It may come as a surprise to many, but jellyfish do exist in freshwater environments. These intriguing creatures, distinct from their marine relatives, inhabit lakes, ponds, and even slow-moving rivers across the globe. Their presence often goes unnoticed due to their small size and intermittent appearances, leading to curiosity about their nature.
Meet the Freshwater Jellyfish
The specific type of jellyfish found in freshwater is known as Craspedacusta sowerbii, often referred to as the freshwater jellyfish or peach blossom jellyfish. These delicate creatures are quite small, measuring between 5 to 25 millimeters (about the size of a nickel to a quarter) in diameter when fully grown. They possess a translucent, bell-shaped body that can have a whitish or greenish tint, blending seamlessly with their aquatic surroundings.
Their bell margin is fringed with a large number of tentacles, ranging from 50 to 500, which vary in length. Four longer tentacles provide stability for swimming, while numerous shorter ones are used for feeding. Unlike many marine jellyfish, C. sowerbii possesses a muscular, shelf-like structure called a velum on its underside, which aids in propulsion.
Where They Are Found
Craspedacusta sowerbii is indigenous to the Yangtze River valley in China, where it naturally occurs in shallow pools. This species has spread to nearly every continent, except Antarctica. Its global distribution is largely attributed to human activities, such as the accidental transport of dormant polyps or resting bodies with aquatic plants, stocked fish, or ship ballast water.
Freshwater jellyfish are found in calm, still, or slow-moving freshwater bodies. These habitats include ornamental ponds, reservoirs, gravel pits, and quarries. Their appearance in a particular location can be sporadic and unpredictable, sometimes appearing in large numbers one year and being absent the next.
Are They Dangerous?
A common concern is their potential danger to humans. Fortunately, Craspedacusta sowerbii is harmless to people. While these jellyfish possess stinging cells, called nematocysts, their stinging apparatus is too small and weak to effectively penetrate human skin.
Most individuals will not feel any sensation if they come into contact with a freshwater jellyfish, though some very sensitive people might experience a slight tingle. These nematocysts are designed to capture tiny prey, such as zooplankton, which constitute their main diet.
How They Survive in Lakes
The ability of Craspedacusta sowerbii to thrive in freshwater environments is linked to its complex life cycle, which involves both a polyp and a medusa stage. The polyp stage is sessile, meaning it attaches to underwater surfaces like rocks, vegetation, or submerged wood. These polyps are microscopic, only a few millimeters long, and are the dominant and more resilient form of the organism, often persisting year-round.
Polyps primarily reproduce asexually through budding, forming colonies or producing frustules that can develop into new polyps. During colder months or unfavorable conditions, polyps can enter a dormant state, contracting into resting bodies called podocysts, which are resistant to cold and drought. When environmental conditions become favorable, particularly rising water temperatures, these polyps can then bud off the free-swimming medusa stage, which is the bell-shaped form most people recognize as a jellyfish. Medusa blooms are most common in late summer and early fall, requiring water temperatures of at least 15 to 25 degrees Celsius to appear.