Eels rank among the most enigmatic animals on the planet, fish whose reproductive lives remained so hidden that Aristotle believed they arose spontaneously from mud. Despite centuries of study, no human has ever witnessed a European or American eel spawning in the wild, and it was only in 2022 that satellite tags confirmed an individual European eel actually reaching its suspected breeding ground in the Sargasso Sea. That single data point underscores how much remains unknown about a creature found in rivers, lakes, and coastal waters across much of the Northern Hemisphere and beyond.
A Life Cycle Like No Other Fish
Freshwater eels of the genus Anguilla go through a series of dramatic physical transformations over lifespans that can stretch beyond twenty years. They begin life as tiny, leaf-shaped larvae called leptocephali, transparent and so flat they look nothing like an adult eel. The bodies of these larvae are filled with a gelatinous material that likely serves double duty: making them nearly invisible to predators and storing energy for the long drift across open ocean and eventual metamorphosis into the next stage.
1Canadian Journal of Fisheries and Aquatic Sciences. The ecology of oceanic dispersal and survival of anguillid leptocephaliAfter months to years of drifting on ocean currents, leptocephali metamorphose into “glass eels,” tiny, translucent versions of their adult form that congregate near coastlines and begin pushing into estuaries and rivers. As they settle into freshwater or brackish habitats, they gain pigmentation and become “elvers,” then grow into “yellow eels,” the sedentary feeding stage that can last a decade or more. When the time finally comes to reproduce, yellow eels undergo one last transformation into “silver eels,” developing enlarged eyes, a silvery belly, and changes to their swim bladder and fat stores, all preparing them for a one-way journey back to the open ocean to spawn and die.
Genomic research has identified signs of natural selection acting on the transition between these life stages. A study scanning the European eel genome found that signal transduction pathways, including those involved in hormone signaling, were among the most overrepresented groups of genes showing evidence of selection pressure between larval and later life stages.
2BMC Genomics. Signatures of natural selection between life cycle stages separated by metamorphosis in European eelThe Spawning Migration That Took Centuries to Confirm
For the European eel, the spawning destination is the Sargasso Sea, a region of the western Atlantic bounded by ocean currents rather than coastlines. Scientists suspected this for over a century based on the discovery of the smallest leptocephali there, but direct proof of adults reaching the area was agonizingly slow to arrive. A large-scale electronic tagging study tracked more than 700 European eels released from four regions across Europe. Of those, the researchers recovered data from 206 tags and reconstructed detailed migration routes for more than 80 individuals. The routes extended from western mainland Europe past the Azores, covering more than 5,000 kilometers toward the Sargasso Sea, with migration speeds ranging from 3 to 47 kilometers per day.
3PubMed Central. Empirical observations of the spawning migration of European eels: The long and dangerous road to the Sargasso SeaAll tagged eels displayed a striking daily rhythm: they dove to deeper water during the day and rose to shallower water at night, a behavior known as diel vertical migration. This pattern likely helps them avoid visual predators during daylight hours and possibly assists with thermoregulation during a journey that can last many months. Average tag durations in a follow-up study ranged from about 40 to 366 days, and five of the satellite tag pop-up locations landed within the Sargasso Sea boundaries, with one eel located inside the presumed breeding area. Average migration speeds in that dataset came out to roughly 7 kilometers per day.
4Scientific Reports. First direct evidence of adult European eels migrating to their breeding place in the Sargasso SeaThe Japanese eel faces a parallel mystery with a different destination. Its spawning grounds lie along the western side of the West Mariana Ridge in the Pacific, where researchers have collected eggs, newly hatched preleptocephali, and even a few spawning adults, always during new moon periods.
5PubMed Central. Spawning sites of the Japanese eel in relation to oceanographic structure and the West Mariana Ridge Yet even with these collections, actual spawning behavior has never been observed in the wild, and the precise locations appear to shift between years.
