Freshwater Jellyfish

Craspedacusta sowerbyii

Olindiidae (a jellyfish family) in the class Hydrozoa and the phylum Cnidaria (corals, jellyfish, hydras, anemones)


This jellyfish has 2 life phases, a polyp form and a medusa form, each giving “birth” to the other.

The polyp is tiny and sessile (attached to a surface; not free-floating), like a very simple sea anemone or hydroid with only a few branches. The polyps form buds on their sides that separate to become new individuals. In this way, the polyps can form in colonies.

The more commonly seen phase of this animal is the free-swimming medusa, which has the typical jellyfish form: an umbrella-like body with a stomach (manubrium) extending downward from the center. At the bottom of the manubrium is the mouth opening, with 4 frilly lobes. A fringe of up to 400 tentacles lines the edge of the “umbrella.”

This creature is transparent or translucent, sometimes faintly tinted tan, gray, white, green, or blue. Four white, opaque patches sometimes appear in the body; these are the gonads (organs that produce sperm and eggs). The medusa phase is most abundant in late summer.


Diameter: when fully grown, about ½ to 1 inch (medusa).


Photo of a freshwater jellyfish
Freshwater Jellyfish


Photo of a freshwater jellyfish
Freshwater Jellyfish


Photo of freshwater jellyfish
Freshwater Jellyfish
Habitat and conservation

These delicate creatures are gentle swimmers and cannot tolerate much of a current, so they usually occur in calm or standing waters. They are most often found in ponds, lakes, reservoirs, and quiet or sluggish pools next to flowing water. They usually float just below the surface and when they appear are often seen in great numbers. They swim by pulsating contractions of the bell-like body.


The medusae wave their tentacles slowly in the water. When a daphnia, copepod, or other tiny prey touches a tentacle, special stinging cells discharge to help subdue the prey. The tentacles draw the food into the jellyfish’s mouth. Any tiny animal is fair game, but zooplankton (microscopic animals that float in the water) are the staple.

image of Freshwater Jellyfish Distribution Map
Distribution in Missouri


Life cycle

The two phases of this animal each reproduce in different ways. The polyp phase, during the growing season, reproduces asexually by “budding”: A new individual starts as a bud forming on the side of the polyp’s body; it grows and eventually breaks away as a new polyp. Some, however, break away and develop into the medusa form, which is capable of creating sperm and eggs and therefore can reproduce sexually. In winter, the polyps contract into a podocyst, or resting stage.

Human connections

Will these jellyfish sting like their marine cousins? The answer is yes and no: They do have the same basic “stinging cells” on the tentacles (used for feeding), but these probably cannot penetrate human skin. A few people have reported itching or redness, but most people don’t feel them at all.

Ecosystem connections

Most of us know that tiny plants and animals form the base of the food chain. Freshwater jellies are an important link between the tiny animals they eat, and their own predators — crayfish, turtles, and more — which are big enough for us to see without magnification.