Jellyfish can indeed be found in lakes and other bodies of freshwater, a fact that surprises many who associate these organisms exclusively with the ocean. This is a rare and sporadic event, as the freshwater species are distinctly different from their large, well-known marine cousins. The freshwater variety is tiny and poses no threat to humans, making their presence a phenomenon of natural curiosity.
The Freshwater Species
The primary species responsible for these lake sightings is Craspedacusta sowerbii, sometimes called the peach blossom jellyfish. Individuals of this species are remarkably small, typically measuring between 5 and 25 millimeters (about one inch) in diameter when fully grown.
They possess a bell-shaped body that is nearly transparent, often exhibiting a slight whitish or greenish tinge. The translucent bell is fringed with up to 400 fine tentacles, which are used to capture microscopic prey. Inside the bell, four distinct opaque-white patches are often visible; these are the gonads.
Global Distribution and Preferred Habitat
The freshwater jellyfish is a globally distributed species, having spread from its native range in the Yangtze River valley of China to every continent except Antarctica. Despite this wide distribution, their appearance in any given body of water is often sporadic and unpredictable. They may appear in great numbers one year and then vanish for several years.
These organisms prefer still or slow-moving freshwater environments, such as natural lakes, reservoirs, quarry pits, and calm sections of rivers. The medusa, or adult jellyfish stage, is most often observed during the late summer and early fall months. The appearance of the free-swimming form is strongly correlated with warm water temperatures, typically above 70°F (21°C).
The Unique Life Cycle
The sporadic appearance of the free-swimming jellyfish is directly linked to its complex, two-stage life cycle. Most of the time, the organism exists in its sedentary, or benthic, form as a tiny polyp, which is the most common stage of the species. These polyps are minute, measuring only about one to two millimeters long, and attach to submerged surfaces like rocks or vegetation.
The polyp stage is primarily focused on asexual reproduction, where the polyps form small colonies by budding off new individuals. Under specific, favorable environmental cues, usually involving prolonged warm temperatures, the polyps will asexually bud off the free-swimming medusa. This medusa is the familiar bell-shaped “jellyfish” stage that is visible to the naked eye.
The appearance of the medusa is a temporary stage lasting only a few weeks, designed for sexual reproduction. The medusae release eggs and sperm into the water, and if fertilization occurs, the resulting planula larva settles to the bottom and develops into a new polyp, completing the cycle. To survive harsh conditions like drought or freezing temperatures, the polyps can also form dormant, cyst-like resting bodies called podocysts. These podocysts are incredibly resilient and are believed to be the main mechanism by which the species is passively transported around the world.
Do Freshwater Jellyfish Sting
A primary concern for anyone encountering a freshwater jellyfish is the potential for a sting, but these tiny creatures are entirely harmless to people. Like their marine relatives, the freshwater jellyfish does possess stinging cells called cnidocytes, which contain harpoon-like structures called nematocysts. They use these cells to paralyze and capture their prey, which consists of microscopic organisms like zooplankton.
However, the nematocysts of Craspedacusta sowerbii are too small and weak to penetrate human skin. There are no reported cases of freshwater jellyfish stinging humans or pets. If a bloom of these translucent bells is observed in a local lake, their presence is only an indication of high water quality and warm conditions, not a cause for concern for swimmers.