A baby eel goes by several different names depending on exactly how old it is and what stage of development it has reached. In order, the terms are leptocephalus, glass eel, and elver. Each name marks a dramatically different-looking creature, and the transformations between stages are so extreme that scientists once classified different life phases of the same species as entirely separate animals. The naming convention applies across all freshwater eel species in the family Anguillidae, from the European and American eels of the Atlantic to the Japanese eel of the Pacific, though the journey each species takes varies enormously in distance.
Leptocephalus, the Leaf-Shaped Larva
The youngest stage of an eel’s life is called a leptocephalus, a Greek-derived term meaning “thin head.” The name is apt: a leptocephalus looks nothing like an eel. It is a flat, transparent, leaf-shaped larva, sometimes compared to a willow leaf made of glass. These larvae are so unlike adult eels that when the Danish scientist Johannes Schmidt identified them in the Sargasso Sea in the early twentieth century, it resolved a longstanding mystery about where eels actually come from.
1PubMed Central. A century of research on the larval distributions of the Atlantic eels: a re-examination of the dataLeptocephali drift on ocean currents for months or even years, depending on the species. European eel leptocephali, for instance, ride the Gulf Stream and related currents from the Sargasso Sea all the way to European and North African coastlines. What they eat during this long journey remained a puzzle for decades because their guts, when examined, rarely contained identifiable food. Researchers eventually worked out that leptocephali feed on marine snow, a catch-all term for the slow rain of tiny organic particles that drifts downward through the water column. Gut-content studies and DNA analysis have shown that the material inside leptocephali includes discarded structures from tiny marine animals called appendicularians, zooplankton fecal pellets, protists, and a wide range of microorganisms.
2CrossRef. The mysterious feeding ecology of leptocephali: a unique strategy of consuming marine snow materialsNitrogen isotope analysis confirmed this picture. Natural leptocephali feed at a very low level in the oceanic food web, consistent with eating particulate organic matter rather than actively hunting prey. This makes leptocephali something unusual in the ocean: they are a previously unrecognized part of how organic material gets recycled in deep water.
3Europe PMC. A low trophic position of Japanese eel larvae indicates feeding on marine snowGlass Eels, the See-Through Arrivals
When a leptocephalus reaches coastal waters, it undergoes a metamorphosis into a glass eel. The flat, leaf-shaped body compresses and elongates into a recognizable eel shape. During this process, the skull forms rapidly, producing a primitive but functional head structure, and the overall body plan shifts from passive drifter to something capable of directed swimming.
4Europe PMC. Japanese eel jaw and vertebra ossification occurring respectively during the larval stage and metamorphosisThe name “glass eel” comes from the animal’s appearance at this point: it is still almost entirely transparent, with only the eyes visible as dark spots. Glass eels are tiny, typically a few centimeters long, and they arrive at estuaries in large numbers during predictable seasonal windows. For European eels, glass eels typically reach the coast roughly six to eight months after hatching.
5Elsevier / Estuarine, Coastal and Shelf Science. Habitat use by the European eel Anguilla anguilla in Irish watersThe glass eel stage is also where one of the most impressive physiological shifts in the animal kingdom takes place. Glass eels must transition from saltwater to freshwater, which requires a complete reversal of how their bodies handle salt and water balance. In the ocean, glass eels actively excrete excess ions. In freshwater, they switch to actively absorbing ions to compensate for what they lose to the surrounding dilute water. Studies on European eel glass eels show that this switch involves large changes in the activity and expression of key ion-transport proteins in the gills, and that glass eels can handle an abrupt transfer from saltwater to freshwater without apparent difficulty.
6CrossRef. Osmoregulatory plasticity of the glass eel of Anguilla anguilla: freshwater entry and changes in branchial ion-transport protein expressionHow Glass Eels Find Their Way Upstream
Glass eels do not simply get swept into rivers by the tide. They use a remarkably sophisticated strategy called selective tidal stream transport. In the Penobscot River estuary in Maine, researchers found that glass eels of the American eel stay near the bottom during ebb (outgoing) tide, then rise into the water column during flood (incoming) tide, riding the landward current while avoiding being washed back out to sea. In areas where the water column was layered by salinity, the glass eels positioned themselves at or below the boundary between salt and fresh water, getting maximum push from the incoming tide without overshooting into the wrong layer.
