Taxonomy & naming
Anguilla japonica was formally described in 1846 by the Dutch naturalists Coenraad Jacob Temminck and Hermann Schlegel, working from specimens collected in Japan during the Siebold expedition. The name is straightforwardly geographical — japonica, of Japan — and the species has remained stable in the genus Anguilla ever since. The Catalog of Fishes (Eschmeyer, the governing authority for valid names) places it in the family Anguillidae, the true eels, within the order Anguilliformes.
This taxonomic placement is the essential fact about the fish: Anguilla japonica is a true eel in every meaningful sense, not a spiny eel (Mastacembelidae, order Synbranchiformes) nor a swamp eel (Synbranchidae). It shares its order with the moray eels, the conger eels, and the garden eels — all marine — but the freshwater eels of the genus Anguilla are the only anguilliform group that routinely colonise continental river systems. The genus Anguilla contains roughly 19 species distributed across the temperate and tropical zones; A. japonica is the most commercially important member in East Asia, occupying the ecological and cultural niche that A. anguilla (European eel) fills in Europe.
Phylogenetic studies place the Pacific Anguilla species as a distinct lineage within the genus, with A. japonica and A. marmorata as the two species most likely to be encountered in the western Pacific. The African and Indo-Pacific species — A. marmorata, A. bengalensis, A. mossambica — are morphologically cryptic and reliably distinguished only by DNA barcoding.
Morphology
The Japanese eel is a large, elongate, serpentine fish reaching up to about 47 in total length, though specimens of 23.5–39.5 in are most commonly encountered in wild catches and aquaculture operations. The body is cylindrical anteriorly and increasingly compressed toward the tail, covered in tiny, deeply embedded cycloid scales arranged in a characteristic basketwork pattern that is invisible to casual inspection — the skin appears smooth and is coated in heavy mucus. The pectoral fins are small and rounded; the dorsal, caudal and anal fins are fused into a single continuous fin fringe around the rear of the body.
Colouration in the freshwater growth phase (the "yellow eel" stage) is olive to brownish-yellow on the dorsum, grading to yellowish or cream on the belly. As adults undergo the pre-spawning transformation to the migratory "silver eel" phase, the belly whitens, the flanks become silvery, the pectoral fins enlarge, the eyes increase markedly in size (an adaptation for deep-sea navigation), and the digestive system degenerates — the fish stops feeding. Females grow larger than males; adult females typically range 19.5–39.5 in, while males are smaller, usually under 23.5 in. The species closely resembles the European eel (A. anguilla) but differs in the number of vertebrae and in minor pigmentation details.
Habitat
Anguilla japonica is catadromous: it spends its growth phase in fresh and brackish continental waters and migrates to the open sea to reproduce. During the freshwater phase — which may last anywhere from a few years to more than two decades — it occupies a wide range of habitats including rivers, streams, lakes, ponds, coastal lagoons, estuaries, and rice paddies. It is particularly associated with cool to warm temperate rivers with clean, oxygenated water, and it shows a preference for well-vegetated, substrated, or rocky reaches where it can shelter by day.
The species is broadly tolerant of salinity gradients and readily moves through brackish estuarine water, but it is fundamentally a freshwater-growth animal. It is most abundant in Japan, Korea, Taiwan and eastern China, and extends to the Philippines. Water temperatures during the growth phase range from roughly 55–82 °F across its natural range, with the warmer parts of the range (subtropical Taiwan and the Philippines) supporting faster growth. Adults do not feed during the spawning migration and are not associated with any particular freshwater habitat at that stage.
Feeding
Anguilla japonica is a nocturnal opportunistic predator and scavenger during its freshwater growth phase. Juveniles and sub-adults feed primarily on benthic invertebrates — insect larvae, crustaceans, worms, and molluscs — worked out of the substrate and from beneath stones and root mats at night. Larger fish take small fishes and carrion in addition to invertebrates. Feeding is accomplished using a short but powerful jaws and a robust dentition; the eel does not stab prey but grips and swallows it, sometimes rotating the body to tear pieces from larger items.
In aquaculture, Japanese eels are fed high-protein pelleted diets and grow rapidly under warm-water conditions. Under natural conditions growth is seasonal and tied closely to water temperature: feeding and growth slow or stop entirely in winter in the cooler parts of the range. The pre-migratory silver eel phase is characterised by cessation of feeding and progressive physiological transformation; by the time adults enter the sea, the gut has atrophied and digestion is impossible.
