The presence of jellyfish in a local lake or pond can be a surprising and often alarming discovery for many people enjoying inland waters. These unique organisms are typically the species Craspedacusta sowerbii, a small hydrozoan that exists in freshwater environments across the globe. While their sudden appearance in large numbers, known as a bloom, may cause concern, they are a fascinating example of marine life adapting to a new environment. The primary question that arises upon encountering these translucent creatures is whether they pose any risk to human swimmers.
Stinging Threat to Humans
Freshwater jellyfish do not pose a stinging threat to humans. Like their marine relatives, C. sowerbii possesses specialized stinging structures called nematocysts, which are coiled, harpoon-like filaments contained within cells called cnidocytes. These are the mechanism used to capture prey in the water column.
However, the nematocysts of the freshwater jellyfish are far too small and structurally weak to penetrate the relatively thick outer layer of human skin. They are specifically designed to immobilize much smaller, microscopic organisms that serve as the jellyfish’s food source. The target prey for C. sowerbii consists almost entirely of zooplankton, such as tiny crustaceans and rotifers.
The venom injected by these cells is effective at paralyzing small invertebrates and micro-fish, but it cannot reach the sensitive tissues beneath human skin. This stands in stark contrast to many marine jellyfish species, whose larger and more robust nematocysts can easily pierce human skin and deliver a potent sting.
Identifying Freshwater Jellyfish
These aquatic inhabitants are easily identified by their distinct physical characteristics in their free-swimming stage. The mature form, known as a medusa, has a transparent, bell-shaped body that is typically small, ranging from 5 to 25 millimeters in diameter. Their delicate structure is made up of approximately 99% water, often giving them a whitish or pale greenish tinge, which makes them difficult to spot until they are present in high numbers.
The bell margin is lined with a large number of fine, string-like tentacles, which can number between 50 and 500 in a fully grown individual. These tentacles are used for feeding and stabilization as the jellyfish pulsates through the water column. The medusa stage is only one part of the organism’s complex life cycle, which also includes a less visible benthic stage.
The organism spends most of its existence as a tiny, sessile polyp, which is a small, cylindrical structure measuring only a few millimeters in height. These polyps attach to submerged surfaces like rocks or logs on the bottom of the water body. The sudden “blooms” of jellyfish observed by the public occur when these hidden polyps asexually bud off the free-swimming medusae, which then rise to the surface.
Global Distribution and Habitat
Craspedacusta sowerbii is believed to be native to the Yangtze River basin in China, but it has now spread and established populations globally. It can be found on every continent except Antarctica. The organism’s dispersal to new regions is often attributed to human activity, such as the global transport of aquatic plants and fish.
The appearance of the medusa stage is notoriously sporadic and unpredictable, even in bodies of water where they are known to exist. These jellyfish favor calm water environments, such as man-made impoundments, reservoirs, quarry ponds, and slow-moving backwaters of rivers. The transition from the hidden polyp stage to the highly visible medusa stage is largely dependent on environmental triggers.
The most significant factor for the appearance of the free-swimming form is warm water temperature, with blooms typically occurring in the late summer or early fall when water temperatures rise above 25 degrees Celsius. These warmer conditions, combined with abundant food sources, prompt the polyps to begin budding medusae. Although the polyp stage is likely far more widespread, its tiny size means the species’ presence is only confirmed when the larger, free-swimming jellyfish suddenly appear.