Several years ago I found myself freezing cold in the middle of Walden Pond. It was a crisp October evening, and I was floating face down in the middle of the lake, gazing through my goggles at something completely magical. Thousands of nickel sized jellyfish floated through dancing sunrays, their tiny bodies like cherry blossoms in the wind. I’d heard rumors that these jellies had been spotted in the pond, and took a hopeful trip from Rhode Island to see them for myself. You too can likely find a place where they live nearby–they’ve been reported all over the world.
This species I saw at Walden is known as Craspedacusta sowerbii. It likely first arrived in the United States on aquatic ornamental plants from China. Like many jellies, Craspedacusta has a complex life cycle. Unlike most jellies, the life cycle of this species also includes a tough, dormant stage, that can survive being dried out and rehydrated. Resembling microscopic seeds, these dormant “frustules” can be transported on plants, boats, and even shoes. For the Walden Pond jellies, I suspect this population started from just one or a few frustules. Why? During my visit I collected nearly 300 jellies, hoping to study their lifecycle in the lab, but when I returned to Rhode Island I discovered that every single jelly was female. When frustules hatch they form polyps, which can grow and spread by cloning themselves. I suspect Walden Pond may be filled with a single clone population, founded from only a few frustules. Over time, a few small polyps can become thousands, with each polyp making one or several small jellies when the season is right.
Craspedacusta sowerbii has now been reported in 44 of the 50 US states, three provinces in Canada, and countries in Europe, Africa and Asia. Thanks to the hard work of Dr. Terry Peard, who is curating this list of reports at his website, freshwaterjellyfish.org, you may even discover freshwater jellies near you. So far this fall, freshwater jellies have made the news in Michigan, Arkansas, South Carolina, and Pennsylvania. And if you see some, please report them to Terry at freshwaterjellyfish.org. Very little is known about these jellies, the impact they are having on ponds like Walden, and the reasons they bloom when and where they do. As the season of freshwater jellies rolls on, we can learn a lot from citizen scientists who document their findings, and study the mysteries of these tiny freshwater jellies.