Have You Seen A Freshwater Jellyfish?

Published: Thursday, April 5, 2018
By: Jerry Smith, Dock Consultant, Bay Harbor Lake Marina

Many boaters and freshwater enthusiasts go about the enjoyment of their favorite body of water oblivious to the small creatures lurking in the water with them. Believed to have originated in the Yangtze River in China, the freshwater jellyfish (Craspedacusta sowerbii) is a little known variety of invasive species that exists in 44 states and all five Great Lakes and their tributaries (www.freshwaterjellyfish.org/location).

With records going back as early as 1880, the freshwater jellyfish is here to stay. While most water enthusiasts have no idea they are swimming with these small aquatic creatures, you will be happy to know that they have not been proven to cause harm to humans, possibly because their stinging tentacles do not have the ability to penetrate human skin or the toxin is not strong enough. It is widely believed that the freshwater jellyfish were introduced into the United States in ballast from international shipping and still others believe they came from importing aquatic plants. It is also thought that the species could have been transported around the United States and Canada attached to the feet of aquatic birds. It really doesn’t matter how they arrived at this point, what is important is that they are here and thriving.

Freshwater jellyfish are most often found in calm freshwater lakes, reservoirs, man-made impoundments, and water-filled gravel pits or quarries. They can also be found in recreational fishing and boating areas. They have been seen in large river systems such as the Allegheny River, the Ohio River, and the Tennessee River. The jellyfish prefer standing water rather than currents, so they generally are not seen in fast flowing streams or rivers. Some reports as early as June have been noted, but August and September seem to be the peak months for jellyfish sightings when lake water is warm and food is abundant. The jellyfish can be seen floating or swimming gently just below the surface of the water. They are mostly translucent, about the size of a penny, and can be easily seen by the naked eye. They often surface in large numbers called “blooms” and sunny days are especially good for spotting jellyfish.

Like many jellyfish, this species has a complex life cycle that includes an attached polyp phase, two larval phases, eggs, and, the most familiar, the medusa (the one that looks like a jellyfish). When conditions get tough, they can wait it out in a dormant resting phase at the bottom of their habitat. The jellies live off copepods and other zooplankton, which they paralyze with their stinging cells or their ring of up to 400 slender tentacles. They pull their prey into their mouth, which hangs below the translucent bell. Freshwater jellyfish may have a whitish or greenish tinge. These small animals have been reported in many countries around the world, from Thailand to India to Brazil, and in most U.S. states and Canadian provinces.

So, next time you take a dip in an old quarry or your favorite swimming hole, see if you can find any one-inch freshwater jellyfish. They won’t sting you, and they’re part of a healthy ecosystem.

Terry L. Peard, Ph.D. was also a contributor to this article. His email is [email protected].