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The double life of the Jellyfish

After 80 years, Drymonema dalmatinum, a giant and very rare jellyfish, reappeared in the Adriatic Sea only two more times after its discovery in 1880. The mystery behind its reappearance is to be found in the biology of these animals

It is easy to say jellyfish. This is well known by the marine biologists of the University of Salento and Cnr-Ismar who receive reports of the presence of these animals from all over the Mediterranean. The last ones concern Drymonema dalmantium, the cauliflower jellyfish sighted three times in the last days in the Gulf of Trieste. Described for the first time in 1880 by the German naturalist Heckel who observed it on the coast of Dalmatia, it was not sighted again until mid-20th century and then reappeared in 2014, and in recent days in the northern Adriatic. The exceptionality of this sighting lies not only in the rarity of this jellyfish but also in its majestic beauty: the specimen, captured in the images of Miramare Marine Protected Area’s researchers, measures about 40 cm in diameter, but an adult individual can reach even one meter. Moreover, other images immortalized it while it was feeding on two specimens of another type of jellyfish (a sea lung, Rhizostoma pulmo). The Drymonema, like many jellyfish species, feeds on other jellyfish, catching it with its (very) stinging tentacles.

It is natural to wonder how it is possible for an animal to appear in our seas only every 80 years and where it stays hidden for the rest of the time. The answer, no less fascinating than the mystery that often surrounds these animals, is to be found in their biology. 
Jellyfish belonging to the Drymonema genus are part of the class of "scyphozoa", which comprehend all organisms called Cnidarians (or coelenterates), i.e. jellyfish, corals, and sea anemones. All these animals have in common a particular characteristic: the presence of tentacles covered by very small stinging cells, called "cnidocytes". These cells are responsible for the annoying and, at times, dangerous stings, but equally indispensable for keeping these incredible animals alive. It is, in fact, thanks to their "paralyzing" power that these organisms, which lack a real muscular tissue and are unable to actively swim, can nourish themselves by capturing their preys. The Cnidarians are extremely ancient organisms, they have lived on this Planet for at least 500 million years and therefore represent one of the most incredible examples of evolutionary success. Their life cycle is also the answer to the mystery of the rare and unexpected appearance of the giant jellyfish Drymonema.

Scifozoi in fact lead a real double life in which they pass from a form of sexual reproduction to asexual reproduction. The giant cauliflower jellyfish spends a lot of time in one of these two forms, the less visible one.
Indeed, the adult jellyfish has the typical "parachute" shape with the tentacles hanging downwards. The mouth of these animals is located exactly in the centre of the disc - called umbrella - in the middle of the oral arms, which have the function of catching and bringing food to the mouth. Form the mouth, in addition to entering, once digested the food comes out as waste. From the mouth cavity, however, come out also the gametes (the reproductive cells, male or female according to the sex of the jellyfish) which, once expelled, will begin to travel carried by the current until they reach a gametic cell of the "opposite sex", by which point the fertilization will take place. In most of the scyphozoa species, a larva called planula (a very small "ciliate bean") will result. If lucky, the larva will not be eaten and it will arrange itself on the sea bottom to develop in the benthic form. During the benthic form it lives attached to the substratum, in which it can spend a long time, depending on the species of jellyfish to which it belongs. In this phase of its life, it takes the name of "polyp". 

The solution to the mystery of the jellyfish that suddenly appears in our seas after being absent for almost 100 years is therefore in this very small creature: the “polyp”. In this phase of life, in fact, scientists think that Drymonema dalmatium spends most of its time, waiting for the best conditions to continue its life cycle and turn into an adult jellyfish. And so, the cycle continues and the small polyp, whose shape resembles a tiny little flower, begins to "stratify”, and grows according to a process that biologists call strobilation. It is in this way that what resembled to a small flower becomes a sort of stack of discs which, once ripe, detach from the body of the polyp (the column) in the form of "efìre": these, growing up, will become adult jellyfish. And so the cycle begins again, like so for around 500 million years.
Given its rarity, the probability of a meeting is so low that it does not represent a real danger: on the contrary, people who meet it should feel lucky, for the privilege of observing a real treasure of the Mediterranean.

Photo: Liza Gomez Daglio / CC BY-SA

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