6Fisheries Science. Several possible spawning sites of the Japanese eel determined from collections of their eggs and preleptocephaliHow Eels Navigate Thousands of Miles
Finding a patch of open ocean thousands of kilometers away without any landmarks is a feat that would challenge the best human navigator without instruments. Eels appear to rely heavily on Earth’s magnetic field. Experiments with glass eels showed that about 98 percent of individuals tested in seawater exhibited a preferred orientation linked to the tidal cycle, and when the same eels were tested in a laboratory under a manipulated magnetic field with no other cues available, about 71 percent maintained the same directional preference during ebb tide. The finding demonstrates that glass eels carry an internal magnetic compass and that this compass is coupled to a biological clock tuned to the tides.
7PubMed Central. Glass eels (Anguilla anguilla) have a magnetic compass linked to the tidal cycleWater temperature appears to modulate this compass. In displacement experiments with European eels, individuals in water below about 12°C oriented significantly toward their home direction using magnetic cues. Above that temperature, the same magnetic orientation broke down and the eels’ headings became essentially random or shifted perpendicular to the expected direction.
8PLoS ONE. Magnetic Compass Orientation in the European EelMagnetism is not the only tool in the kit. A comprehensive review of glass eel navigation proposed a hierarchical model: in the open ocean, eels rely on lunar and magnetic cues; as they approach the coast, they add chemical signals and salinity gradients; and once inside estuaries, they layer on odor detection, water current direction, and salinity alongside the ever-present magnetic sense. The result is a flexible navigation system that shifts depending on what type of environment the eel is moving through.
9PubMed. A comprehensive hypothesis on the migration of European glass eels (Anguilla anguilla)A Body Built for Two Worlds
Eels live in freshwater rivers, brackish estuaries, and the open ocean at different points in their lives, and sometimes they cross between these environments rapidly. That means their bodies need to handle wildly different salt concentrations, a challenge that would kill many fish. In seawater, where a fish constantly loses water by osmosis, eels drink the surrounding water and then face the problem of getting rid of all the excess salt that comes with it. Research on the eel esophagus showed that in seawater-adapted eels, the esophagus becomes permeable to sodium and chloride, allowing these ions to diffuse passively out of the swallowed seawater into the blood before the diluted fluid reaches the intestine. The effect is substantial: ingested seawater is diluted roughly three times more efficiently in the esophagus alone than in the stomach.
10PubMed Central. Eel esophagus as an osmoregulatory organFarther down the digestive tract, the rectum plays an unexpectedly important role. Studies on Japanese eels showed that rectal water absorption was significantly greater in seawater-adapted individuals than in freshwater ones, driven by an osmotic gradient created by active ion transport. The rectum expressed high levels of a water channel protein called aquaporin 1, making it a key site for reclaiming water from the gut contents. Together, the esophagus strips salt out of swallowed seawater while the intestine and rectum reclaim the desalinated water, an elegant assembly line for survival in the ocean.
11PubMed. Rectal water absorption in seawater-adapted Japanese eel Anguilla japonicaEels are also famously capable of brief overland excursions. American eels have been documented crawling across wet grass and climbing damp surfaces to bypass obstacles. Laboratory tests of their terrestrial movement found that they were more effective on soft, compliant surfaces but struggled with steeper inclines, which makes intuitive sense for an animal with no limbs.
12Integrative and Comparative Biology. Terrestrial Locomotion in American Eels (Anguilla rostrata): How Substrate and Incline Affect Movement PatternsWhat Eels Eat and Where They Fit in the Food Web
During the yellow eel stage, when they settle into rivers and estuaries for years, eels are opportunistic predators with broad diets. Gut content analyses of European eels in estuarine habitats found that crustaceans and fish dominated their meals, with the amphipod Gammarus zaddachi and the green crab Carcinus maenas among the most frequently consumed prey. Eels also eat other eels: smaller yellow eels of their own species appeared in gut samples, along with juvenile European flounder.
13PubMed Central. Role of estuarine habitats for the feeding ecology of the European eel (Anguilla anguilla L.)This generalist feeding strategy makes eels important mid-level predators in the habitats they occupy. They help regulate populations of crustaceans and small fish, and in turn they provide food for birds, larger fish, and mammals. In freshwater systems where eels have declined sharply, those ecological roles go unfilled, with cascading effects that are difficult to predict but easy to worry about.
