One of the specimens of the freshwater jellyfish (Craspedacusta sowerbyi), appeared in a mine lake, Középbánya lake near Salgótarján, but populations of it also appeared in other calm, freshwater reservoirs, lakes, and quarries. They are about a few centimeters in size, transparent creatures, and appeared in the lake due to the higher water temperature and good water quality.
Freshwater Jellyfish present no danger to humans at all. Although they have small, microscopic-sized nettle cells that contain toxins, they are only dangerous to single-celled, tiny organisms, so people can even hold them without any harm.
Although not native in Europe and originally from the Yangtze basin in China, the freshwater jellyfish can be found in several bodies of fresh water such Lake Mályi, Lake Omsk, Gyékényes, Lake Kis-Tokaj, Lake Palatinus, Lake Dorogi, and the Szigetköz section of the Danube and Dráva – but they can practically occur in any clean, high-quality waters of the country. They appeared and spread in Europe sometime in the early 1920s from Southeast Asia, when they probably got into natural waters from aquariums either as adult specimens or in egg form.