Migratory birds have remarkably consistent departure times from their breeding and wintering areas year after year, so much so that we refer to “migratory calendars” that define their different populations. Long-distance migratory species are especially punctual in their arrival and departure, often occurring on the same day or within the same week. It is no coincidence that the well-known proverb says, “On St. Benedict day, the swallow is under the roof.” This is because their migration is not controlled by simple, changeable weather and climate events but by an internal mechanism driven by biological clocks that regulate and time their annual life cycle.
This is not the case, however, for other vertebrate groups, which are more dependent on local weather conditions, as is the case for the movements of the many species of herbivorous mammals in the African savannahs. There, it is the rainy seasons and the vegetation renewal they bring that trigger the movements of buffalo, zebras, giraffes, gazelles, and so on.
However, if a species must migrate because it is the optimal survival strategy sooner or later it will move, and once it begins the journey, it will complete it. At least that was the common belief, but fortunately, we were forced to change our minds due to the behavior of eels. They are perhaps the most complex and spectacular example of migration known. Once they reach sexual maturity, eels descend European rivers and begin a long journey to the Sargasso Sea. There, they reproduce and die, while their larvae make the return journey, carried by sea currents. These small, translucent “elvers,” once returned to the rivers, will begin a new migratory cycle (see The mysterious eel). The eel’s spawning grounds were discovered by J. Schmidt in 1922, and many questions about its biology remain unanswered. Among these, nothing was known about the complex strategies used to release mature specimens from rivers, the so-called capitoni of the Christmas table.
The Undecided Eels of the Narva River
The Narva is a peaceful, beautiful river that marks the disputed border between Estonia and Russia. It is a drain from Lake Peipsi that flows 77 km into the Baltic Sea. Its eels, now sexually mature and migrating toward the sea, were equipped with acoustic transmitters that could detect their movements even over long distances. Once released, they continued their journey, supposedly one-way, reaching the mouth of the Narva and then disappearing into the sea. Unexpectedly, after 10-11 months, 21% of those released returned to the river from which they had departed, swimming upstream again. Some even traveled up to 15 km from the mouth. Their stay in the river lasted about thirty days, after which they set out again, this time without hesitation. The fastest among them covered about 1300 km in 68 days until it left the Baltic Sea, when its signal was lost.
The story of the Estonian researchers had a happy ending, as after much deliberation, the eels finally migrated. However, their research has not explained the reasons for this behavior. A stopover at sea before their final departure is expected, as the functional changes required for the transition from freshwater to marine environments are significant. These include changes in morphology, skin coloration, and, most importantly, tissue reorganization and renal function, transforming a freshwater fish into a saltwater one. In saltwater, the kidney must excrete the abundant magnesium and sulfate ions, as well as sodium and chloride. To survive, the eels must modify both the renal corpuscle and the proximal tubular part of the nephron, a process that takes time to perfect.
But why turn back, and what’s more, not to a generic river, but to the one they started from? This is truly intriguing… After a year at sea, pinpointing it requires a precise orientation mechanism, given that the percentage of eels returning to the Narva compared to those initially excludes the possibility that it is a random event. How could such a result be possible? I can think of a mechanism for reversing the outward journey, which many species are capable of completing. However, the individual eel must record the route with a compass, which then allows the return route to be determined and followed. Too complex, if not impossible, in open sea conditions.
One could also consider an orientation system based on olfactory stimuli, similar to that of salmon fry, which, through a process of imprinting on the olfactory characteristics of the waters of their native river, can find them again for reproduction after 3-4 years of growth in the sea. In this case, it obviously cannot be a question of imprinting, but the smell of the waters in which the eels grew up could have been memorized during their past life in the river they entered as larvae and where they lived for at least 15-18 years. Why not?
The eel has certainly guarded the secrets of its behaviour for many years and perhaps has not yet grown tired of doing so!
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Original data from: M. TAMBETS and collaborators. Biology Letters 17, DOI: 10.1098/rsbl.2021.0346. Migrating silver eels return from the sea to river of origin after a false start.
Credits
Author: N. Emilio Baldaccini, Former Professor of Ethology and Conservation of Zoocenotic Resources at the University of Pisa. He has published over 300 scientific papers in national and international journals. Actively engaged in scientific education, he is also a co-author of academic textbooks on Ethology, General and Systematic Zoology, and Comparative Anatomy.
Translated by Maria Antonietta Sessa
