Taxonomy & naming
Anguilla bicolor was described by the Scottish physician and naturalist John McClelland in 1844, the type locality being Sulawesi (then known as Celebes). It belongs to the family Anguillidae — the true, catadromous freshwater eels — placed within the order Anguilliformes, the real eels of the world's oceans and rivers. This is a critical point: unlike the spiny eels (family Mastacembelidae, order Synbranchiformes) or the swamp eels (family Synbranchidae), which are eel-shaped by convergence only, Anguilla bicolor is a genuine eel, sharing its order with congers, morays, and the European and American eels that have become conservation emblems.
The species has a complex taxonomic history. For much of the twentieth century, "Anguilla bicolor" was applied broadly across the Indo-Pacific and Indian Ocean, encompassing populations from East Africa, Madagascar, and India that are now increasingly treated as Anguilla marmorata or as additional distinct taxa. Modern taxonomy, anchored in DNA barcoding of cytochrome oxidase I (COI) and other mitochondrial markers, restricts A. bicolor sensu stricto to the Indonesian and Philippine form. The African populations that were historically placed under A. bicolor are now recognised as A. marmorata, which itself is a remarkably widespread and morphologically variable species. The name bicolor — Latin for 'two-coloured' — refers to the contrasting dorsal and ventral colouration common across the genus.
Morphology
Anguilla bicolor is a typical Anguilla in body plan: greatly elongate, nearly cylindrical anteriorly and somewhat compressed toward the tail, with a continuous median fin running from well behind the pectoral fins around the tail tip. The scales are minute, deeply embedded, and set in a characteristic basket-weave pattern at an angle to those of adjacent rows — the arrangement that gives Anguilla its common descriptor 'shortfin' in this species, a reference to the pectoral fin length that is one of the traditional characters used to distinguish species in the genus before genetic tools became available.
Reported maximum length from FishBase is approximately 39.5 in total length, though most individuals encountered in the growth phase are considerably smaller — 15.5–31.5 in is the common size range. Colour in the growth phase is typically dark brownish or greenish-grey on the dorsum, shading to yellowish or pale ventrally. As in all Anguilla, the appearance changes at sexual maturity: migrating silver eels develop enlarged eyes, thickened lateral fat stores, and a silvery-metallic sheen. Distinguishing A. bicolor from the sympatric A. marmorata in the field requires attention to vertebral counts, fin-ray counts, and ultimately genetic confirmation — the two species are morphologically cryptic wherever their ranges overlap in Indonesia.
Habitat
Through its growth phase Anguilla bicolor is a freshwater and brackish-water fish. It inhabits rivers, streams, mountain torrents, lowland floodplains, coastal lagoons, estuaries and lakes across Indonesia (including Sulawesi, Java, Borneo, and the eastern archipelago) and the Philippines. Like all Anguilla it is euryhaline — tolerant of a wide salinity range — and juveniles (glass eels and elvers) are frequently encountered in brackish coastal zones before ascending into fresh water, where they may remain for a decade or more during the growth phase.
Temperature across its tropical range is broadly 73–86 °F, and the species occupies a variety of water chemistries, from the soft, slightly acidic highland streams of Sulawesi to the harder, more turbid lowland rivers. It is a benthic, nocturnal animal that shelters in bank burrows, root masses, and soft substrate during the day, emerging at night to forage. Elvers show remarkable ability to climb wet rock faces and pass barriers that defeat other fishes, allowing them to colonise upland waters.
Feeding
Anguilla bicolor is a carnivore throughout its life. Juveniles in the glass-eel and elver stages feed on small invertebrates — zooplankton, chironomid larvae, small crustaceans and worms. As the fish grows it shifts toward larger prey: insect larvae, crustaceans, small fishes, and worms form the bulk of the adult diet in freshwater. The hunting method is typically bottom-oriented, the eel nosing through substrate, leaf litter, and root masses after dark, using olfactory cues to locate prey rather than vision.
In estuarine and coastal zones, particularly during early life stages, the diet broadens to include marine invertebrates. The species is an opportunistic generalist in this respect, a trait shared across the genus and one reason that Anguilla eels tolerate a wide range of degraded or modified habitats during their growth phase, even where food availability varies seasonally.
