As anglers, swimmers and other water recreation fans flock to Michigan lakes and streams this summer, some may catch a glimpse of a common, though rarely seen invertebrate known as the freshwater jellyfish. Don’t be concerned though — these jellyfish are not harmful to humans and are believed to have negligible adverse effects on the aquatic environment.
Freshwater jellyfish (Craspedacusta sowerbyi) are related to the various marine species of jellyfish, but in essence are not true jellyfish since they have a membrane called a velum that the marine species do not. Despite this difference, the appearance and movement of freshwater jellyfish mimic those of a true jellyfish. They are not native to Michigan or even North America and are believed to be native to China and Asia. However, this species has been found throughout most of North America for well over a century and is considered to be a nonharmful member of our aquatic communities, and certainly an intriguing one.
The first documentation of freshwater jellyfish in Michigan occurred in 1933 from the Huron River in southeast Michigan. Today, they can be found in most Michigan lakes and streams, and are most frequently spotted in lakes in the late summer and early fall.
Freshwater jellyfish are polymorphic, meaning that there are multiple forms of the same entity within a population.
The jellyfish life cycle includes the following steps:
The jellyfish spend winter in bodies of water in a podocyst resting stage.
As conditions become more favorable, such as during spring, the podocysts develop into polyps and continue the life cycle with asexual reproduction.
By late summer, polyps develop into the medusa stage, which most resembles a free-swimming jellyfish. Jellyfish in this stage take an umbrella-shaped form and can range in size from a penny to a quarter. This is the life stage that is visible to the human eye and often observed during late summer.
“These are the sightings that often trigger calls to local DNR offices from curious spotters who have caught the rare glimpse of the invertebrate,” said Tim Cwalinski, the DNR’s northern Lake Huron manager. “Though freshwater jellyfish do have stinging cells like the marine species, their tiny size means they lack the ability to sting, and so they’re not harmful to people. It is also believed to be unlikely that freshwater jellyfish could consume enough zooplankton in our bodies of water to negatively compete with fish species.”
Cwalinski said that we generally think our waterways are inhabited only by fish, aquatic plants, waterfowl and various invertebrates. However, there is an entire other spectrum of life under the surface, such as plankton, bryozoans and jellyfish.
“If you’re among those to observe a single medusa stage or colony of freshwater jellyfish this year in our lakes or slow-moving streams, consider yourself fortunate, don’t panic, and understand this is simply an invertebrate that has inhabited most of our waterways for a century.”
