Eels shift through a remarkable sequence of colors over their lifetime, starting as transparent larvae and ending as dark-backed, silver-bellied adults ready for an ocean crossing. These changes are not cosmetic quirks. Each color phase corresponds to a different habitat, diet, and stage of sexual development, and the transitions are driven by hormones, water temperature, and the demands of survival in radically different environments. The story gets stranger when you learn that some eels glow green under the right light, others turn bright orange in the deep sea, and a few can lighten or darken their skin within minutes.
The Glass Stage
The first form most people never see is the leptocephalus, the larval eel. These larvae are leaf-shaped, paper-thin, and almost completely transparent. The transparency is a survival strategy: drifting in open ocean currents for months or even years, a see-through body makes it very difficult for predators to spot them against the featureless blue water. Their bodies contain very little blood pigment and almost no melanin at this stage. This is not unique to eels; many ocean larvae are transparent for the same reason. But eels maintain this glassy look longer than most fish, because their ocean drift phase is unusually extended.
As larvae approach coastal waters and begin to metamorphose, they shrink slightly and become “glass eels,” a term that describes their still-translucent appearance. At this point, pigmentation starts creeping in. Tiny melanophores, the cells that produce dark pigment, begin appearing along the body. Researchers classify the progression of glass eel pigmentation into substages, tracking how dark spots spread from the head backward along the body. The speed of this darkening depends heavily on water temperature: warmer water accelerates pigmentation, while cooler water slows it down. Salinity plays a secondary role, with higher salt concentrations slightly delaying the process.1Journal of Fish Biology. A stage‐structured model to predict the effect of temperature and salinity on glass eel Anguilla anguilla pigmentation development Work on Japanese eels confirmed that temperature, not the size of the fish, is the main driver of how quickly pigment develops.2Journal of Fish Biology. Growth, pigmentation and activity of juvenile Japanese eels in relation to temperature and fish size
Yellow Eels and the Long Freshwater Phase
Once glass eels swim into rivers, estuaries, or coastal lagoons and settle in, they become “yellow eels.” The name is slightly misleading. They are not bright yellow. Their bellies take on a yellowish or cream color, and their backs darken to olive, brown, or greenish-brown, depending on the species and the local habitat. This countershading, dark on top and pale below, is a standard camouflage pattern in fish. Viewed from above, the dark back blends with the murky river bottom. Viewed from below, the lighter belly blends with the brighter surface water.
Studies of European eel skin structure have shown that the integument’s reflective properties at this stage are tuned for freshwater camouflage. Spectral measurements of the skin confirm that the color pattern has genuine concealment value, and changes in that coloration track with changes in habitat as eels eventually move toward the sea.3Journal of Fish Biology. Structure and colour of the integument of the European eel Anguilla anguilla (L.) Yellow eels can spend anywhere from five to over twenty years in this phase, feeding and growing without reproducing. The color stays relatively stable throughout, though eels in darker-bottomed habitats tend to be darker overall, and those in sandy or silty environments can be paler.
Silvering and the Preparation for Migration
The most dramatic color change in a freshwater eel’s life is the shift from yellow to silver. When an eel reaches a critical size and fat reserve, it begins a transformation that overhauls nearly every system in its body. The belly goes from yellowish to bright metallic silver. The back deepens from olive-brown to near black. The pectoral fins darken. The eyes enlarge, sometimes doubling in diameter. The digestive tract shrinks because the eel will stop eating entirely and rely on stored fat for a migration that can cover thousands of kilometers.
Researchers working with European eels have identified five distinct stages for females and two for males during this transition, ranging from the growth phase through a pre-migrant stage to two fully migratory stages. An important period of growth appears to trigger silvering, possibly through the production of growth hormone.4Journal of Fish Biology. The silvering process of Anguilla anguilla: a new classification from the yellow resident to the silver migrating stage But growth hormone is only part of the picture. The process is deeply tied to sexual maturation. In Japanese eels, levels of thyroid-stimulating hormone and the thyroid hormone thyroxine rise during silvering, suggesting the thyroid axis plays a coordinating role.5PubMed. Cloning of the cDNA for thyroid stimulating hormone beta subunit and changes in activity of the pituitary-thyroid axis during silvering of the Japanese eel, Anguilla japonica Meanwhile, experiments have shown that testosterone can induce some of the hallmarks of silvering on its own, including enlarged eyes and a shrinking digestive tract, while thyroid hormone and estrogen alone do not have these effects.6PubMed. Endocrine evidence that silvering, a secondary metamorphosis in the eel, is a pubertal rather than a metamorphic event That finding led researchers to argue that silvering is better understood as a puberty-like event than a simple metamorphosis.
