Chad, one of our Service Managers with 25 years of lake management experience, took this interesting photograph of freshwater jellyfish at a lake he was working on this week. There were quite a few of the jellyfish and he said they looked a little spooky gliding through the water near the surface of the lake. About the size of dimes and nickels, he had seen them a few times, but rarely. I did a little research since I didn’t know much about them.
Apparently there is only one species on earth and it is native to the Yangtze River in China. Nevertheless, it has spread widely throughout temperate areas of the world. They are believed to have entered the United States with aquatic plants, fish and other aquatic animals sold for ornamental purposes during the early 1900s. These freshwater jellyfish now exist in at least 43 states, but are most prevalent in the temperate regions.
Much like saltwater jellyfish they are predators and have stinging cells along their tentacles that allow them to capture zooplankton and perhaps fish larvae. Crayfish are believed to eat them, but not much is known about their feeding habits. They are not harmful to humans, but sensitive people may feel some irritation if they rub against the jellyfish.
Freshwater jellyfish can reproduce in several ways, but the adults swimming in the photograph are usually single sex, either male or female. I found little information about their abundance. Sometimes they are abundant in a pond one year and are not seen again. Sometimes, they reoccur for several years. There were also reports that sometimes they appear for a year or two, disappear only to reappear a few years later. Water quality does not seem to be an indicator of where they might be found.
Lakes and ponds are fascinating complex ecosystems.