Taxonomy & naming
Anguilla dieffenbachii was described by the British zoologist John Edward Gray in 1842, with the species named in honour of Ernst Dieffenbach, the German naturalist who collected specimens during his survey of New Zealand in the late 1830s. The Catalog of Fishes (Eschmeyer, CAS — the governing authority for valid names) places it in the genus Anguilla Schrank, 1798, the type genus of the family Anguillidae, within the order Anguilliformes. These are the true eels — a fundamentally different lineage from the spiny eels (Mastacembelidae, order Synbranchiformes) and the swamp eels (Synbranchidae, also Synbranchiformes) that share the freshwater aquarium trade under the name 'eel'. The confusion is one of trade names, not biology.
New Zealand has two Anguilla species: A. dieffenbachii (longfin eel) and A. australis (shortfin eel). The names refer to the relative length of the dorsal fin compared to the anal fin — in the longfin, the dorsal fin begins well forward of the anal fin origin, whereas in the shortfin the two fins originate at roughly the same point. The longfin is the larger, longer-lived, and more endemic of the two; A. australis also occurs in south-eastern Australia. The genus Anguilla encompasses catadromous species across the North Atlantic (A. anguilla), eastern North America (A. rostrata), East Asia (A. japonica), Africa and the Indo-Pacific (A. marmorata, A. bengalensis, A. mossambica, and others), Australia and the Pacific, placing A. dieffenbachii within a globally distributed but taxonomically cohesive group.
Morphology
The longfin eel is serpentine and muscular, with a body that tapers gradually toward the tail and is covered in tiny cycloid scales embedded so deeply in the skin that the fish appears scaleless. The skin is thick and rich in mucus. The lower jaw projects beyond the upper, reflecting its role as an active predator, and the pectoral fins are small and rounded. The defining character shared by all Anguilla is the elongate dorsal fin that begins well ahead of the anal fin — in A. dieffenbachii this forward-placed dorsal origin is particularly pronounced.
Colouration in the growth phase is dark dorsally — olive-brown to nearly black — fading to a yellowish or creamy white ventrally. When sexually mature fish enter the so-called silver-eel phase in preparation for their spawning migration, the flanks turn silver, the eyes enlarge noticeably (a functional adaptation for deep-ocean life), and the digestive system degenerates as the body prepares to fuel a migration on stored fat alone. Females grow substantially larger than males: females in New Zealand rivers may exceed 5 ft in total length and weigh several kilograms, whereas males rarely exceed about 19.5 in.
Habitat
Anguilla dieffenbachii is endemic to New Zealand, occurring in rivers, streams, lakes, estuaries, and wetlands of both the North and South Islands, with additional records from the Chatham Islands and some offshore islands. It occupies a wider altitudinal range than the shortfin eel, penetrating headwater streams and high-country lakes that are beyond the reach of A. australis, and it tolerates cool, fast-flowing upland torrents as well as slow lowland rivers and tidal estuaries.
During the continental growth phase the longfin eel is an inhabitant of both freshwater and brackish environments — it moves freely through estuaries and can tolerate a wide range of salinity, temperature, and water chemistry. New Zealand rivers are generally cool, with temperatures in the lowlands spanning roughly 46–68 °F across the seasons, and upland habitats substantially colder. The species is broadly tolerant of these conditions and has been recorded over a wide range of pH. By day it shelters under rocks, logs, bankside debris, and in burrows; at night it leaves cover to hunt.
Feeding
Longfin eels are opportunistic carnivores over a wide size range of prey. Small juveniles feed on aquatic invertebrates — insect larvae, amphipods, worms, and small crustaceans — while larger individuals progressively incorporate fish, freshwater crayfish (kōura, Paranephrops spp.), frogs, and occasionally small birds or mammals that fall into the water. The species has a well-documented reputation among New Zealand anglers and observers as an able and bold predator.
Foraging is largely nocturnal; during daylight hours the eel rests sheltered under cover and is rarely seen. Its olfactory system is highly developed, as in all Anguilla, and plays a central role in locating prey in the dark and in turbid water. The long gut is capable of handling a very wide range of prey, and the eel grows steadily through decades of feeding before body condition is sacrificed in the terminal spawning migration.
