Craspedacusta sowerbyi – Freshwater Jellyfish
Scientific Name: Craspedacusta sowerbyi; synonym Craspedacusta sowerbii
Common Name(s): freshwater jellyfish, peach blossom fish (China)
The freshwater jellyfish Craspedacusta sowerbyi is easily recognized when in the medusa form, or jellyfish form, of its life cycle. The small bell-shaped medusa form is translucent with a white to green coloring and ranges from 5-25mm (0.2-1inch) in diameter. Jellyfish do not have a digestive system with two openings like most animals. Instead, they have only one orifice for both ingestion and expulsion of waste. The freshwater jellyfish has a digestive system, called the gastrovascular cavity, composed of four radial canals and one central canal, which are easily visible with the naked eye, and a mouth on the ventral surface of the bell. Between 50 and 500 tentacles surround the bell of the medusa, each containing specialized stinging cells called nematocysts which immobilize prey.
Craspedacusta sowerbyi is native to the Yangtze River valley in China where it coexists with another freshwater jellyfish, C. sinensis. However, it has become globally cosmopolitan, being found in nearly all temperate climates. In Bella Vista, freshwater jellyfish have been observed in all of our lakes.
Craspedacusta sowerbyi occupies a variety of freshwater habitats. It inhabits shallow pools along the Yangtze River in its native range, while in Bella Vista it has no problems occupying the relatively deep lakes.
Freshwater jellyfish reproduce both sexually and asexually. Sexual reproduction happens in the medusa life stage where gametes, eggs and sperm, are released into the water. Once a larva has formed from the gametes it can settle into the polyp stage. The polyp is a sessile stage that can undergo budding, a type of asexual reproduction, to produce a clone of itself which can be used to form a colony group or move away and settle in a new location. When conditions are right for sexual reproduction, the polyp will undergo budding to produce a medusa. The medusa stage is not produced regularly, and populations may wait several years before producing medusa again.
The main prey of freshwater jellyfish, both medusa and polyps, are zooplankton such as copepods and daphnia. Being opportunistic feeders, they wait until a prey organism is tangled in their tentacles.
The freshwater jellyfish is not considered dangerous to humans or larger organisms. The stinging cells in this species are very small and unlikely to penetrate human skin.
The ecological impacts of Craspedacusta sowerbyi remain unclear. Although unlikely to influence the remainder of the ecosystem, it is thought that large numbers of C. sowerbyi may shift the balance of zooplankton from one species to another, depending on feeding preference.
If you observe one of these unique specimens while participating in water activities around our lakes, do not be alarmed, just consider yourself lucky to have had the opportunity to see one.