Joliet, IL – Illinois residents may be surprised to learn that tiny freshwater jellyfish are occasionally spotted floating in area lakes and ponds, especially during late summer as water temperatures rise.
According to the Illinois Department of Natural Resources, the freshwater jellyfish species Craspedacusta sowerbii—sometimes called the peach blossom fish—has been documented throughout the state and much of the eastern United States. Although native to the Yangtze River valley in China, these jellyfish have been present in the United States for more than a century, likely introduced with aquatic plants and stock fish, according to the U.S. Geological Survey.
Despite their exotic origins, these jellyfish are no cause for concern. The Forest Preserve District of Will County reports that the creatures are about the size of a penny, translucent, and harmless to humans. According to the Indiana Department of Natural Resources, there have been no reports of people being stung in the Midwest. With up to 500 tiny tentacles, they are designed to capture small zooplankton, not affect swimmers.
The Missouri Department of Conservation notes that freshwater jellyfish are most often seen in still, clean waters—such as lakes, ponds, and quarries—during August and September, when water temperatures hover around 80 degrees Fahrenheit. For much of their lives, they remain attached to hard surfaces as polyps and only appear in their jellyfish form when conditions are right.
Experts emphasize that while the appearance of Craspedacusta sowerbii in local waters can surprise lakegoers, these jellyfish have not been shown to harm native species or disrupt ecosystems where they are found.
Although sightings are rare, experts stress that the presence of Craspedacusta sowerbii does not pose a threat to native species or ecosystems. According to the Pennsylvania Sea Grant, these jellyfish are believed to have spread through the movement of aquatic plants and stock fish, but their impact remains minimal.
Residents are encouraged to appreciate these unique visitors if spotted, but there is no need for alarm or concern for water safety.
