A Mnemiopsis leidyi - Guillaume Marchessaux

  • Mnemiopsis leidyi does not present any danger for the man, and is not stinging.
  • On the other hand, this particularly invasive species colonizes the ponds of Occitania at great speed, and rots the life of fishermen and bathers.
  • Guillaume Marchessaux is working on an inventory of this species in the region.

Do you know the Mnemiopsis leidyi? If you happen to cool off in the ponds around the Mediterranean, perhaps you have already met this cousin of the jellyfish. Do not panic: it does not present any danger for the man, and is not stinging. On the other hand, this transparent, little stewing invertebrate, with a gelatinous appearance, invades the lagoons of the South. And that poses big problems.

Guillaume Marchessaux, researcher in marine biology, knows this animal well. For years, it has been closely studying its expansion and the damage it generates. "This species is native to the coasts of Mexico, and the Atlantic coasts of the United States," notes Guillaume Marchessaux. Since the 1980s, it has been observed in the Mediterranean, as it has been transported by tanker ballast water. At the time, there was no legislation on this subject. Since then, it has completely invaded the Mediterranean. "

A Mnemiopsis leidyi - Guillaume Marchessaux

It breeds at breakneck speed

The Mnemiopsis leidyi only likes salt water: this funny species dabbles in the sea, but it adapts perfectly to pond water, "brackish water", although it is less salty. "These are typical lagoon waters, a mixture of sea and fresh water, which are at the interface between the continent and the sea. They are salt water, but not as much as the sea," explains Guillaume Marchessaux. Invertebrates, on the other hand, do not like the fresh water of rivers.

In 2017, this doctor of environmental sciences had already demonstrated how particularly strong was the invasion of Mnemiopsis leidyi in the Etang de Berre, in the Bouches-du-Rhône. This summer, Guillaume Marchessaux set out to establish an inventory of the presence of this creature in all the ponds of Occitania. Because this invertebrate, 5 to 6 cm long, flows smoothly in the ponds of Gold and in the Rhône canal in Sète, in Hérault, in the ponds of Bages or Leucate, in Aude , or Canet-en-Roussillon, in the Pyrénées-Orientales.

This species, which feeds on plankton, multiplies there at breakneck speed. "It has no predator in the areas where it is introduced, so it is not naturally controlled, says Guillaume Marchessaux. And his physiological abilities make him a super invader. Each organism is hermaphroditic, and self-fertilizes, and can lay up to 10,000 euros per day. And between the egg and the adult form, there are only 14 days. The species also has a strong ability to eat. She has no satiety threshold, she eats everything and anything, continuously ... "

"Under the weight, the nets tear"

The problems it poses are numerous. First, it steals plankton from fish. Then she is the bane of fishermen. "It clogs the nets, the fish die asphyxiated," notes the researcher. A plague, for eel fishermen, a species which must absolutely be sold alive. "Sometimes even, under the weight, the nets tear," says the marine biologist. Economic losses can be very significant. "It's a huge problem," says a 20-minute  fisherman who works on the Bages pond, and has seen a drop in activity since the invertebrate invasion. The fillets fill up, and the eel, she doesn't like it. Every year, it gets worse and worse. It is there approximately from May, until it is really cold, "he continues. "We were full of nets," notes a former fisherman from Aude. This prevents fish from entering, it is very problematic on a daily basis. "

Finally, the Mnemiopsis leidyi also sabotages swimming for lovers of lagoons. It does not sting, but when it swarms in the ponds, it has enough to scare away. "From a certain threshold, we notice a desertion of the beaches of the ponds," continues Guillaume Marchessaux. Some people are afraid to put their heads under water for fear of swallowing it. At certain times of the year, it is a real soup of Mnemiopsis leidyi. "

The problem is that it is very complicated to fight against the invasion of this species. "It is there, and it will stay there," says the doctor of environmental sciences. You cannot eradicate a species. However, we can try to control it. "You would probably have to confront a predator," but it will be very complicated. We must be sure that it works, but also be sure that this predator will eat Mnemiopsis leidyi well and not of another species. To try to better understand the invasion of this gelatinous invertebrate, Guillaume Marchessaux will meet, until the end of July, professionals and users of ponds in Occitania.

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