The increased proliferation of jellyfish is a growing concern.Indeed, some scientists observed a very strong global growth while others believe that the variations depend on natural cycles.Nevertheless, the fear remains the same: that one day the mass of jellyfish exceeds the mass of fish, dominant thereafter the extent of the oceans.Today, swarms of jellyfish can already reach large sizes if they are visible on satellite images and cause serious damage to fisheries, aquaculture or cooling plants. In November 2007 a dense web of Pelagia noctiluca 16 km2 x 11m deep submerged cages from a farm in Ireland, killing 100 000 salmon for a loss of about £ 1 M.This proliferation of jellyfish is all the more worrying that they are voracious predators of eggs and larvae of fish.In addition, fewer fish due to fishing reduces fish predation on larvae of jellyfish. A vicious circle that could lead to the dominance of jellyfish.In the western Mediterranean jellyfish Pelagia noctiluca same saw lurking in the depths, not going back to that engage in “raids” in very large swarms before returning to the depths, leaving a big gap behind them. These raids are not visible to the public. Those who do evil swimmers are the same jellyfish end of life dating back to the surface to be washed ashore on the coast sandstone currents. Unfortunately for bathers, even at the end of life, their stinging cells still function perfectly.To understand and possibly prevent these phenomena, it is essential to follow the life cycle of jellyfish in the seasons through planktonic samples, observations / identifications and counts throughout the year.The objective is to define the cycles of Mediterranean jellyfish, their impact on other living beings, including plankton also bringing attention to predators of jellyfish.