Dear Javad,

Because of their small size (the size of a shilling), freshwater jellyfish are less terrifying than sea jellyfish some of which can reach up to two meters wide.

Freshwater jellyfish are harmless. Of course they sting. It's part of their survival system. They use this property to paralyze their prey before eating. But because of their small size, the freshwater jellyfish does not affect humans. If so you do not have to worry as found in the lake.

It does one find freshwater jellyfish? In the calm blue waters. Our fishing and recreational lakes for them are prime habitat and artificial lakes formed by old pits and quarries abandoned. And believe it or not, as it is reported to have found in crystalline waters as few as murky water color small ponds mashed peas which are frequently seen on farms. Even found in the still waters of certain rivers, but never in the rapids.

The freshwater jellyfish feed primarily on animal plankton, microscopic animals that are found suspended in the waters of our lakes. However, it should be noted that an increase in plankton populations due to over-fertilization of water does not necessarily indicate that there will be an increase in populations of freshwater jellyfish.

This is especially toward the end of summer, in August, they become visible to the naked eye in our lakes, when the waters are warm and abundant food. They float just below the water surface.

The appearance of jellyfish in a lake is sporadically. They can appear suddenly in large numbers when we had never seen before. And the following year, they may completely disappear and reappear several years.

In the first stage of their life, jellyfish live on the beds of lakes and move very little. They bear a different name. These polyps! Some believe that the polyps would be transported from one lake to another on the legs of waterfowl, which would explain why more and more lakes are colonized freshwater jellyfish in Quebec.

What to do?

Nothing! Because there is nothing to do. If conditions are favorable to your lake freshwater jellyfish, they will one day appear. Nothing can be done! It is nature! We must especially avoid making collective hysteria as we did with zebra mussels or as is done even today with the Eurasian water milfoil, as if it were a diabolical plant. We live with nature or you go back to town, a privileged place where there is neither zebra mussels nor jellyfish nor ears to milfoil.

With my best regards

Prof. Bachir ACHOUR

Dear Bachir Achour many thanks for your interesting and useful information.  The size of these Jellyfish (JF) is different in comparison with sea water Jellyfish (SWJF). Are they two species? I was thinking about the source of these JF in my lake, and I found two way for presence of these JF here:

1- Transporting by migrant birds

2- enter by visitors (people)

َAnd can these JF reflex the quality of the water, bad quality or good quality?

Best Regards.

Javad Bayat.

Dear Javad,

This species has the "pretty" name craspedacusta sowerbii Lankester. A native of the Yangtze River in China, it would have been transported around the world by water hyacinths, legs and feathers of birds or by introduced fish.

Their presence proves the quality of the water. But they can appear in clear water or in cloudy water.

With my best regards

Prof. Bachir ACHOUR

Many thanks for your answers. They were really useful for me.

Best regards.

Javad,

There is a factor just mentioned by Prof. Achour, which I feel should be stressed because it is an important driver or these jellyfish "blooms". It has to do with nutrient enrichment in the water, namely, Nitrogen and Phosphorous. These nutrients may be supplied in sufficient amounts to sustain an exponential growth of phytoplankton (jellyfish food), which in turn drives a jellyfish bloom. I do not know the specific conditions and size of your lake, but it should be good idea to verify if the bloom is related to abnormal nutrient input by agricultural activities, fertilization of golf courts, septic tanks, etc. If so, actions would be needed to cut that nutrient input. Otherwise your lake could end up as an anoxic water mass unable to sustain a healthy ecosystem

Jellyfish lives  at the same time in a polluted environment;because of this phenomenon , there are/must be many many  causes in our explanation

Freshwater hydroid jellyfish seem to be cosmopolitan, as they have been recorded in water bodies of all continents, in temperate as well in tropical waters. As Bachir Achour explained, Craspedacusta sowerbii is probably the species you collected, which is the most known and diffused in the world. No one knows if the presence of JF, and articularly C. sowerbii is linked to water quality in some way, in my experience it can occurs both in eutrophic and oligotrophic waters. Probably, the exceptional abundances of jellyfish, when it happens, is linked to particular conditions of warm water and food (i.e. microzooplankton) availability. It means that their appearance is often unpredictable, disconnected fron any biogeochemical cycle, at least as we know at present.

Dear many thanks for your answers.

I must say that the lake is at oligotroph stage. We found them at the temperature of 29 C at the 50 cm below surface. The meanconcentration of Phosphorus is about 0.05 mg/ll P and NO3 is about 4 mg/l.

Dear many thanks for your answers.

I must say that the lake is at oligotroph stage. We found them at the temperature of 29 C at the 50 cm below surface. The meanconcentration of Phosphorus is about 0.05 mg/ll P and NO3 is about 4 mg/l.

Dear many thanks for your answers.

I must say that the lake is at oligotroph stage. We found them at the temperature of 29 C at the 50 cm below surface. The meanconcentration of Phosphorus is about 0.05 mg/ll P and NO3 is about 4 mg/l.

Dear many thanks for your answers.

I must say that the lake is at oligotroph stage. We found them at the temperature of 29 C at the 50 cm below surface. The meanconcentration of Phosphorus is about 0.05 mg/ll P and NO3 is about 4 mg/l.

I have found them in many different lakes and ponds.  But they all occur at this time of year.  They are mainly present as hydroids during the rest of the year.  Enjoy them.  They are often absent as JF for many years before reappearing again.

Same in harbours of Lake Constance or gravel pits in the Rhine valey. They occur in late summer and dissapear when water temperature decreases. They are not native, but did not provoke any problems.

Dear Javad, maybe it's a writing mistake, but it's hard to consider as oligotrophic a lake with a mean concentration of phosphorous (Total P?) around 0,05 mg/l. It means 50 micrograms, which is in the field of eutrophy. Perhaps are they micrograms? Have you other parameters such as annual mean transparence, or chlorophyll a concentrations?

Dear Marco Seminara, the mean concentration of chlorophyll a is 2.5 microgram and the transparency is about 4.5 m where the depth is more than 4.5 m. 

This looks like the medusa stage of Craspedacusta sowerbii.  This has a polyp stage that is very small and once temperatures attain 20C+ the polyps stobilize and produce small medusa which do not normally become recognised until they are larger.  These may attain the size of an Euro coin.  Their appearance may be sporadic and appear usually are recognised in the late summer.  The polyp stage occurs normally in shallow water.  By removing stones from the shallows of a lake and placing them in an aquarium with a heater you might be able to produce the medusa stage. 

Interesting that the medusa is now widely spread about the world.  There are some other freshwater medusa that your specimens might just be so the aquarium rearing could confirm the species once you have medusa.

all the best

Dan