Freshwater Jellyfish

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

This jellyfish has 2 life phases, a “polyp” form and a “medusa,” 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.

Size: 
Diameter: when fully grown, about ½ to 1 inch (medusa).
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.
Foods: 
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.
Distribution in Missouri: 
Statewide.
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 ecosystems are based on the tiny plants and animals that form the base of the “food chain.” Freshwater jellies form 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.”