Mating
Longfin eels are semelparous — they reproduce once in a lifetime and die. Sexual maturation is triggered after an extended freshwater growth phase that may last 20 to 60 years or more (with exceptional females exceeding 80 years recorded in some accounts), after which a fundamental physiological transformation takes place: the so-called silver-eel change, in which the eyes enlarge, the skin thickens and silvers, the gonads develop, and the digestive tract regresses. In this transformed state the eel stops feeding and begins the downstream migration to the sea.
Females grow far larger and live far longer than males before maturing, which is why the largest longfin eels in any river system are invariably female. The precise spawning destination of A. dieffenbachii is not confirmed with certainty, but evidence points to a deep-water site somewhere in the tropical Pacific Ocean — a pattern consistent with other Southern Hemisphere Anguilla that spawn in warm, deep offshore waters far from their growth habitat. Spawning aggregations have never been observed in the wild for any Anguilla species.
Breeding
Anguilla dieffenbachii cannot be bred in captivity under any circumstances, and no managed breeding programme exists. The spawning migration spans thousands of kilometres of open ocean, the spawning depth is likely abyssal, and the physiological cues that trigger and sustain the migration are not replicable in an aquarium or farm setting. This is not a gap in technique — it is a fundamental biological constraint shared by all Anguilla. European eel (A. anguilla) hatcheries have spent decades attempting captive breeding with only the most limited success under experimental conditions using hormone injections, and the effort has not been commercially viable.
All eel 'farming' — and there is a substantial aquaculture industry in New Zealand and East Asia based on Anguilla — depends entirely on the capture of wild glass eels (newly arrived leptocephalus larvae that have metamorphosed at the coast and begun migrating upstream) which are then grown to market size in captivity. The leptocephalus larvae of A. dieffenbachii — leaf-shaped, transparent, and carried by ocean currents back toward New Zealand from the Pacific spawning ground — settle in coastal waters and ascend rivers as elvers, beginning the growth phase anew. Given the species' protected status in New Zealand, taking glass eels for aquaculture or the aquarium trade is heavily regulated.
In the aquarium
The New Zealand longfin eel is not an aquarium species in any practical sense. Adult fish are enormous — females regularly exceed one metre — long-lived to a degree that defies typical aquarium commitments, legally protected in New Zealand, and dependent on a complex catadromous life cycle that cannot be sustained in captivity. It is mentioned here to be clear: this is not a fish to acquire, and it is not available through legitimate aquarium trade channels.
For hobbyists drawn to the Anguilla genus by genuine interest, the relevant practical notes are that all true eels are scaleless and highly sensitive to copper-based medications and poor water quality; they are powerful, nocturnal, escape-prone animals that require fully sealed tanks with no gaps; and most are sizeable fish requiring large, mature, well-filtered systems with plenty of cover. Those characteristics apply to any Anguilla species if one were to be kept, but the longfin eel specifically should be known as a wild animal of conservation concern, not a trade commodity.
Conservation
Anguilla dieffenbachii is assessed as Endangered (EN) on the IUCN Red List, and it faces serious conservation pressures that have caused significant population declines since European settlement of New Zealand. Dams and water diversions block the upstream migration of juvenile eels and the downstream migration of spawning adults; drainage of wetlands and modification of lowland rivers have reduced available habitat; and decades of commercial and recreational harvesting — eel fishing was a major industry in New Zealand through the twentieth century — removed very large numbers of the slow-maturing adults that sustain the population. Because females do not mature until they are very large and very old, removing the largest individuals disproportionately impacts reproductive output.
The species is now protected under New Zealand's Wildlife Act, and commercial freshwater eel fishing is managed under quota. Passage at dams remains an ongoing challenge, and programmes to trap and truck eels around major barriers exist at some sites. The IUCN assessment status for A. dieffenbachii was not confirmed in the sources available for this article; the species is the subject of ongoing New Zealand government monitoring. Among Maori, the longfin eel (tuna) holds deep cultural and traditional significance — it features in oral traditions and was a staple food obtained through elaborate weir and trap fisheries — and iwi (tribal groups) are active participants in contemporary management decisions.