Freshwater Jellyfish in the National Capital Region's Streams

The freshwater jellyfish Craspedacusta sowerbii. Credit: U.S. Geological Survey / Dave Head.

Partners monitoring aquatic macroinvertebrates in the streams of the National Capital Region's parks recently found an unexpected animal: a freshwater jellyfish. Exotic-sounding and easy to overlook, these jellies have been seen in North America since the early 1900s.

Craspedacusta sowerbii, also called the peach blossom jellyfish, is no bigger than a quarter and harmless to people. It has spread into many North American fresh waters, yet little is known about it.

Where they come from

Peach blossom jellies are non-native. They are found across North America in slow-moving streams, larger rivers, lakes, ponds and artificial water bodies. They are native to the Yangtze River in China, where they are adapted to changing water conditions, and some speculate they arrived with imported ornamental water plants.

From 2019 to 2022, collaborators at the University of Maryland Center for Environmental Science sampled environmental DNA and found preliminary evidence of freshwater jellyfish in every park in the National Capital Region.

Freshwater jellyfish are usually about the size of a penny. Credit: U.S. Geological Survey / Myriah Richerson.

Biology

Despite the name, jellyfish are not fish; they are cnidarians. The freshwater jelly has a flattened bell-shaped body, four longer tentacles and many smaller ones — up to 500 in all — and is usually 5 to 25 millimetres across. This adult stage, the "medusa," is the easiest to recognise, but jellyfish pass through several forms over their complex life cycle that look and behave very differently.

How they fit in

Like marine jellyfish, C. sowerbii is an opportunistic predator, eating zooplankton and small insects that stray within reach of its tentacles. It can eat fish eggs and other macroinvertebrates, but scientists have not found it to be a significant predator of them.

Are they dangerous? No. They hunt with stingers on their long tentacles, but they are too small to sting larger animals.

Freshwater jellyfish from several angles. Credit: U.S. Geological Survey / Dave Head.

Macroinvertebrates and healthy streams

Freshwater jellies are aquatic macroinvertebrates: small animals without backbones that can be seen with the naked eye, a group that also includes insects, snails, crayfish, worms and mussels. Many fish, amphibians, reptiles, birds and mammals eat them. Some are important indicators of water quality — their presence, numbers and variety reflect a water body's overall condition — and they play a large part in nutrient cycling, both consuming and breaking down waste in the food web.

The region's streams hold many macroinvertebrate species, tracked over the long term by the National Capital Region Inventory & Monitoring Network. Besides jellyfish, the more unusual ones include freshwater mussels and freshwater sponges. Understanding and monitoring these animals is an important part of managing and conserving the parks.

For more, see the U.S. Geological Survey's species profile of Craspedacusta sowerbii (McKercher and others, 2024) and the Smithsonian Environmental Research Center's summary (Light, Grosholz and Moyle, 2005).

Sources

Based on "Freshwater Jellyfish," by Hannah Harkness, National Capital Region Inventory & Monitoring Network science communications intern, published by the National Park Service; rewritten in hubnx's own words.

• The article places sea urchins and sea stars among the cnidarians; they are echinoderms, so that line is left out. Its statement that C. sowerbii is the only known freshwater jellyfish species is also left out.
• The article's life-cycle diagram, credited to the Smithsonian Ocean Portal, is not reproduced.
• Words on this page from people and organisations outside the federal government are paraphrased; rewritten in hubnx's own words.