They've been in the Great Lakes Basin for decades, but recently entered lakes Erie and St. Clair.
Freshwater jellyfish — about the size of a penny or quarter — are translucent, slimy blobs that inhabit lakes, streams, ponds, and some rivers in Pennsylvania and much of the northeastern United States.
Like their saltwater counterparts, freshwater jellyfish will sting people, but their stingers are so small they would not penetrate human skin.
— GoErie (@GoErie) July 24, 2017
Some biologists categorize freshwater jellyfish — their scientific name is Craspedacusta sowerbyi — as an invasive species, while others, like Sara Stahlman, an invasive species specialist with Pennsylvania Sea Grant, place them in the "non-native species" realm.
Whatever their classification, the invertebrates have been floating around portions of the Great Lakes ecosystem for decades, and the Ontario Ministry of Natural Resources and Forestry recently reported that freshwater jellyfish appear to have entered Lake Erie and Lake St. Clair, a smaller lake between lakes Huron and Erie.
"They've been around for quite some time in Pennsylvania, since the early 1990s," Stahlman said. "They prefer more slowly moving, stagnant waters. They're found in ponds, lakes and slower moving rivers in Pennsylvania."
This species exists in two forms: a polyp form and a hydromedusa, or adult form, Stahlman said.
"In the polyp form, they would not be as recognizable to people," she said. "They can be dormant in sediment for a long time. They seem to be concentrated more in the eastern part of the U.S. In Pennsylvania, they're all over the place. It's a temperature-dependent type of situation. You see more of the adult forms in warmer-temperature waters."
More than 180 aquatic invasive and non-native species have become established in the Great Lakes Basin, according to the 2017 State of the Great Lakes Highlights Report, a study conducted by the U.S. Environmental Protection Agency and Environment and Climate Change Canada.
Some of the more well-known invasive species that inhabit the Great Lakes Basin include zebra and quagga mussels, round goby, rusty crayfish, sea lamprey and the predatory northern snakehead fish.
Most researchers and authorities do not believe freshwater jellyfish pose any threat to humans or the Great Lakes ecosystem.
"I would categorize these jellyfish as a non-native species and not necessarily as an invasive species," Stahlman said. "Invasive species are very harmful from a health standpoint to humans, from an ecological standpoint and from an economic standpoint. It's still to be determined how the jellyfish are impacting the ecosystem, but they do not really seem to be impacting the ecological balance. We're not seeing rapid spread or rapid numbers."
Native to China's Yangtze River, it is believed the invertebrates were transported to the United States with ornamental aquatic plants and fish stocks, Stahlman said.
Freshwater jellyfish were discovered in the Huron River near Ann Arbor, Michigan, in 1933, according to the U.S. Geological Survey. They have since been recorded in lakes Huron, Ontario, St. Clair and Erie, and in dozens of inland lakes and streams in Illinois, Indiana, Michigan, Wisconsin, Minnesota, New York and Ohio.
Freshwater jellyfish in Canada have been recorded in Quebec since 1955 and in portions of Ontario since 1980, according to the U.S. Geological Survey.
The jellyfish eat zooplankton and use their stingers to catch prey, and are preyed upon by turtles and crayfish, Stahlman said.
Even though the tiny invertebrates do not appear to pose a threat to people, Stahlman urges the public to report jellyfish sightings to authorities.
"It's very important to be diligent, and if you see something that doesn't look right, report it," she said.