Mysterious movements and the secret life of European eel ( Anguilla anguilla )

The movement of European eels (Anguilla anguilla) has long been a mystery keeping scholars on their toes. Where European eels come from, for example, has been the source of some serious lateral thinking through the ages. In the fourth century BCE, Aristotle concluded that eels spontaneously generated from ‘the entrails of the earth’, with Pliny the Elder having another crack at the question in the 1st century AD, suggesting that eels rubbed against rocks and their ‘scrapings come to life’ (Christina Couch, 2022). Understanding of the phenomenon grew with time, and in 1912, Danish zoologist Johannes Schmidt published his findings into the location of the spawning area of the European eel, with the Sargasso Sea pinned as ‘perhaps a principal spawning region’ (Schmidt, 1912). We now know that adult European eel living in freshwater and coastal environments migrate (becoming silver eels) to the Sargasso Sea to spawn. Eel larvae (leptocephali) then migrate away from the Sargasso Sea, drifting with the Gulf Stream and North Atlantic Current, metamorphising (becoming glass eels) until they reach the European continental shelf, where (as elvers) they establish themselves in coastal, estuarine and freshwater habitats (becoming yellow eels). Although, it was not until 2022, with the satellite tagging of 26 eels, that the first direct evidence of silver eels migrating to the Sargasso Sea to spawn was obtained (Wright et al., 2022). A remarkably recent finding, considering their cultural importance and established conservation concerns (Righton & Walker, 2013). Indeed, while there has been progress made in understanding the movements of European eels in recent decades, including the impact of initial migration on female silver eels' fat reserves (Clevestam et al., 2011), migration route choices of silver eels through large river systems (Breukelaar et al., 2009) and migratory behaviour in the early stages of silver eel migration (Davidsen et al., 2011), there are still many unanswered questions. This is particularly true of eels which spend their growing days in coastal waters (rather than the better-known portion which reside in freshwater systems). However, in this issue, Rohtla et al. (2026) reveal more of the secret life of eels. By tagging (with acoustic transmitters) one hundred yellow and pre-migratory eels, the movement of these entirely marine-dwelling eels was reconstructed within a hydrophone array in Sandnesfjord, Norway. Of those hundred eels, it was possible to observe silvering and related behaviour in 22 eels. From these efforts, Rohtla et al. (2026) showed that: (i) migration spanned October to December (a much narrower period than silver eels emigrating from freshwater), (ii) pre-migratory yellow eels can stay in the marine environment for several years before migrating, (iii) migration occurred after sunset (like those from freshwater), but was not influenced by the moon phase or tidal cycle. Then there were also some findings, which require a little more inference – although, seemingly more plausible compared to Aristotle's attempts! For example, several individuals made a last minute dash into more brackish waters before migrating out to sea, and while the cause is unknown, Rohtla et al. (2026) hypothesise that this could be part of a reorientating of their ‘internal compass’ before migrating. Pre-migration was also characterised by occasional deep dives, the cause of which is also unknown, with Rohtla et al. (2026) hypothesising that this behaviour could be part of navigation or preparation for diel vertical migrations that eels exhibit in the open ocean. Together, these results give a fascinating insight into a part of the eel life stage that has been left relatively unexplored. The results demonstrate how far our understanding of the European eel has come since Aristotle, while also highlighting how much still remains unknown. Therefore, despite its somewhat celebrity status in the fish world, the European eel continues to hold many mysteries, offering fertile ground for future research.