Jellyfish Aren’t Just for Saltwater

By Adrienne Montgomerie

“Mom, we saw jellyfish!”

“No way. We’re in a lake. Jellyfish are saltwater creatures.”

“No, really, honey," his dad said. "There were jellyfish.”

“Well that’s cool,” I said with total skepticism.

The next day, I said I wanted to see the jellyfish. Totally bracing for the “we’re just kidding” punchline, off we went.

Canoeing into a little bay of a medium sized lake in eastern Ontario, my son dipped a pail in the water, and pulled up several jellyfish about the size of a quarter. White, but mostly transparent. They looked almost like large contact lenses. Delicate, undulating in the green bucket.

The bay was full of them. A bloom of jellyfish. In fresh water.

They didn’t sting. Or if they did, they were so small that it was hard to tell. I didn't want to touch them because they are so delicate. I had to learn more about this.

It turns out that these jellyfish (C. sowerbii) are an invasive species, and they are quite widespread across North America. You can report sightings of them on the Freshwater Jellyfish website.

How Long Have Jellyfish Been in North America? 

This year there are thousands of news reports about the freshwater jellyfish, but there have been confirmed sightings in Canada and the USA as far back as 1934. There are even reports from the 1800s in London, England. Sightings are reported throughout Canada and the USA, but that 1934 sighting was in Horseshoe Lake near Ste. Agathe-des-Monts, Quebec.

How Can I Get a Closer Look?

If you want a closer look at these jellyfish, you can scoop some up in a clean bucket for a few minutes. Be sure to gently put them back where you found them. Check the sightings at FreshwaterJellyfish.org to find a likely lake.

They are big enough to see with just your eyes, but a magnifying glass will help you get a closer look.

The adult stage only lasts several days, so you may not find them when you go to look. It takes about a month for a jellyfish to grow. I saw them bloom on a very warm weekend one Thanksgiving but there are many reported sightings in mid-summer.

Where Did Freshwater Jellyfish Come From?

They probably came in the water inside a ship from South America, or on imported water plants from China.

They are found in crystal clear water, in slimy ponds, and in every kind of water in between.

How Can We Help Prevent Them from Spreading?

Clean boats and water toys in hot soapy water, just as you would to stop the spread of zebra mussels and gobies.

Don’t take plants from an infested place and put them in a new place.

Can you Keep at Jellyfish at Home?

Delicate and eerie, jellyfish have a mesmerizing appeal. Like any wild animal, they are healthiest when left in the wild. Like any invasive species, it’s best not to transport them to new areas, where they can infest more lakes.

Aquariums that keep jellyfish find it very difficult. None have been able to keep freshwater jellyfish on display. They usually only live a few days in captivity.

It takes special equipment to keep the pH correct and the water circulating continuously, and they need a constant supply of fresh plankton. Raising jellyfish takes a lot of attention. An aquarium is hazardous itself, as the delicate jellyfish can get sucked into filtration systems and pumps, and air bubbles can get trapped inside the jellyfish, holding them at the surface.

Are Jellyfish Harmful?

Freshwater jellyfish do sting, but their stinger is so small that it can’t seem to penetrate human skin. Some people do feel an irritation but it seems to be easily washed off.

It’s not yet known how these creatures will affect the ecosystem. They do eat plankton, which other species rely on, creating competition for the food. They are also eaten by gulls, crayfish, and turtles, providing new food that may help those species thrive (which in turn can affect other species that compete and rely on those animals). Few organisms have a zero sum impact on the environment they live in. We just haven’t seen the full effect yet.