The Question: I believe we have freshwater jellyfish in the lake where we live. I have never seen anything like them in Massachusetts before. Do they sting and are they safe to swim with?

Submitted by: Anonymous, Massachusetts, USA

The Short Answer: Although freshwater jellyfish (Craspedacusta sowerbii) have tentacles with stingers, the tentacles are so small they probably can’t penetrate your skin. I have never heard of anyone noticing a sting from freshwater jellyfish, so I would say they are safe to swim with. On, a companion site to, you’ll find the article I wrote some years ago about freshwater jellyfish – and my inability to find them. Because people searching for information on freshwater jellyfish come across that article, I’ve been getting reports all through the month of September this year (2010) from people who have seen freshwater jellyfish all over the country. It must be a good year for them.

I, unfortunately, have still never seen them. One of these days …