Freshwater Jellyfish?!

Until today I didn’t even know such things existed, but this morning one of the science teachers showed up to work with a jar of water from a local lake containing about four small, live jellies. (I love how every time I’m running out of ideas for a good blog post, the universe provides something awesome!) Getting my camera to take a good photograph of a such a tiny, almost transparent, moving subject proved impossible, so I settled for a short video clip instead.

According to this fact sheet from the USGS, they’re called Craspedacusta sowerbyi and were originally native to China but are now found pretty much throughout the world. However, adult medusae like this (which are only one stage of their complex life cycle) aren’t seen all that often, only in sporadic irruptions. (An irruption is a sudden, sometimes unpredictable increase in population.) It isn’t clear whether they cause any particular problems as an invasive species – they do eat plankton, of course, which could potentially screw with aquatic ecosystems – but they’re harmless to people.

Vilas County, the county in northern Wisconsin where I live, contains more than 1300 natural freshwater lakes, the highest density of lakes found anywhere in the world. That’s literally about one lake for every fifteen people living here. I guess it’s not surprising that with so much freshwater habitat around, we occasionally find weird and surprising things!

[Update: Check out the lengthy comment below by O’fieldstream, who provides a lot more information on these creatures.]

6 thoughts on “Freshwater Jellyfish?!”

  1. Rebecca … the farther south you go the more common the ‘C. sowerbii’ become. We have them in most of the water systems in Hoosierland. Though they are rarely found in moving water, as they prefer lentic systems (still), they are found in larger river systems that have large ‘slack water’ – such a oxbows and low-water ponds. They have been found in such environments in the Wabash River.

    They are rarely found because of their miniscule size and transparency. When people do find them, it’s usually during a ‘bloom’ (when populations rise to feed; high photo-periods (sunny days) are best. When they are found, most just think they are some kind of ‘egg sac’.

    Fortunately they are so small that the venom cnidocytes (small exploding cells that deliver venom – they do have (they are, after all, jellyfish!) are -fortunately- too small and thus believed incapable of penetrating healthy human skin. However, there have been reports of people being stung by C. sowerbii. No substantiation has yet been recorded, but … since they have venom … who knows ..? A person who is acutely allergic to bee venom might want to be careful. Just good to be aware.

    The C. sowerbii is a member of the Hydrozoa class, which includes the abundantly common Hydra (; another very aggressive and predatory member of the micro-animal kingdom.

    Interesting to note that _irruptions_ are also found in the saltwater species of jellyfish as well. In the past few years, increased ocean temperatures and lower water quality ( have caused the populations of the Giant Jellyfish (Nemopilema nomurai – in the Pacific and Nomad jellyfish(Rhopilema nomadica – in the Mediterranean to explode, as well as other species worldwide. Increased human injury, cost to fishing and recreation is a growing problem.

    One more thing. Since the populations of freshwater jelly seem to vacillate widely; appearing one year and not again for several years, if at all; and with their appearance in other water bodies with no means of being introduced than by ‘carriers’, this give rise to attention toward the need for carefulness with regard to invasive species. The #1 transport tool for invasive species of all kinds: human apathy.

    We must be far more knowledgeable, aware and caring in order to protect native species and prevent the invasion of unwanted – and unnatural – species from other regions/lands/countries ever getting a foothold on ‘native’ soil/water/air.

    Fine article. Keep up great reporting.


    1. Thank you for all the great information! I wrote this post in a bit of a hurry, since our internet has been cutting in and out lately, so I didn’t take the time to do as much research as I could have. No one I work with had ever heard of freshwater jellyfish before we saw them today, including a number of people who have lived here and paddled these lakes for decades. We were all amazed.

  2. Hi Rebecca–I learned about these creatures not too long ago from reading Stephen Lyn Bale’s “Nature Calling” blog. You might be interested in checking out what he wrote–the blog post is at and the linking newsletter article (he works at a nature center in Tennessee) is

    I bet you’ll like his blog!

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