Eel

An eel is any ray-finned fish belonging to the order Anguilliformes, characterized by an elongated, snake-like body, a scaleless or minimally scaled skin, and the absence of pelvic fins. The order comprises roughly 800 known species distributed across about 19 families, ranging from the familiar freshwater eels of the genus Anguilla to deep-sea specialists such as gulper eels, snipe eels, and morays. Eels occupy a significant place in human history as a source of food, a subject of scientific fascination, and a recurring motif in folklore, yet many aspects of their biology—particularly their reproduction—remained mysterious for millennia. In recent decades, dramatic declines in several commercially important species have made the eel a prominent focus of international conservation efforts.

Taxonomy and Classification

The order Anguilliformes is a well-defined group within the infraclass Teleostei. Its members are subdivided into suborders including Anguilloidei, Muraenoidei (moray eels), Congroidei (conger eels), Nemichthyoidei (snipe eels), and Saccopharyngoidei (gulper and pelican eels). The true freshwater eels belong to the family Anguillidae, which contains a single genus, Anguilla, comprising approximately 15 to 20 species distributed across temperate and tropical regions of the world, with the notable exceptions of the South Atlantic and the eastern Pacific.

Among the best-studied species are the European eel (Anguilla anguilla), the American eel (Anguilla rostrata), and the Japanese eel (Anguilla japonica). These species bear a striking superficial resemblance to one another, which long complicated their identification and fueled scientific debate about whether they represented distinct populations of a single species. Modern genetic analysis confirmed their status as separate species, though hybridization between American and European eels has been documented.

It is important to note that several animals commonly called "eels" are not members of Anguilliformes at all. The electric eel (Electrophorus electricus) of South America is a neotropical knifefish more closely related to catfish, while the swamp eels (Synbranchidae) and the "wolf eel" of the North Pacific belong to entirely separate lineages.

Anatomy and Morphology

Eels possess one of the most distinctive body plans among fishes. The body is extremely elongated and laterally compressed toward the tail, giving it a serpentine appearance. The dorsal, caudal, and anal fins are typically fused into a single continuous fin ribbon that runs around the posterior half of the body. Pectoral fins may be present or absent depending on the species, and pelvic fins are universally absent—a defining characteristic of the order.

The skeleton is highly flexible, with numerous vertebrae that may exceed 100 in some species, permitting the sinuous swimming motion characteristic of the group. Eels lack the ventral swim bladder connection to the gut found in many fishes, and in many species the swim bladder is modified for buoyancy regulation during extensive vertical migrations. The gill openings are reduced to small slits, and the skin, often covered by a thick layer of mucus, contains minute, deeply embedded scales in most freshwater species. These traits allow eels to burrow into sediment, wriggle through crevices, and even travel short distances over damp ground, which explains their ability to colonize isolated bodies of fresh water.

Life Cycle

The life cycle of freshwater eels is among the most complex and remarkable in the animal kingdom. Eels are catadromous, meaning they spend the majority of their lives in fresh water but migrate to the ocean to spawn. The canonical example is the European eel, whose reproductive grounds were located in the Sargasso Sea, a region of the North Atlantic Ocean.

The life history proceeds through a series of distinct stages. Spawning takes place in deep oceanic water, after which the eggs hatch into flattened, transparent, leaf-shaped larvae called leptocephali. These larvae drift with ocean currents for months to years—for European eels, a transatlantic journey of up to two to three years—gradually moving toward continental shelves. Upon reaching coastal waters, the leptocephalus metamorphoses into a miniature, transparent "glass eel." As glass eels enter estuaries and rivers, they acquire pigmentation and are known as elvers. The juvenile eel then grows in fresh water or coastal habitats as a yellow eel, a phase lasting anywhere from several years to decades, before undergoing a final transformation into the sexually mature silver eel. The silver eel's body darkens, its eyes enlarge, and its digestive tract degenerates in preparation for the long spawning migration, after which the adult is presumed to die.

The process by which eels reproduce remained a scientific enigma for centuries. Aristotle proposed that eels arose spontaneously from mud; Sigmund Freud, early in his career, dissected hundreds of eels in search of reproductive organs without success. The Danish biologist Johannes Schmidt, working over a period of nearly two decades, published in the 1920s his landmark finding that the smallest leptocephali of the European eel were collected in the Sargasso Sea, providing the first strong evidence of the species' spawning location. Direct observation of spawning in the wild was not achieved until 2014, when researchers documented Japanese eels spawning in the Pacific.

