Freshwater Jellyfish Are Too Tiny to Sting Humans

Freshwater jellyfish may sound like an oxymoron—because jellyfish live in the ocean, right? Not all of them. Craspedacusta sowerbii, also called the peach blossom jellyfish, thrives in rivers, lakes and even gravel pits.

Originally from the Yangtze River valley in China, these bell-shaped jellyfish have spread across every continent except Antarctica. They’re considered nonindigenous in most places and often turn up in the U.S. Geological Survey’s Nonindigenous Aquatic Species Database.

They’re small—usually less than an inch (about 2 cm) in diameter—with a translucent body that can have a whitish or greenish tinge. Long tentacles trail from the edge, armed with stinging cells to capture prey like zooplankton. But don’t worry: They’re too weak to penetrate human skin.

Life Cycle of Craspedacusta Sowerbii

Freshwater jellyfish have a two-stage life cycle. Most of the time, they exist as a tiny, plant-like polyp stuck to rocks or aquatic plants. This polyp phase reproduces asexually and can form colonies that persist for years.

When conditions become more favorable—usually in late summer—polyps produce medusae, the free-swimming form people recognize as jellyfish. This medusa stage is short-lived, sometimes lasting just a few days or weeks.

The medusae reproduce sexually, releasing sperm and eggs into the water. These fertilized eggs develop into larvae that settle and become new polyps. They also produce resting bodies, or podocysts, which help the species survive tough conditions like freezing winters or droughts.

How They Travel So Far

Freshwater jellyfish are easily transported. Their tiny polyps and resting bodies can hitch rides on aquatic plants, stocked fish, crayfish or even migrating birds. These travelers unknowingly help jellyfish colonize new habitats like ponds, reservoirs and larger river systems.

The aquarium trade contributes too. Ornamental aquatic plants often carry hidden polyps. Under ideal conditions, jellyfish blooms may occur even in artificial lakes and gravel pits.

This adaptability makes them an ideal invader. They've even been found in shallow pools with minimal water movement, a testament to their ability to exploit nearly any fresh water environment.

Feeding and Role in the Ecosystem

Craspedacusta sowerbii are carnivores. They float around using their long tentacles to snare small organisms like copepods and other zooplankton populations.

These animals are part of the phylum Cnidaria, which means they share traits with marine jellyfish like a single mouth opening and stinging cells called cnidocytes.

While their impact is still being studied, they may disrupt local trophic relationships by competing with native predators for food. Some researchers, including J. Liebig, J. Larson and T.H. Makled, argue that they can temporarily shift the food chain during blooms.

Still, they’re hard to spot. Most people have never seen one, even if they swim in waters where freshwater jellies are present. The American Midland Naturalist journal has published several papers calling for more research on their ecology.

How To Identify Freshwater Jellyfish

These jellyfish are easily identified when in medusa form. Look for a small, umbrella-like body with a greenish tinge and about 100 tentacles arranged in a ring. The body contracts rhythmically, pushing water out for movement.

They often appear suddenly in late summer when water temperatures rise. Warm, calm conditions in a water body can trigger rapid development from polyp form to medusa.

Unlike some dangerous jellies, their stingers can’t hurt humans. You’d need a microscope to see the damage they do to their prey. They don’t threaten swimmers but can spark curiosity among those who notice these jelly-like blobs floating by.

We created this article in conjunction with AI technology, then made sure it was fact-checked and edited by a HowStuffWorks editor.