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Freshwater Jellyfish in New Hampshire

After the 2018 annual meeting, some Lake Association members brought to our attention the sighting of small clear “jellylike” beings floating near their beaches, and were concerned what they might be. They are freshwater jellyfish, they do reside here in NH, and the good news is, they are nothing to worry about. In fact, freshwater jellyfish have been seen in lakes and ponds in all 50 states except Alaska, Montana, Wyoming, and the Dakotas.



The adult freshwater jellyfish is about the size of a quarter. Normally they are clear, but can be white or green. In its adult form, it is referred to as a Medusa. A juvenile is a “polyp.” As a polyp, these jellyfish often hide under rocks or floating pieces of wood, and along a mucky bottom of water bodies, hence making them hard to spot. As they move along, tentacles capture prey. Their diet consists mainly of tiny invertebrates, such as zooplankton. Due to their small size, they do not pose any threat to humans or pets swimming nearby. Sightings on the 53 New Hampshire Lakes that have them, typically occur between July and October. They prefer warmer water, which is why all but one of the lakes where they have been spotted, are in the southern part of the state. That exception is White Oak Pond in Holderness.


Freshwater jellyfish lay eggs which grow into a sort of larva, and then into polyps. The polyps can also reproduce asexually. According to the DES fact sheet, small buds are formed, which become new jellyfish. If this happens they are all the same sex. The jellyfish become dormant during the winter, and if temperatures are not too cold, can survive until warmer weather. However, if a “bloom” of jellyfish is seen on a lake one year, it is not at all unusual for them to not be seen the following year. This is one reason for the difficulty experienced by scientists attempting to document these colonies. When and where they will appear is hit or miss.


Although these jellyfish were originally noted in England in 1880, they weren’t seen here in the US until 1908. Currently, under the direction of Dr. Terry Peard, biology students at Indiana University of Pennsylvania are doing research on these elusive creatures. Their findings, as well as other links can be seen on their website: http://freshwaterjellyfish.org/history.html.


If you see these jellyfish in Swains Lake, (or any lake or pond in our state), the NH Department of Environmental Services wants to hear from you. They document each sighting, and update their list of lakes with them. You can call the DES at: (603)271-0698, or email them at: < beaches@des.nh.gov >. Be sure to include the lake or pond name, the town, and date, plus any other information you feel is important.

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