Distribution and Habitat

Freshwater eels of the genus Anguilla are found on every continent except Antarctica and in most major river systems of the temperate and tropical world. Marine eels are even more broadly distributed, occupying coral reefs, rocky coastlines, abyssal plains, and hydrothermal vent communities. Moray eels, the most diverse group in terms of species richness, are predominantly tropical reef dwellers that shelter in crevices and ambush passing prey.

Eels are highly tolerant of a wide range of environmental conditions, including substantial salinity and temperature variation, and can survive periods of low oxygen by gulping air. This adaptability, combined with their migratory capacity, has enabled them to inhabit waters from mountain streams to ocean trenches exceeding several thousand meters in depth.

History of Human Interaction

Eels have been harvested by humans since prehistoric times. Archaeological evidence indicates that eel fishing was practiced in Neolithic Europe, and eels formed a staple of the medieval European diet. In England, eels were so economically important that some monastic leases and tithes were paid in eels; the Domesday Book of 1086 records eel rents in considerable quantities across eastern England. The Dutch and the Maori of New Zealand developed sophisticated traditional techniques for catching migrating eels, including fixed weirs and elaborate traps.

Culinary traditions involving eels are numerous and culturally significant. In Japan, freshwater eel, particularly Anguilla japonica, has been consumed since at least the Edo period; the dish kabayaki—grilled eel glazed with a sweet soy-based sauce, served over rice as unadon or unaju—remains an iconic element of Japanese cuisine. In Europe, smoked eel is a delicacy in the Netherlands, Scandinavia, and Germany, while jellied eels were a traditional feature of working-class London cuisine. Italy boasts a long tradition of eel fishing in the Comacchio lagoons, and Spain celebrates angulas, juvenile glass eels served as an expensive delicacy.

Scientific interest in eels has also shaped intellectual history. The failure of naturalists, including Freud in 1876, to locate eel reproductive organs contributed to ongoing debates about spontaneous generation, and the eventual solution of the eel life cycle mystery is regarded as one of the classic achievements of twentieth-century marine biology.

Economic and Ecological Significance

Eels constitute a commercially valuable fishery worldwide. Historically, they supported substantial fishing industries in Europe, East Asia, and North America, with catches measured in the tens of thousands of tonnes annually. Because eels do not reproduce reliably in captivity on a commercial scale—breeding in aquaculture remains largely experimental—the industry depends entirely on wild-caught juvenile glass eels for stocking of eel farms. This dependence has created a lucrative and often illicit international trade in glass eels, with juveniles harvested in Europe and East Asia and smuggled to farms in East Asia, where demand is greatest.

Ecologically, eels function as both predators and prey across multiple trophic levels and habitats. As voracious carnivores, they consume invertebrates, fish, and amphibians, helping to regulate populations in both freshwater and marine ecosystems. Their migrations transfer nutrients and energy between oceanic and continental environments, and they serve as prey for larger fish, birds, and mammals.

Conservation Status and Threats

Populations of several eel species have undergone severe declines since the late twentieth century. Recruitment of European eel glass eels to continental waters fell by an estimated 90 to 95 percent from 1970s levels, and the species is currently classified as Critically Endangered by the International Union for Conservation of Nature (IUCN). The Japanese eel is listed as Endangered, and the American eel is considered of significant conservation concern.

The causes of decline are complex and include overfishing of glass eels and adult eels, habitat loss and fragmentation caused by dams and other barriers to migration, pollution, parasitic infections such as the invasive swim bladder nematode Anguillicola crassus, climate-driven changes in ocean circulation affecting larval transport, and hydroelectric turbine mortality during downstream migration. The European Union adopted the European Eel Recovery Plan in 2007, mandating member states to reduce fishing pressure, restore migration routes, and stock suitable habitats. The Convention on International Trade in Endangered Species (CITES) has listed the European eel in Appendix II, requiring that international trade be regulated and documented. Enforcement remains challenging, however, due to the scale of illegal glass eel trafficking, which has been described as one of the largest wildlife smuggling operations in the world by volume.

Cultural Significance

Beyond their economic role, eels have occupied a distinctive place in human culture and mythology. The elusive nature of their reproduction gave rise to enduring legends in European folklore, including tales of eels springing from horsehair dropped into water. In Polynesian and Maori tradition, eels appear as ancestral figures and supernatural beings, and eel weirs featured prominently in pre-colonial resource management. In Japanese culture, eating eel on the midsummer day known as Doyō no Ushi no Hi is a custom believed to restore vigor during hot weather. The eel's serpentine form and mysterious life history have also made it a recurring subject in literature, art, and popular science writing, symbolizing secrecy, transformation, and the persistence of natural enigmas.

See also

• Fish migration
• Sargasso Sea
• Catadromy
• Anguillidae