Is there such a thing as a true freshwater jellyfish? No, not really, but there is a species that looks like a jellyfish, reproduces like a jellyfish and is known under the common name freshwater jellyfish: Craspedacusta sowerbyi. This species is distinguished from true jellyfish by a structure called a velum that freshwater jellyfish has and marine jellyfish lacks. However, most people still look at Craspedacusta sowerbyi as a jellyfish and I will henceforth just refer to it as freshwater jellyfish.
The freshwater jellyfish Craspedacusta sowerbyi originates from the Amazon area in South America but can today be found on all continents. The freshwater jellyfish has mainly been introduced to new areas unintentionally, e.g. with shipments containing other plants and animals from the Amazon area. They are not very common and are primarily found in slow moving and standing waters, especially in reservoirs, dams and slow moving channels. They feed on microscopic zooplankton.
The freshwater jellyfish is quite small and only reaches about 2 cm / 0.8 inch in diameter. It is unsure whether these jellyfish can sting humans; some scientist believe that they can’t penetrate human skin with their stinger cells while there are humans swearing that they have been stung by freshwater jellyfish.
Freshwater jellyfish can be kept in aquariums as long as they are kept in aquariums without any fish in them. They are however quite sensitive and it is hard to keep them alive to adulthood. The aquarium should only be slowly circulated and decorated with suitable places for the polyps to attach themselves such as rocks or ceramic pots. Freshwater jellyfish can in aquariums be fed rotifers if young and desalted brine shrimp when they grow larger.
Freshwater jellyfish can be breed in aquariums and goes through the same stages as all other jellyfish. Freshwater jellyfish can reproduce asexually as polyps and sexually as medusas. The polyps form "frustules" which are able to move although slowly before attaching themselves to a suitable substrate and becoming polyps themselves. A newborn polyp formed in this way is about 0.3 mm long. Each new polyp usually forms a colony of 2-3 polyps by undergoing budding (creating new polyps at the base of itself).
The freshwater jellyfish reproduction in the medusa state is as earlier mentioned sexual and requires a male and a female. The female releases her eggs in the water which are then hopefully fertilized by the male. The fertilized eggs then enter the cleaving state where the egg cell divide to form something called a planulae which is best described as a creeping and swimming freshwater jellyfish larvae. The planulae later settles down on a suitable substrate and forms polyps and the entire circle can start over.
Freshwater jellyfish eggs can be artificially inseminated however this process is very hard and it is easier to just keep males and females together if you want to reproduce this species.
Freshwater jellyfish are not among the easiest freshwater creatures to keep and breed in aquarium but they are one of the most unusual and can be a fascinating experience to deal with.
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