7Oxford Academic. Selective tidal stream transport in the estuarine migration of glass eels of the American eel (Anguilla rostrata)This behavior raises an obvious question: how do glass eels know when the tide is flooding? The answer appears to involve an internal clock tuned to tidal rhythms and a built-in magnetic compass. When European eel glass eels were tested in the sea, almost all of them oriented in a direction related to the tidal cycle. When the same eels were tested in the lab with an artificial magnetic field and no other environmental cues, about seventy percent maintained the same orientation they showed during ebb tide in the wild. Glass eels, in other words, carry a compass linked to an internal tidal clock, giving them a navigational toolkit that works even when they cannot smell the river or feel the current.
8Europe PMC. Glass eels (Anguilla anguilla) have a magnetic compass linked to the tidal cycleElvers, the Pigmented Climbers
Once glass eels move into estuaries and begin entering freshwater for good, they start to gain pigmentation and are then called elvers. The shift from “glass eel” to “elver” is not a sharp biological boundary; it is a gradual darkening as melanin develops in the skin. In practice, a glass eel becomes an elver once it has visible pigmentation and has committed to freshwater life. The transition from partially pigmented elver to fully pigmented eel takes place over the summer months following the spring arrival into freshwater, and is accompanied by significant growth.
9CrossRef. Pigmentation, otolith rings, and upstream migration of juvenile American eels (Anguilla rostrata) in a coastal Rhode Island streamElvers are driven to move upstream, and they do so with surprising determination. Small Japanese eels have been documented climbing vertical weirs over a meter and a half tall, moving only at night, and doing so more frequently when water temperatures rise above about 15°C. Warmer water and rain both increase the probability of climbing, which makes intuitive sense: warmer, wetter conditions mean more water flow and better conditions for the mucus-covered skin that lets them grip wet surfaces.
10PLOS ONE. Water temperature and precipitation stimulate small-sized Japanese eels to climb a low-height vertical weirElvers are also the life stage most entangled in the global eel trade, a point covered in more detail below. In Maine, the elver fishery is one of the most lucrative per-pound fisheries in North America, with prices that have at times exceeded a thousand dollars per pound. The animals caught in these fisheries are shipped to aquaculture operations in East Asia, where they are raised to market size.
Yellow Eels, the Long Growth Phase
After the elver stage, young eels settle into their freshwater habitat and are known as yellow eels, named for the olive-to-yellowish color of their belly and flanks. This is the longest phase of an eel’s life, lasting anywhere from a few years to several decades depending on the species, sex, and conditions. Female European eels, which grow larger than males, may spend twenty years or more in this stage before maturing.
Yellow eels are sedentary compared to every other stage. They establish home ranges in rivers, lakes, estuaries, or coastal lagoons and feed on whatever is available: insects, worms, crustaceans, small fish. Some yellow eels never fully commit to freshwater and spend their entire growth phase in brackish or even coastal marine habitats. Research in Ireland and elsewhere has found substantial variation in habitat use, with some individuals staying in estuaries rather than penetrating far inland.
5Elsevier / Estuarine, Coastal and Shelf Science. Habitat use by the European eel Anguilla anguilla in Irish watersFrom the standpoint of eel terminology, the yellow eel stage is where people stop using “baby eel” language. Yellow eels are juveniles in a reproductive sense, since they have not matured sexually, but they are fully formed eels in appearance and behavior. The remaining name change comes only when they prepare for their final journey.
Silver Eels and the Final Transformation
When a yellow eel’s body is ready to reproduce, it undergoes one last metamorphosis into a silver eel. The changes are dramatic: the skin turns silvery, the eyes enlarge substantially, the skull reshapes, and the digestive tract begins to degenerate because the eel will never eat again. Respiratory muscles grow larger, and the overall cranial structure shifts to accommodate bigger eyes and potentially stronger jaw muscles.
11PubMed Central. From yellow to silver: Transforming cranial morphology in European eel (Anguilla anguilla)The silvering process also involves something remarkable and grim: the eel begins breaking down its own skeleton. European eels redistribute phosphorus and calcium from their bones to soft tissues and developing gonads. In females, this bone loss can become so severe that it threatens the mechanical stability of the skeleton. Along with the calcium and phosphorus, toxic metals stored in bone and muscle tissue get mobilized and transferred to the ovaries, potentially harming the eggs.
12PubMed Central. Bone resorption and body reorganization during maturation induce maternal transfer of toxic metals in anguillid eelsSilver eels then migrate downstream and out to sea. European eels travel more than 5,000 kilometers toward the Sargasso Sea, a journey that satellite tagging suggests takes many months at average speeds of roughly 3 to 12 kilometers per day.