Mating
Like all members of the genus Anguilla, the Japanese eel is semelparous — it reproduces once, at the end of its life, and dies. Mating takes place in the open ocean at spawning grounds estimated to lie in the subtropical western North Pacific, broadly in the region of the Mariana Islands, at depths likely exceeding several hundred metres. Adults reach the spawning grounds after a migration of at least 1,311 mi from the nearest river systems, navigating by geomagnetic, chemical, and oceanographic cues across open water without feeding.
The mechanics of spawning in wild Anguilla are poorly known for any species; no spawning aggregation has been observed directly in the field for A. japonica. What is known comes from the capture of ripe adults and newly-fertilised eggs, and from the distribution of the earliest-stage leptocephalus larvae. Spawning appears to coincide with a narrow seasonal window and with specific ocean temperature and current conditions. Both sexes die after spawning.
Breeding
The Japanese eel cannot be bred in the conventional aquarium sense, and this is the central fact governing its relationship with human commerce. After decades of intensive research by Japanese government laboratories and universities, captive spawning has been achieved experimentally using hormone injections, but the resulting larvae have proved exceptionally difficult to rear: leptocephalus larvae require specialised diets (in the wild they likely feed on marine snow and particulate organic matter in deep water) that have only recently begun to be replicated in the laboratory, and survival rates remain extremely low. No commercially viable captive-breeding programme exists for any Anguilla species, including A. japonica.
The eel-farming industry in Japan, China, Taiwan and South Korea is therefore entirely dependent on catching wild glass eels — newly metamorphosed juvenile eels, roughly 2–3 in long, that arrive in river estuaries after drifting for one to two years as leptocephalus larvae from the oceanic spawning grounds. These glass eels are transferred to warm, artificially fed grow-out tanks where they mature over one to three years. The global aquaculture output of Japanese eel (hundreds of thousands of tonnes per year at peak) has never reduced pressure on wild glass-eel stocks; if anything, the industry's growth amplified it. Anyone keeping a Japanese eel is almost certainly keeping a fish that was caught wild as a juvenile.
In the aquarium
Anguilla japonica is not a mainstream aquarium fish, and for good reason: it reaches a metre in length, is a powerful nocturnal predator, and will eat any fish or invertebrate it can catch. It is kept occasionally by specialist hobbyists and more often encountered in Asian food markets and aquaculture facilities than in ornamental tanks. Those who do keep it report a hardy, long-lived animal that tolerates a range of water conditions and is surprisingly responsive to its keeper once settled, but the size and feeding requirements make it unsuitable for any ordinary community aquarium.
Like all true eels, Anguilla japonica is a determined and capable escape artist. The skin is heavily mucous and the body extraordinarily muscular; these fish will find any gap in a lid, force open loosely-weighted covers, and exit tanks through filter inlets. A perfectly sealed lid is essential. They are also sensitive to copper-based medications (a property shared with all scaleless or thin-scaled fishes), so any chemical treatment in the tank must be chosen carefully. In Japan and some other parts of its native range, the Japanese eel is a protected or regulated species and its possession may require permits; prospective keepers should confirm the legal status in their jurisdiction before acquiring one.
Conservation
Anguilla japonica is assessed as Endangered (EN) on the IUCN Red List, reflecting one of the most severe population collapses documented for any commercially fished species. Glass-eel recruitment into Japanese rivers has declined by approximately 99 percent since the 1960s — a figure that encompasses wild-caught and farming-destined fish together and points to a systemic failure of the species' reproductive pipeline rather than a localised depletion event.
The causes are multiple and interacting. Overfishing of glass eels at estuaries, driven by the enormous demand from the aquaculture industry, has removed a disproportionate fraction of each year class before those fish can grow and eventually reproduce. Dam construction across East Asian river systems has blocked adult migration routes and cut off spawning-migration access to upstream habitat. Pollution and the loss of estuarine nursery habitat have reduced juvenile survival. The parasitic nematode Anguillicola crassus, a swim-bladder parasite native to A. japonica and introduced into European eel populations where it is now a major conservation threat, causes direct physical impairment of the migratory fish.
The species is not listed on CITES appendices, but trade in glass eels is restricted or banned in Japan and heavily regulated in other range states. International glass-eel smuggling — eels moving from European or American sources to Asian aquaculture markets — has become a significant wildlife-trafficking issue linked indirectly to A. japonica demand. Recovery will require simultaneous reductions in fishing pressure, restoration of river connectivity through fish-pass infrastructure, and protection of estuarine nursery habitat; the experimental captive-breeding research underway in Japan offers a long-term alternative to wild seed, but has not yet reached operational scale.