A rare and wonderful observation. August 2023. Bass Lake, California.

I was watching my grandson fish off the dock at Miller's Landing on Bass Lake. "Papa, there's a jellyfish". I think that most people would have said, "jellyfish are in the ocean". I ran for a collecting container. There is one genus of freshwater jellyfish. The species that is seen in the U.S. is Craspedacusta sowerbyi. Apparently it arrived from China in the 19th Century. It pops up in various lakes and then disappears for a decade or more. It's thought (according to Pennak) to be existing in a "hydroid" form most of the time and only occasionally appears in the medusa form. What causes these changes in the life cycle is unknown. Twenty years ago I encountered someone who had observed one in Lake Oroville. I was unable to see a specimen myself. That was a great disappointment. I was fortunate to have my sharp-eyed grandchildren around this Summer. The medusa is only about 1.5 cm in diameter and is not easy to spot in lake water.