The Great Lakes have faced their fair share of invasive species, including Asian carp and zebra mussels, but freshwater jellyfish appear to be a low concern. According to the Indiana DNR, they are most common in late summer, between August and September. However, since jellyfish can live for several months under water, this suggests that there may be more than one type of jellyfish living in the lakes that are able to migrate toward warmer waters to avoid winter temperatures.
While freshwater jellyfish do occur in the Great Lakes, they are not considered a threat to human health or the environment. The only current concern is that they may interfere with scientific studies of lake ecology because of their tendency to stick to boats, cages, and other floating devices. Scientists use electric currents to capture and study aquatic organisms such as fish and bugs, so if too many jellyfish are present, this could prevent them from being captured.
Asian carps were originally introduced into the United States as a food source for humans and animals, but now they are also used in fishing contests. Carp grow extremely fast and can reach 4 feet long within three years. As adults, they weigh about 50 pounds and can survive in both fresh and salt water.
Zebra mussels were originally native to Europe, where they caused problems for farmers by clogging water pipes.
According to the US Geological Survey, freshwater jellyfish were discovered in the Huron River in Ann Arbor, Michigan, in 1933. They've since been discovered in Lake Huron, Ontario, and St. According to Stahlman, jellyfish consume zooplankton and utilize their stingers to hunt food, and they are preyed upon by turtles and crayfish. Their stings can be harmful to humans who don't know how to treat them properly.
Jellyfish are members of the class Cnidaria. There are five main groups of jellyfish: cubozoa (or box jellies), cydippa, hydromedusae (or water plants), pagulidae (or bubble jellies), and salinariids (or sea nettles). The majority of jellyfish in coastal waters are cubozoa, which include Stony Brook University's mascot, the jellyfish. Cubozoans are typically transparent when young but develop brown or black skin as they grow older. The skin of a mature box jelly bears several layers with hairs on the outside and these help it catch wind and float. Jellyfish also have small fins near their bell that they use to swim short distances.
Cubozoans are found in all seas but are most common in tropical areas where they contribute significantly to the nutrient cycle and play an important role in the ecosystem. For example, they provide food for larger animals such as fish and sharks. They're also important for recreational fishing because people enjoy catching them with rod and reel!
The list of Wisconsin waterbodies where freshwater jellyfish have been documented is available at http://dnr.wi.gov/lakes/invasives/. These "jellyfish waters" range in size from small ponds to lakes covering 9,842 acres (Lake Mendota) and reaching depths of 236 feet (Big Green Lake). Freshwater jellyfish have been discovered in a wide range of water types. They have been reported in fresh, brackish, and saltwater habitats.
In addition to these invasive species reports, there are also numerous sightings of native jellyfish in Wisconsin waters every year. For example, in 2013 researchers with the Wisconsin Department of Natural Resources (DNR) observed 1 million Mediterranean sea nettles (Urticina dioica) in one lake during an annual survey of aquatic invasive species. They estimated that up to 5 million more jellyfish may have been present in the lake but were undetected because they die when submerged.
Native jellyfish include Aipyrenea spongia, which can grow to be 30 inches long; Cassiope mertensii, which can reach 15 inches in diameter; and Cyanea nocturna, which can grow to be 6 inches long.
Although jellyfish do not cause any serious diseases in humans, people sometimes get scared when they see large numbers of jellyfish in the water. If you come across a jellyfish in Wisconsin waters, please follow local fishing regulations and don't try to catch the jellyfish.