Parasites, Dams, and Pollution
The threats facing eels are stacked. One of the most insidious is a swim-bladder parasite called Anguillicola crassus, a nematode native to Asia that was introduced to European waters through the aquaculture trade. The parasite feeds on blood inside the swim bladder and causes mechanical damage that compromises buoyancy control. In swim-tunnel experiments, infected eels had lower cruising speeds and higher energy costs per kilometer traveled. Nearly half of eels with damaged swim bladders stopped swimming entirely at low aerobic speeds. Simulated migration trials confirmed that heavily infected eels failed to complete distances under 1,000 kilometers, far short of the 5,000-plus kilometer journey to the Sargasso Sea.
14Journal of Experimental Marine Biology and Ecology. Swimming performance of silver eels is severely impaired by the swim-bladder parasite Anguillicola crassusThe mechanical damage is not trivial even after the parasites themselves are gone. Biomechanical testing of infected swim bladders showed that as the bladder wall thickened in response to repeated infection, its tensile strength actually decreased, meaning scarred bladders are both stiffer and weaker.
15Journal of Experimental Biology. A mechanical approach to understanding the impact of the nematode Anguillicoloides crassus on the European eel swimbladderHydroelectric dams are another serious barrier. A study on the Penobscot River in Maine tracked downstream-migrating American eels and found that cumulative mortality through dammed sections was roughly 9 percent over just 4.4 kilometers, compared with the same 9 percent spread across 58 kilometers of undammed river. Fish that had already passed one dam upstream suffered even higher mortality at the next one downstream, suggesting a latent or cumulative toll. More than 10 percent of tagged eels were delayed at dams for over 24 hours, and low water flows made things worse.
16Canadian Journal of Fisheries and Aquatic Sciences. The consequences of dam passage for downstream-migrating American eel in the Penobscot River, MaineAt conventional hydropower turbines, the mortality numbers can be grimmer. A study on five dams along the Shenandoah River recorded turbine-related mortality rates ranging from about 16 to 41 percent at individual dams, with overall project-specific mortality rates (combining all passage routes) between 3 and 14 percent per dam.
17Transactions of the American Fisheries Society. Downstream Passage and Impact of Turbine Shutdowns on Survival of Silver American Eels at Five Hydroelectric Dams on the Shenandoah River There is some hope in new turbine designs, however. Testing of a novel hydropower turbine found zero mortality among eels that passed through it over a 48-hour monitoring period, suggesting that fish-friendly engineering can make a real difference.
18Transactions of the American Fisheries Society. Safe passage of American Eels through a novel hydropower turbinePollution adds yet another layer. European eels sampled from two Polish river basins showed measurable accumulation of mercury, lead, arsenic, and cadmium in their tissues, with higher concentrations in eels from the river system exposed to more industrial input. While the levels in that study did not exceed food safety thresholds, eels are long-lived and fatty, two traits that make them efficient accumulators of fat-soluble contaminants and heavy metals over time.
19PubMed Central. Heavy Metal Bioaccumulation in European Eels (Anguilla anguilla) from the Odra and Vistula River Basins (Poland): Implications for Environmental and Food SafetyThe Black Market for Glass Eels
The European eel is critically endangered, and international trade in its glass eels has been restricted since the species was listed under CITES in 2009. That has not stopped a lucrative smuggling industry. Glass eels, barely the length of a finger, can fetch thousands of dollars per kilogram on Asian markets, where they are grown out for the booming unagi trade. Spain is one of the leading source countries for trafficked eels and also accounts for the highest number of seizures.
20European Journal of Criminology. Inside the slippery world of glass eel trafficking: Lessons learned from Spain to prevent the illegal trade of European eelsEnforcement is catching up, slowly. Researchers developed a rapid genetic test that can distinguish European eel tissue from other species in a shipment, and its first real-world application in Hong Kong led to a prosecution for eel smuggling, the first of its kind in that jurisdiction.
21Conservation Science and Practice. Development and application of a novel real‐time polymerase chain reaction assay to detect illegal trade of the European eel (Anguilla anguilla)Why Nobody Has Closed the Breeding Cycle
Eel aquaculture currently depends entirely on catching wild glass eels and growing them in farms, because closing the breeding cycle in captivity has proved fiendishly difficult. Eels do not reproduce spontaneously in tanks. Their bodies seem to require specific and poorly understood environmental triggers to fully mature sexually, triggers that may involve pressure changes, temperature shifts, and prolonged swimming that mimic the open-ocean migration.
