Almost everywhere on Earth, entire sections of coastlines are occasionally invaded by jelly-fish. While the periodicity of such phenomena has been relatively well-known (sometimes several centuries!), the mechanisms involved are relatively unknown. It is to be noted that the physiology of these “Cnidaria”, which are sometimes smaller than is generally thought, remains an important area for scientific investigation. In her doctoral thesis, Amandine Collignon describes (for the first time) the entire development cycle of a key species, Chelophyes appendiculata, showing reproduction that is both sexual and asexual! Even more worryingly, she has also shown that jelly-fish, like other local marine species, are largely contaminated by microplastic residues.  The work was carried out at the Stareso oceanographic station (University of Liege) in Calvi bay (Corsica).  

Stories about jellyfish are not often told to children on long winter nights... In most adults they evoke a kind of repulsion tinged with dread. Their tentacles which carry cells that can inflict stings (only a minority of which are actually urticating), do nothing to allay the antipathy aroused by these flaccid viscous masses that invade beaches at certain times.  

These same jellyfish, which are part of a phylum or branch called cnidaria, can elicit gasps of wonder from the same public that yesterday hated them so intensely whenever they are displayed in an aquarium. The ethereal way in which these invertebrates with neither a shell nor skeletal structure drift through the water, is a delight to watch, just like the extreme diversity of their umbrella-like shapes: these shapes can be round or angular, flat, or dome-shaped, transparent or colored etc. These strange animals, which belong to the same phylum as coral and sea-anemones can therefore also become objects of curiosity or even fascination. Comprised of 95 % water, these animals are without a heart and nervous system as we define them. They are capable of “melting” whenever there is a lack of food only to regenerate themselves and reform when a food supply becomes available again. Some species (almost a thousand of them and forming a grouped into 300 different kinds) have up to 800 tentacles per individual.

Existing in micro, macro and meso forms, there are jellyfish to suit all tastes

What the wider public does not know is that apart from the “typical” jellyfish that exist, there is a multitude of others that are infinitely smaller, some only measure a few centimeters in length, and others are practically invisible to the naked eye. These less spectacular species are interesting to study from the point of view of achieving a better classification of a phylum that is still less well-known and subject to frequent reorganizations, but also from the point of view of better understanding the proliferation phenomena concerning these species that have been recorded almost everywhere throughout the globe over the last three decades or so.  

The tourist industry will still recall the episode in 2006, which saw veritable clouds of pelagia invade the bathing waters of sea resorts. Pelagia is a stinging species with a pink umbrella of around 10 to 12 centimeters in diameter and which is also found along the North-sea coastline. Over recent years, other proliferations of jellyfish have been recorded from the Mediterranean as far as Ireland, the Gulf of Mexico, the Black Sea, the Caspian Sea, the United States and Japan.  

Episodes like this make the front headlines of newspapers due to the fact that they make life difficult for holidaymakers. They can also cause problems for fishermen. Due to the fact that there are millions of them and they can contain up to 20% of zooplankton in volume which can clog up fishermen’s nets. They are also predators - jellyfish are carnivores - they eat the eggs of fish both in the natural environment and on fish farms. In other regions, particularly in Scandinavia, large volumes of jellyfish referred to as “swarms” have been known to clog up the cooling pipes of electric power stations. This shows the impact such proliferations can have on the economy!