Freshwater Jellyfish

For years, my friend Erik insisted that there were freshwater jellyfish in the reservoir near where he lives in southern Connecticut, and for years I pretended I didn’t believe they existed, even after he showed me the illustration in the Pond Life Golden Guide.  I had been down to visit a few times, and he always said you had to be there at just the right time to catch them.  So this past August when he told me the jellyfish “bloom” was happening, I decided to go down and settle this once and for all.  Unfortunately, since they don’t really exist, I wasn’t well prepared to photograph them, so in the photos below you can see the scratchy bottom of the sandwich box I used to scoop up some water, complete with some little chunks of peanut butter.

Freshwater jellyfish (Craspedacusta sowerbii), about 12 mm across.

Detail of the fringe of tentacles.

Apparently these are found in calm waters throughout the world.  A freshwater jellyfish uses its several hundred tentacles to paralyze prey and guide it into its mouth, which is in the middle and is the same opening through which it expels waste.  Those four things radiating from the center are the sex organs.  This sexually reproducing form that we recognize as a jellyfish is called a medusa, and the rest of the life cycle is spent as an asexually reproducing form called a polyp.  Polyps become encysted and dormant in the winter, and it is thought that they can be transported to new water bodies accidentally by other animals.  I still think this is all some kind of hoax though.

About Charley Eiseman

I am a freelance naturalist, endlessly fascinated by the interconnections of all the living and nonliving things around me. I am the lead author of Tracks & Sign of Insects and Other Invertebrates (Stackpole Books, 2010), and continue to collect photographs and information on this subject. These days I am especially drawn to galls, leaf mines, and other plant-insect interactions.
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8 Responses to Freshwater Jellyfish

  1. Another bit of fascinating information — thanks!!!!

  2. Stephen Gifford says:

    A naturalist from our local metro park provided this link re this species. Its apparently an invasive.

  3. Lynne Roberson says:

    I’m always learning stuff from Northern Naturalists!

  4. Susan Sawyer says:

    These occur in quite a few places in Vermont — my local site is Greenwood Lake in Woodbury. I found I could keep them alive only a few days in an aquarium, but they are magical to watch. We see them when August is warm, around Labor Day. And did you find the website of the man in Indiana who keeps track of where they are?

    • Yes, I made the word “polyp” a link to that website, but I suppose I should have mentioned the site explicitly. Good to know they’re in Vermont–I guess there’s no reason I shouldn’t be able to find them in Massachusetts too then.

  5. Marvin says:

    Interesting. I never knew there were fresh water jellyfish.

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