Freshwater Jellyfish

by Carl Strang

Thirty years ago I was an assistant professor of biology at a small college in Pennsylvania. In my office I had set up an aquarium with tiny relatives of these north Atlantic jellyfish.

It was, however, a freshwater tank, as these were freshwater jellyfish. I have no photos, but did make some drawings as I observed their interesting behavior.

These are transparent creatures, reaching roughly the size of a quarter, and can occur in large numbers. They swim by pulsing their bodies, compressing water within the bell to propel themselves forward. When swimming they retract their tentacles. At some point, having lifted themselves near the surface, they stop swimming, turn on their sides, extend their tentacles and allow gravity to pull them down.

This brings the tentacles in contact with tiny planktonic prey. Freshwater jellyfish are said to occur most commonly in manmade lakes and large ponds. The ones in Pennsylvania came from a small reservoir. The first year or two I was in DuPage County, a number of them appeared in Silver Lake at Blackwell Forest Preserve, which is a manmade lake. On the other hand, Dad told me he has seen them in Lake Maxinkuckee at Culver, Indiana, which is a glacial lake.

When I was researching freshwater jellyfish three decades ago, I found that there was uncertainty regarding their geographic origin. They first appeared in Europe in 1880 among freshwater organisms transported from South America. Apparently they were not known in North America prior to that time, so some assumed they are non-native here, as well. I have not seen any in more than 20 years, but perhaps one of these years, in May’s Lake or Trinity Lake, I’ll again be reciting the poem I wrote in those days in Pennsylvania:

Blown-glass crystal, simple and free,

Pulsing symmetrically.

Changing, timeless, moving, mindless,

Combing a freshwater sea.

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