Aggregations of Rhopilema nomadica, the commonest jellyfish in the southeastern Mediterranean, direct themselves to the west while swimming, thereby influencing their course. This finding has emerged from a new study undertaken at the Leon H. Charney School of Marine Sciences at the University of Haifa and published in the prestigious journal Current Biology (Directional swimming patterns in jellyfish aggregations). “Until now, it was assumed that jellyfish aggregations swim in a random manner so that their direction is dictated solely by water currents. Thanks to innovative research methods, we discovered that along the coast of Israel, jellyfish swim toward the west, i.e., against the direction of the waves,” said Dr. Yoav Lehahn of the Department of Marine Geosciences at the University of Haifa, the editor of the study.
Jellyfish belong to the phylum Cnidaria and have existed for around 500 million years without any morphological changes. Researchers’ knowledge about jellyfish is still very limited in many respects. One of the biggest unknowns about jellyfish aggregations is the nature of their movement and migration patterns. The lack of understanding of this aspect impairs the ability to prepare for the arrival of jellyfish at various facilities, such as power stations and desalination plants. “If the jellyfish were passive – in other words, if they were not able to move independently and simply drifted along in sea currents, we could predict the arrival of aggregations. We cannot do so, and this suggests that jellyfish have swimming capabilities that we do not yet understand,” the researchers explained.
In the current study, Dr. Lehahn and a team of researchers from the Leon H. Charney School of Marine Sciences, together with researchers from the Technion, the Hebrew University of Jerusalem, and Tel Aviv University, set out to examine the migration patterns of aggregations of Rhopilema nomadica jellyfish along the coasts of Israel and thereby to learn more about their swimming abilities.
The data collection stage in the study was complex and the researchers used innovative methods. Firstly, a light airplane flew along the coastline from Ashkelon in the south to Nahariya in the north, mapping the distribution of aggregations and identifying the largest ones. Next, the researchers monitored and photographed the direction of progress of the aggregations using drones operated from a nearby research boat. Lastly, the researchers examined the direction in which the aggregations swam relative to the sea currents and waves. “The use of unique research methods based on the collection of data from light airplanes, drones, and research boats allowed us to identify the swimming and movement patterns of thousands of jellyfish and to analyze these patterns using mathematical models,” explained Dr. Lehahn.
As mentioned, the researchers discovered that the accumulations of Rhopilema nomadica jellyfish show an ability to navigate. They swim collectively at a speed of around 10 cm a second, moving to the west in a counter-wave direction. By so doing, they distance themselves from the coast and increase their chances of survival. The researchers added that the swimming patterns may vary between different jellyfish species and different maritime environments. “When the strong tidal currents are dominant, it is possible that the jellyfish would swim against the current rather than against the waves, to reduce the risk of being swept onto the beach. In other cases, jellyfish may prefer to stay close to the coast, in inlets or estuaries. Swimming toward the open sea may not always be the first choice – the underlying objective is to reach a place where they have the best chances of survival.”
Given these capabilities, why do jellyfish end up by the coast, where in many cases they are swept onto dry land and die? The researchers explain that the final speed and direction of movement are determined by a combination of the jellyfish’s swimming and the influence of currents and waves. When there are no waves and the current is weak, movement will be determined solely by swimming. But when there is a strong undertow, this will significantly affect the direction. “After storms, for example, large numbers of jellyfish are swept onto the beach, because they were unable to overcome the undertow created by the waves. But it is clear that jellyfish accumulations do not just passively drift in random directions. The study advances our understanding of their movements, and in the future may allow us to predict when accumulations will arrive on coasts and when they will depart,” the researchers concluded.