The new coloration is not just for show. The silver belly and black back create a more extreme version of oceanic countershading, optimized for deep, open water rather than the murky rivers the eel has been living in. The silver acts like a mirror, reflecting the dim ambient light from below and making the eel harder to see from underneath. The dark dorsal surface absorbs light from above. During the metamorphosis, the cranial musculoskeletal system also changes: jaw and respiratory muscles enlarge, with the respiratory muscles potentially helping the eel breathe in the deeper, less oxygenated waters it will encounter during its ocean crossing.7PubMed Central. From yellow to silver: Transforming cranial morphology in European eel (Anguilla anguilla)
How Eels Change Color in Minutes, Not Months
On top of the slow, life-stage color transitions, eels can also lighten or darken their skin rapidly, on the scale of minutes to hours. This is physiological color change, driven by hormones that control how pigment is distributed inside melanophores. When the dark pigment melanin spreads out within the cells, the skin darkens. When it clumps together into a tight ball at the center of each cell, the skin looks lighter.
The system works through two opposing hormones. Alpha-melanocyte-stimulating hormone (α-MSH), released from the pituitary gland, causes melanin to disperse, darkening the skin. A second hormone, melanin-concentrating hormone (MCH), does the opposite, pulling pigment together and lightening the eel. When researchers moved eels from a dark background to a white one, the levels of α-MSH in their blood dropped rapidly, and the skin lightened. Removing the pituitary gland altogether caused the skin to lighten permanently, confirming that the pituitary is the main source of the darkening signal. Injecting pituitary extracts back into those eels re-darkened them. And when MCH was injected into dark-adapted eels, their skin lightened, an effect that became even stronger when α-MSH was blocked with an antibody.8Journal of Endocrinology. Evidence for the participation of a melanin-concentrating hormone in physiological colour change in the eel
This rapid system gives eels the ability to fine-tune their shade to match their immediate surroundings, separate from the slower developmental changes that happen over months and years. An eel sitting on a pale sandy bottom can lighten within the hour; one hiding in dark crevices or weedy areas can deepen its tone. It is essentially a real-time camouflage adjustment layered on top of the life-stage color scheme.
Eyes That Change with the Skin
Eel color changes do not happen in isolation from other sensory systems. As the skin darkens and shifts to oceanic countershading during silvering, the eyes are undergoing their own transformation. In the freshwater yellow phase, eel eyes are relatively small and suited to the light conditions of rivers and estuaries. During silvering, they enlarge dramatically and become better adapted to the blue-shifted, dim light of the deep ocean.
Recent work on Japanese eels has shown that the types of light-detecting proteins (opsins) expressed in the eyes shift as eels mature. Immature eels show patterns of opsin expression that respond to the mix of blue and green light found in shallow coastal waters. Mature eels, by contrast, ramp up expression of opsins suited to deep-sea conditions, particularly those tuned to blue wavelengths and broad-spectrum sensitivity. The shift essentially remodels the eye to match the new environment the silver eel is headed for, just as the new skin coloration matches the open ocean’s visual demands.
Eels That Glow Green
One of the more unexpected discoveries about eel color came in 2013, when researchers identified a green fluorescent protein in the muscle tissue of Japanese freshwater eels. Named UnaG (after “unagi,” the Japanese word for freshwater eel), it was the first fluorescent protein ever found in a vertebrate. Unlike the famous green fluorescent protein from jellyfish, UnaG does not need oxygen to glow. Instead, its fluorescence is triggered by bilirubin, a byproduct of the normal breakdown of hemoglobin.9PubMed. A bilirubin-inducible fluorescent protein from eel muscle The protein belongs to the fatty-acid-binding protein family and is found specifically in small-diameter muscle fibers.10Cell. A Novel Vertebrate Fluorescent Protein and Its Application to a Bilirubin Assay
UnaG is not limited to Japanese eels. Researchers later found closely related fluorescent proteins in false moray eels from a completely different family. These proteins, called Chlopsid FP I and II, share the same structural quirk as UnaG and produce strong green fluorescence when bilirubin is present, with excitation and emission spectra closely matching UnaG.11PLoS ONE. Adaptive Evolution of Eel Fluorescent Proteins from Fatty Acid Binding Proteins Produces Bright Fluorescence in the Marine Environment The fact that the fluorescence evolved independently in distantly related eel families suggests it may serve some biological function, though what that function might be is still debated. Some researchers have speculated about a role in signaling or camouflage, but there is no firm evidence yet. In the meantime, UnaG has found a practical use in medicine: because it binds bilirubin so tightly and specifically, it forms the basis of a highly sensitive blood test for bilirubin levels, a clinically important marker for liver function and conditions like jaundice.
Deep-Sea Eels and Unusual Colors
Not all eels follow the transparent-to-yellow-to-silver pathway. The eel order is enormous and varied, and species that spend their entire lives in the deep sea have evolved coloration suited to their pitch-dark world. The general rule in the deep ocean is that red and orange pigments are effectively invisible, because the wavelengths of light needed to see red do not penetrate to those depths. Many deep-sea creatures exploit this by being red or orange, achieving near-perfect camouflage against the black water.
