Population structure and spawning area(s) of tropical eels

Eels live up to several decades in freshwater before they travel across the ocean to their marine spawning areas. The relentless hunt of researchers for these areas roots in the fact that the eels reproductive cycle puzzled science since centuries and still does. At the close of the 19th century, when the migration loop of European eel (Anguilla anguilla) across the Atlantic was still unknown, the search for its spawning area was termed the eel question. This quest was so important for scientists at that time that the famous German zoologist Max Schultze (1825-1874) supposedly claimed at his deathbed that all important questions in biology have been solved except the eel question! As ripe spawning eels had never been caught, the eels reproductive cycle had remained beyond full comprehension since Aristotle (384-322 B.C.) who in lack of a natural explanation had claimed that eels are spontaneous creations from the entrails of the earth. With the discovery of newly hatched larvae of American (A. rostrata) and European eels in the Sargasso Sea, the eel question was finally answered in 1922 by the Danish scientist Johannes Schmidt (1877-1933). Only about 30 years ago, a comparable milestone discovery was made by Katsumi Tsukamoto (born 1948) locating the spawning area of Japanese eel (A. japonica) near the Mariana Islands. However, even today the spawning areas for the majority of the in total 16 eel species remain unknown, although considerable effort has been made to find them. Habitat destruction, overfishing, pollution and introduction of parasites have driven several eel species near extinction. In eastern Africa and the Indian Ocean islands, eels are seen as mythical creatures supporting subsistence fisheries. Countries in these areas have recently entered the global eel trade heedless of the contribution of these migratory fishes to the livelihoods of vulnerable human societies. Worldwide, the Indian Ocean eels are the least studied stocks and little is known about the marine phase of their life cycle. We will tag endemic Anguilla mossambica with satellite transmitters to study their marine migration routes from Madagascar and possibly locate their spawning area, which they may share with one or more of the co-occurring species (A. marmorata, A. bicolor bicolor, A. bengalensis labiata). We will explore the oceanographic conditions along the eels migration path to identify landmarks that the eels may use to find their spawning area. Furthermore, we will study the population structure of all four species throughout their distribution range and screen for interspecific hybrids to conclude about shared spawning sites. Our results will enable future conservation efforts and help to guide sustainable fishing.

From myth to reason: Progress towards unravelling the mysteries of tropical Anguillid eels Why had the reproduction of a slippery, snakelike freshwater fish fuelled the imagination of humans for millennia? Because nobody found ovaries or testes in freshwater eels! Throughout the Indo-Pacific, eels are mythical creatures: Sacred tales try to explain the eel's world and man's experience with them, but the human horizon ended with the sea surface when the eels commenced their mysterious migration into the deep blue. During four expeditions (2022 - 2025) we hunted for migrating eels in remote rivers of Madagascar and succeeded for the first time to tag and release a total of 15 individuals from three different species into the Indian Ocean. The eels carried the satellite transmitters eastward for over six months and up to 950 km into the South Equatorial Current towards the Mascarene Ridge. The tags surfaced right where French scientists by applying theoretical models had predicted the spawning area to be. The eels performed regular diel vertical migrations, quickly descending beyond 1000 m depth before dawn and ascended to shallower depth of 200 m at night. During the day they searched for and resided at chilly 5 C, potentially needing these frigid temperatures for gonad maturation. Despite the evolutionary importance of this oceanic phase, it remains poorly understood in many of the 16 eel species. Traditionally, Indonesia has been considered the centre of diversity and evolutionary origin for this group. However, our comprehensive analysis of 344 individual whole genomes from all Anguilla species collected at 50 geographically distinct locations worldwide presents an entirely different view of their evolutionary history: Eels may have developed from an ancestral species either off Australia or in the Atlantic. Despite the fact that different eel species have evolved millions of years ago, we found widespread crossbreeding and fertile hybrid offspring among them. Hence, they must spawn at the same time and location boosting the development of new species through genetic exchange. Our study revises the evolutionary history of the genus and sheds light on the mechanisms driving diversification in the marine environment thereby uncovering some of the mysteries of tropical eels.