Freshwater Jellyfish

Mon, September 28, 2015 | Lakes and Parks

Perhaps you’ve heard or seen for yourself that we have jellyfish in our lakes?  It’s true.  One species of freshwater jellyfish is known and its documented distribution is throughout the US except for the north-midwestern states. 

Figure 1.  Map showing distribution of freshwater jellyfish (Craspedacusta sowerbri) in US.




This map was taken from the US Geological Survey webpage about this species,
http://nas2.er.usgs.gov/viewer/omap.aspx?SpeciesID=1068, but you can simply search “freshwater jellyfish” and learn much of what is known about this interesting organism.

The species we have is the same that has spread throughout the US, Craspedacusta sowerbri, and was first found and documented in the US in Philadelphia in 1928 and the great lakes in 1933.  It originated from the Yangzte River valley in China and likely made its way across the globe via shipments of ornamental aquatic plants such as water hyacinth.  It was first described in London in 1880.  Once in the US, it has likely spread through the usual mechanisms including sales and distribution of aquatic plants, fish stocking and distribution, and waterfowl movements. 

Freshwater jellyfish, like the marine species are capable of stinging and paralyzing their prey, such as small fish and invertebrates, but are not considered dangerous to humans simply due to their extremely small size – about the size of a penny in the stage of life that we see in the water, called the hydromedusae. 

Figure 2.  Photo from the above mentioned USGS website showing the hydromedusae phase of the life cycle of freshwater jellyfish found in Bella Vista Lakes in size comparison to a penny. 

 
Most of the time, this jellyfish exists in a “polyp” phase on the lake bottom that we do not see.  When conditions are conducive the polyp form gives rise to the hydromedusae form all at once in the population creating a “bloom” in the lake with lots of individual tiny jellyfish floating around and that’s when we notice them.  I have not kept specific notes over the last eight years that I’ve been in the village but we have for sure seen these organisms “bloom” in Lakes Brittany and Lomond.  This summer, members have been seeing them in Lake Brittany again.  Jellyfish blooms seem to be temporary in our lakes lasting only weeks or a month or so and environmental conditions that promotes a bloom in our lakes has not been established. According to the scientific literature, a bloom can be in response to favorable conditions in zooplankton population, or other water chemistry factors, and only occur in water temperature over 25 degrees Celsius.

The important thing to note for members is that these jellyfish are not harmful to us humans, as far as anyone knows, and that their presence does not indicate any problems such as water pollution or anything like that.  Perhaps they provide some unlearned benefit.  But at least, they provide something different and very interesting for us to experience once every few years in our lakes.  As an example, check out the artful photograph taken of one of the jellyfish I harvested from Loch Lomond several years ago.  POA graphic artist, C.J. Joslin (formerly Sarratt), quickly created this photo in her office using a wine glass and a desk lamp.  She created a great display of a visually interesting organism.

Figure 3.  Photo taken of a freshwater jellyfish harvested from Loch Lomond in 2009.

Perhaps you’ve heard or seen for yourself that we have jellyfish in our lakes? It’s true. One species of freshwater jellyfish is known and its documented distribution is throughout the US except for the north-midwestern states. Perhaps you’ve heard or seen for yourself that we have jellyfish in our lakes? It’s true. One species of freshwater jellyfish is known and its documented distribution is throughout the US except for the north-midwestern states.