Freshwater jellyfish
Craspedacusta sowerbii
The freshwater jellyfish is a cnidarian originally native to the Yangtze River basin in China, now globally distributed as a non-native species. It exhibits a life cycle involving an alternation of generations between a microscopic sessile polyp and a free-swimming medusa. The medusa stage typically appears during summer months when water temperatures exceed 25 degrees Celsius. Despite possessing stinging cells, they are harmless to humans as they cannot penetrate human skin.
Details
Identification
Umbrella-shaped, transparent body; up to 400 fine tentacles at the margin; four clearly visible, whitish gonads arranged in a cross shape.
Social behavior
Often appears in massive aggregations (blooms), but shows no cooperative social behavior.
Diet
Primarily feeds on zooplankton such as daphnia, copepods, and rotifers.
Hunting strategy
Passive capture by extending tentacles; prey is paralyzed upon contact and moved to the mouth.
Spawning substrate
Releases gametes directly into the open water (broadcast spawning).
Overwintering
Overwinters in the form of dormant stages (podocysts) or as polyps in the sediment.
Ecology
Ecological role
Predator in the pelagic food web; can significantly reduce zooplankton density during mass occurrences.
Natural predators
Fish (occasionally), crustaceans, and predatory insect larvae.
Competitor species
Other planktivorous organisms such as juvenile fish or predatory zooplankton.
Threats
Sudden cold spells and chemical water pollution.
Scientific profile
Profile
Distinguishing features
Features hundreds of tentacles of varying lengths at the umbrella margin, arranged in groups. Characteristic are the four whitish, sac-like gonads hanging beneath the radial canals. Polyp stages are tiny (approx. 1 mm), lack tentacles, and form colonies.
Reproduction
Metagenesis with alternation of generations: Asexual reproduction of polyps via budding, frustules (creeping larvae), or podocysts. Sexual reproduction via medusae releasing gametes into the water. In Europe, populations are often single-sex (mostly only males or only females), limiting sexual reproduction.
Role in food web
Top-down predator in the pelagic food web; during mass developments ('blooms'), it can significantly alter the structure and abundance of zooplankton and compete with planktivorous fish.
Protection & threats
Status not on standard scale
Main threats
No known threats; the species is considered non-native (invasive) in Europe and benefits from global warming and the construction of reservoirs.