Although they have been on Earth for over 500 million years, researchers still do not know enough about jellyfish, the reasons for their reproduction, their migration routes, and more, and measures from different scientific fields were required to reach a conclusion
Where do they come from? A new study conducted at the University of Haifa and published in the journal Frontiers in Marine Science Proposes for the first time that the origin of the swarms of jellyfish common in Israel, the wandering threadworms (Rhopilema nomadica) is on the shores opposite the eastern side of the Nile basin in Egypt (not in the river itself). "The complex life cycle of jellyfish makes tracking their origins a challenging task. Using oceanographic models, genetic testing and citizen science observations, we found that the bottom of the Nile estuary, an area rich in fertilizers, may be inhabited by the tiny polyps from which the jellyfish are formed. With the help of the currents that usually come from the south to the north, the jellyfish reach the shores of our country", explained Dr. Dor Edelist, from the University of Haifa, one of the authors of the study.
Although they have been on Earth for over 500 million years, researchers still do not know enough about jellyfish, the reasons for their reproduction, their migration routes, and more. Jellyfish have a complex life cycle that begins with intercourse in the water column, and the formation of a fertilized egg (planula) that swims and settles on the seabed. The planula undergoes a process of metamorphosis on the bottom and becomes a polyp. "The polyps, which are less than a cm in size, grow between 4-10 discs (grey), which are released under the conditions that suit them, and after their release, they become adult jellyfish and this is basically the sexual life cycle of the organs. In addition, the tiny polyps may reproduce asexually by spawning and cloning on the seabed. When the water usually warms up in April, the polyps release the disks, and that is why we see the dense swarms on the coasts of the country at the beginning of the summer when they are already large," explained Dr. Edelist, adding that the tiny size of the polyps, together with the complex life cycle, makes it very difficult to identify Those polyp colonies, at least for the jellyfish found in the Mediterranean, researchers do not know exactly where their mother colonies are. "We have never seen a polyp of a wandering thread in nature because they are very small and sit on the bottom of the sea, which makes it very difficult to identify them," said the researchers.
In the current study, Dr. Edelist, Prof. Dror Angel, and PhD student Hila Dror from the Department of Marine Civilizations at the University of Haifa, together with a team of researchers from Norway, sought the source of life of the polyps from which the migrating filamentous jellyfish of the Mediterranean Sea are formed. To do this, they used a unique combination of biological data, oceanographic models and citizen science.
The researchers wanted to perform a "backward search", that is, to identify when the adult jellyfish reach the shores of Israel, and then "go back" with the currents to find the location from which they came. The first step was to identify precisely when the first jellyfish arrived on Israel's shores. For this purpose, they used the data of the website "Jedusas in the Nation", which was established by Dr. Edelist and Prof. Angel about a decade ago, where citizens report in real time the presence of jellyfish. The researchers used data from reports in June and July of the years 2017 and 2018, in one of which the swarm was long and dense and in the second the swarm was sparse and much shorter. In light of the knowledge from previous studies that the growth time of the jellyfish, from leaving the polyp to reaching its maximum size, is about three months, the researchers have now moved to the complicated stage: monitoring the sea currents, and the northern current specifically, during that relevant period, for three months, to reach a point from which the jellyfish came out.
Monitoring the currents of the sea, with the level of precision that the researchers requested, is of course complicated. While we know the general direction of the currents, the currents can change, stop and even reverse from time to time. The model used is a three-dimensional oceanographic model with a resolution of 0.8 km called SINMOD, developed by the Norwegian SINTEF Institute. The model simulates the release of virtual particles from different points in space and tracking their passive drift in the sea currents.
From the results of the study, it was found for the first time that the polyps of the wandering thread originate from the shores in front of the shallow eastern part of the Nile in Egypt (and not in the Nile itself). "According to our research, the large colonies of the polyps are found in this area of the Nile estuary, which is rich in fertilizers and can be a food source for the polyps and the young graylings. This is the point from which it takes about three months for the current to take the jellyfish to the shores of Israel and arrive during the bathing season in the months of June-July. When the temperature of the water reaches 20 degrees around the month of April, the polyps release the disks and as mentioned, about two or three months later the jellyfish arrive at their full size on our beaches", explained Dr. Edelist.
The present study also found confirmation of the findings from previous studies, that all filamentous jellyfish come from one population that does not have the genetic variation that may characterize different populations in the eastern Mediterranean. "Our jellyfish, of Egypt, of Turkey, of the entire eastern Mediterranean come from one large population of the same species from the same source," the researchers said.
"The jellyfish are a big challenge that needs to be faced and not a problem that needs to be solved, and they are important in the food web in the Mediterranean. History shows that in the sea, a new species invading a new environment is never eradicated. Part of our dealing with the challenge of jellyfish is sharing the location of the jellyfish with the citizens on the 'Meduzos Baam' website, providing information regarding the prevention of burns and the recommended treatment for them. "Jellyfish are part of nature and they also deserve some love," concluded the researchers.
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Finally, after decades, an article about a wandering thread without a word was swallowed from the Galilee