A striking example comes from the saccopharyngiform eels, the bizarre gulper and swallower eels of the abyss. When researchers identified larvae of the previously mysterious family Neocyematidae, they found the larvae had orange pigmentation on the gut and head regions, lacking the black pigmentation typical of related eel larvae. Adults of the species are entirely red or orange.12PLoS ONE. Resolving deep-sea pelagic saccopharyngiform eel mysteries: Identification of Neocyema and Monognathidae leptocephali and establishment of a new fish family “Neocyematidae” based on larvae, adults and mitogenomic gene orders This is a completely different approach from the silvering strategy of freshwater eels. Where a European eel turns into a mirror to swim through dimly lit open water, a deep-sea eel turns red to vanish into darkness. Each solution matches the physics of the local light environment.
Ribbon Eels and Color-Linked Sex Change
Ribbon eels are among the most visually striking members of the eel order. Young ribbon eels are jet black with a yellow dorsal fin. As they grow, they turn brilliant electric blue with a yellow jaw and fin. Eventually, some individuals become entirely yellow. For decades, aquarists and marine biologists assumed this color sequence tracked a sex change: black juveniles were thought to be sexually immature, blue individuals were males, and fully yellow ones were females that had undergone sex reversal, since ribbon eels are protandrous hermaphrodites (they can shift from male to female).
That story is tidy and satisfying, but captive observations have complicated it. A study of ribbon eels at the Vienna Zoo documented fertilized eggs and early larvae for the first time, and noted that body color variations in captivity did not reliably reflect the eel’s sex or sexual maturity.13PubMed. First observations of fertilized eggs and preleptocephalus larvae of Rhinomuraena quaesita in the Vienna Zoo That does not necessarily mean the color-sex link observed in the wild is wrong, since captive conditions differ from reef environments in lighting, diet, social interaction, and stress. But it does suggest the relationship between color phase and reproductive status in ribbon eels is less rigid than often claimed. Environmental factors may influence the color transition independently of sex change, or the hormonal triggers may respond differently to captive conditions. The honest answer is that the biology of ribbon eel color change is still not fully worked out.
When Pollution Changes the Color Equation
Eel color can also be altered by environmental contamination, though the changes are less about surface appearance and more about what is happening beneath the skin. Research comparing European and American yellow eels from estuaries with different levels of pollution found that eels from more contaminated sites showed larger melanomacrophage centers in their organs, particularly the spleen and liver. These are clusters of pigmented immune cells that accumulate when the body is dealing with chronic damage or infection. Eels from the dirtiest sites also showed higher levels of lipofuscin, a yellowish-brown “wear and tear” pigment that accumulates in cells under oxidative stress, and hemosiderin, an iron-storage pigment that increased in eels from sites with higher metal contamination.14PLoS ONE. Biotransformation, antioxidant and histopathological biomarker responses to contaminants in European and American yellow eels from the Gironde and St. Lawrence estuaries
These internal pigment changes are invisible from the outside, but they matter. Melanomacrophage centers, lipofuscin, and hemosiderin are biomarkers that researchers use to assess how much stress an eel population is under from pollution. Higher accumulations of organic contaminants like PCBs and DDT, and heavy metals like mercury and cadmium, correlated with more pronounced pigment deposits. Some of these biomarkers also increased with age, making it tricky to separate the effects of long-term pollution exposure from normal aging, since eels can live for decades and accumulate contaminants the entire time. For a species already in serious decline across much of its range, understanding how pollution affects the health machinery behind color and immunity is part of the broader conservation puzzle.
Why Eel Color Change Matters Beyond the Eels Themselves
The fluorescent protein UnaG has already been adopted as a laboratory tool and a clinical diagnostic aid, but the broader science of eel coloration feeds into several other fields. The silvering process is one of the most complete natural examples of an animal restructuring its body for a completely different environment, and studying the hormonal controls behind it has informed research into how vertebrate puberty and metamorphosis relate to each other. The fact that testosterone, rather than thyroid hormone, drives key aspects of silvering challenged assumptions about how secondary metamorphoses work in fish and opened new questions about the evolution of hormonal regulation in vertebrates.
The rapid background-matching system controlled by α-MSH and MCH in eels is a model system for studying hormonal color change across fish and amphibians. Because eels are relatively large and hardy, they are easier to work with experimentally than many of the small tropical fish that also change shade on short timescales. And the deep-sea eels’ use of red and orange pigmentation has contributed to our broader understanding of how animals exploit the physics of light attenuation in water, a principle that applies to everything from squid to deep-sea shrimp. Eels, in other words, are not just one color story. They are a whole portfolio of color strategies packed into a single order of fish, each solution shaped by a different set of evolutionary pressures.