussels more as a support for human and machine work," Maliszewska said. In water quality control, the main role is played by our laboratories, where numerous water tests are performed: physicochemical or microbiological. Water is tested at each stage of treatment, as well as in the network."
The images frequently shared in posts on this topic come from "Fat Kathy," a documentary by Julia Pelka about the role of mussels in Warsaw's water monitoring system (visible around the 8:02 mark). Fat Kathy, "Gruba Kaśka" in Polish, is the name of a key water intake facility on the Vistula River.
Available on YouTube, the film is described as "a philosophical essay on dependence people on nature and the world around them." You can watch it here:
In the past, we have looked at claims that one adult oyster can filter more than 50 gallons of water in 24 hours.
- YouTube. https://www.youtube.com/watch?v=h1lRDdPbhio. Accessed 6 Mar. 2025.
Gruba Kaśka | Film | 2018. www.filmweb.pl, https://www.filmweb.pl/film/Gruba+Ka%C5%9Bka-2018-824986. Accessed 6 Mar. 2025.
"Gruba Kaśka" pracuje dla Warszawy od 50 lat • MPWiK. https://www.mpwik.com.pl/view/gruba-kaska-pracuje-dla-warszawy-od-50-lat. Accessed 6 Mar. 2025.
Kasprak, Alex. "Can 1 Adult Oyster Filter More than 50 Gallons of Water in 24 Hours?" Snopes, 4 Feb. 2025, https://www.snopes.com//fact-check/water-oyster-filter/.
Małże i ryby testerami wody • MPWiK. https://mpwik.com.pl/view/malze-i-ryby-testerami-wody&i=678. Accessed 6 Mar. 2025.
Warszawska kranówka – jak czytać wyniki badań MPWiK? • MPWiK. https://www.mpwik.com.pl/view/warszawska-kranowka--jak-czytac-wyniki-badan-mpwik. Accessed 6 Mar. 2025.