13Nature. First direct evidence of adult European eels migrating to their breeding place in the Sargasso Sea Spawning likely begins in December and peaks in February, after which the adults presumably die. No one has ever observed eel spawning in the wild. The eggs hatch into leptocephali, and the cycle starts again.14Europe PMC. Empirical observations of the spawning migration of European eels: The long and dangerous road to the Sargasso Sea
Why the Eel Trade Revolves Around Wild-Caught Juveniles
Eel aquaculture is a massive global industry, particularly in East Asia, but it has a fundamental constraint: farmers cannot breed eels in captivity at any meaningful scale. Eel farming works by catching wild glass eels or elvers and raising them in ponds or tanks to market size. This means the entire industry depends on a steady supply of wild juveniles. In Taiwan, declining glass eel availability has directly reduced cultured eel production, and the same pattern holds wherever eels are farmed.
15CrossRef. Sustainable Governance of Japanese Eel Industry in Taiwan Under Glass Eel Supply UncertaintyThis reliance on wild juveniles has driven glass eel fisheries from Maine to the Philippines, where the freshwater eel industry has been growing but remains heavily dependent on wild-caught glass eels as seed stock.
16CrossRef (The Philippine Journal of Fisheries). Mixed Methods Approach in Documenting Aquaculture Practices and Market Dynamics of the Freshwater Eel Anguilla spp. Industry in the PhilippinesThe inability to close the eel’s life cycle in captivity has been one of the longest-running problems in aquaculture. Japanese researchers achieved the first-ever production of glass eels from captive-bred larvae, proving it was technically possible to rear leptocephali all the way through metamorphosis.
17Europe PMC. The first success of glass eel production in the world: basic biology on fish reproduction advances new applied technology in aquaculture The process, however, remains painstaking. Captive-reared leptocephali grew more slowly than wild ones, likely because the artificial diet was nutritionally incomplete and the water temperature was lower than in natural habitats.
18ScienceDirect. Production of leptocephali of Japanese eel (Anguilla japonica) in captivityMore recent work has improved survival and growth by using water conditions closer to what larvae experience in the wild and by switching to artificially feminized adult eels rather than wild-caught silver eels as broodstock. But mass production remains out of reach. The larval diet is still a bottleneck, partly because earlier formulations relied on eggs from a shark species that is itself endangered. Deformities in captive leptocephali and glass eels are also a persistent problem.
19CrossRef. Recent advances in artificial production of glass eels for conservation of anguillid eel populationsConservation and What Threatens Young Eels
European, American, and Japanese eel populations have all declined to the point where they are considered outside safe biological limits. The European eel is listed as critically endangered by the IUCN, and recruitment of glass eels to European rivers has dropped to a small fraction of historical levels.
20SpringerOpen. Do we protect freshwater eels or do we drive them to extinction?The causes are tangled together. Overfishing of glass eels and elvers for the aquaculture trade removes juveniles before they can grow and eventually spawn. Dams and weirs block upstream migration, limiting access to freshwater habitat. Environmental DNA surveys of European rivers have confirmed that migration barriers reduce eel presence at sites farther from the sea, effectively shrinking the usable habitat.
21Elsevier. Environmental DNA analysis indicates that migration barriers are decreasing the occurrence of European eel (Anguilla anguilla) in distance from the seaOn top of these pressures, an invasive parasitic nematode has spread through European eel populations. The worm infects the swimbladder, the gas-filled organ eels use to control their depth. In moderate infections, the swimbladder can still function, but severe infection may damage it past a tipping point where the eel can no longer complete its spawning migration across thousands of kilometers of deep ocean.
22PubMed Central. A mechanical approach to understanding the impact of the nematode Anguillicoloides crassus on the European eel swimbladderNames Beyond the Anguillids
The terminology described above applies specifically to freshwater eels in the genus Anguilla, which includes about sixteen species worldwide. But many other fish commonly called “eels” go through a leptocephalus stage as well, because the larval form is shared across the broader order Elopomorpha. Conger eels, moray eels, and garden eels all produce leptocephali, and their larvae can grow much larger than those of freshwater eels. Some conger eel leptocephali reach lengths that dwarf their freshwater cousins. After metamorphosis, though, the terminology diverges. “Glass eel” and “elver” are terms used almost exclusively for Anguilla species. Juvenile morays and congers are simply called juveniles, or occasionally post-larvae, without the specialized vocabulary that freshwater eels have earned by virtue of their economic and cultural importance.
The richness of eel naming, in other words, says as much about the human relationship with freshwater eels as it does about the animals themselves. These are species that have been fished, traded, farmed, and obsessed over for centuries, and the layered terminology reflects a long history of people watching these creatures show up at riverbanks in stages so different from one another that each seemed to deserve its own name.