Researchers have managed to force maturation using repeated hormone injections. For males, recombinant human chorionic gonadotropin produced the best sperm quality at the lowest cost compared to other hormonal treatments.
22Aquaculture. Study of the effects of thermal regime and alternative hormonal treatments on the reproductive performance of European eel males (Anguilla anguilla) during induced sexual maturation On the female side, maturation can be coaxed with pituitary extracts, but getting viable eggs that survive through hatching and early larval development remains unreliable.
A closed recirculating aquaculture system designed specifically for European eel reproduction achieved spontaneous spawning and egg transfer, but the hydrodynamic conditions in the incubation chambers proved problematic: high flow rates caused constant loss of viable eggs, preventing consistent embryonic development.
23Aquacultural Engineering. A closed recirculating aquaculture system for artificial seed production of the European eel (Anguilla anguilla): Technology development for spontaneous spawning and eggs incubationThe Japanese eel is slightly further along but still far from commercial-scale captive breeding. Despite decades of work, key regulatory mechanisms governing eel reproduction remain unresolved, particularly around the neuroendocrine signals that trigger final maturation and the pathways of sex differentiation.
24Reviews in Aquaculture. From Barriers to Breakthroughs: Toward Sustainable Artificial Reproduction in the Japanese Eel (Anguilla japonica) Until someone cracks this puzzle, every farmed eel on your plate started as a wild-caught juvenile, which means eel farming currently adds to the pressure on wild populations rather than relieving it.
Climate Change and the Ocean Conveyor
One widely discussed theory for the decline in European eel recruitment is that shifting ocean currents, driven by climate change, could be disrupting the drift of leptocephali from the Sargasso Sea to European coasts. A modeling study tested this idea by simulating larval drift patterns before and after 1970, the period when recruitment began its steep decline. The model found that the proportion of larvae carried by the North-East Atlantic Current to northern European coastlines was indeed greater before 1970, while more larvae were entrained into southbound current branches afterward. However, the overall proportion of larvae successfully drifting from the spawning area to the eastern Atlantic was not affected.
25Ecology of Freshwater Fish. Climate change and passive transport of European eel larvaeThat result is important because it suggests the recruitment crash is not simply a matter of larvae failing to reach Europe. The distribution of where they arrive may be shifting, with fewer reaching northern rivers and more drifting south, but the total number making it across the Atlantic does not appear to be the bottleneck. The real drivers of decline likely lie elsewhere: overfishing of glass eels, habitat loss, dam construction, parasites, and pollution, all of which hit eels after they arrive on the continental shelf.
Eels in Human Culture
Long before scientists struggled to unravel the eel’s biology, people around the world were catching, eating, and telling stories about them. Eel fisheries in Europe date back millennia, and the animals have woven themselves into cuisine, folklore, art, and literature across many cultures.
26Ecology of Freshwater Fish. The importance of Anguillids: a cultural and historical perspective introducing papers from the World Fisheries Congress In Japan, grilled eel (unagi) is a summer delicacy with a dedicated holiday, Doyo no Ushi no Hi, believed to boost stamina in the heat. In parts of England, jellied eels were a working-class staple for centuries. The Basque Country prizes elvers cooked in garlic and olive oil, a dish now so expensive it borders on the absurd given how scarce the glass eels have become. In New Zealand, the longfin eel holds deep significance for Māori communities, who consider eels a taonga, a treasured possession, and manage traditional fisheries according to customary law.
The eel’s slippery reputation and shapeshifting life cycle have made it a natural subject for myth. Ancient Greeks debated whether eels had sex at all. Medieval Europeans sometimes classified them alongside snakes. And a young Sigmund Freud, before he turned to the human mind, spent a frustrating season in Trieste dissecting hundreds of eels in an unsuccessful search for their testes, an early scientific failure that some scholars think colored his later thinking about hidden drives. The eel, in other words, has been confounding human understanding for as long as humans have been paying attention.