Mating
Anguilla bicolor is catadromous and semelparous: it mates once, at sea, and dies. The entire adult life is oriented toward a single terminal reproductive migration. After spending years — commonly a decade or more — feeding and growing in fresh and brackish water, sexually mature individuals undergo the dramatic metamorphosis to the silver-eel phase: eyes enlarge, skin thickens, gut degenerates, gonads develop, and fat reserves are mobilised for the ocean crossing. The fish then ceases to feed and migrates downstream and out to sea.
The spawning grounds of A. bicolor have not been conclusively identified, unlike those of the European eel (which uses the Sargasso Sea) or the Japanese eel (the western North Pacific, near the Mariana Ridge). For the Indo-Pacific Anguilla species, the spawning area is presumed to lie in the open Indian or Pacific Ocean, but direct observation of spawning behaviour and the capture of leptocephalus larvae traceable to A. bicolor remain largely elusive, making this one of the outstanding unresolved questions in tropical eel biology.
Breeding
Anguilla bicolor cannot be bred in captivity — this is not a practical limitation but a biological one. The reproductive migration to deep ocean water is triggered by complex physiological cues that have not been replicated in aquaculture conditions for any Anguilla species, despite decades of research on the economically important European and Japanese eels. Captive females can be induced to produce eggs by hormonal manipulation, and experimental fertilisation is possible, but sustaining leptocephalus larvae through their months-long drift back to coastal nurseries has not been achieved for any eel in the genus.
Aquaculture of anguillid eels — where it exists at all, primarily for A. japonica and A. anguilla — depends entirely on catching wild glass eels at river mouths and then growing them in ponds or tanks to marketable size. This practice is under regulatory pressure throughout the range of the better-studied species, but for A. bicolor, where even the basic biology is poorly documented, no commercial eel farming industry has developed. Captive specimens, occasionally kept in public aquaria as demonstration animals, are wild-caught individuals. The species offers nothing for the breeding hobbyist and is not part of the ornamental trade in any meaningful way.
In the aquarium
Anguilla bicolor is not an aquarium fish. It appears in the ornamental trade only rarely, as an incidental catch or local curiosity, and is not suited to conventional hobby keeping. As a true eel it grows to 31.5–39.5 in, is a powerful, scaleless, scaleless animal sensitive to copper-based medications and to dissolved pollutants, and requires a very large, mature system if it is to be kept at all. Like all Anguilla it is a confirmed escape artist — it can squeeze through extraordinarily small gaps, climb wet glass and silicone, and will travel over dry surfaces for short distances — so any housing must have a fully sealed, gapless lid.
Beyond the practical difficulties, Anguilla bicolor is a wild fish of significant ecological and conservation concern, and in many parts of its range its collection and export are restricted or unregulated in ways that make its trade ethically fraught. It is nocturnal, burrows readily in a sand substrate, and would need large, meaty foods such as whole prawns and fish chunks fed after dark. It would not be compatible with smaller tankmates. The honest summary is that this is not a hobbyist fish: it belongs in its river.
Conservation
Anguilla bicolor is assessed as Near Threatened (NT) on the IUCN Red List. Much remains poorly documented — population trends, spawning biology, and the precise limits of the species' range — and the long-running taxonomic confusion with A. marmorata means that historical catch data are unreliable indicators of A. bicolor abundance in the strict sense; but recruitment declines and heavy glass-eel fisheries across its range place it close to a threatened listing.
The broader Anguilla genus is a conservation crisis in slow motion. The European eel (A. anguilla) is Critically Endangered; the American eel (A. rostrata) is Endangered; the Japanese eel (A. japonica) is Endangered. The pressures are shared: hydroelectric dams blocking upstream migration, glass-eel fisheries removing recruits before they can grow, habitat degradation across the freshwater growth range, and in some regions the introduced swimbladder parasite Anguillicola crassus. For A. bicolor specifically, the fragmentary distribution record, the cryptic overlap with other species, and the lack of population monitoring mean that local depletion — whether from fishing, habitat loss, or dam construction across Indonesian and Philippine river systems — would be very difficult to detect before it was severe. A Near Threatened listing is a warning, not a reassurance, and the trajectory of the better-studied Anguilla species argues for precaution.