Taxonomy & naming
Anguilla megastoma was described by the German naturalist Johann Jakob Kaup in his 1856 'Catalogue of apodal fish in the collection of the British Museum', published in London. The species name megastoma is derived from the Greek megas (large) and stoma (mouth), referring to the notably wide gape of this species relative to other members of the genus. The World Register of Marine Species (WoRMS) and the Catalog of Fishes (Eschmeyer, the governing authority for valid names) both recognise Anguilla megastoma Kaup, 1856 as the valid binomial, and the name has remained stable — there are no widely-used synonyms or problematic old combinations for this species.
The genus Anguilla belongs to the family Anguillidae, order Anguilliformes, the true eels. This is a critical distinction: Anguilla species are genuine eels with a fully eel-like, scale-bearing body plan and a life history tied to deep ocean spawning. They are not related to the spiny eels (Mastacembelidae, order Synbranchiformes) or the swamp eels (Synbranchidae), whose superficially similar elongated forms evolved convergently for burrowing in fresh water. Anguilla megastoma is one of the more poorly studied members of a genus that includes well-researched temperate species such as A. anguilla (Europe) and A. japonica (East Asia), meaning some aspects of its biology remain incompletely documented.
Morphology
Anguilla megastoma is a large, elongate eel with the typical Anguillidae body plan: a cylindrical anterior trunk that becomes laterally compressed toward the tail, a continuous median fin running from behind the pectoral fins around the tail, and small embedded scales that give the skin a smooth, almost scaleless appearance to the touch. The head is large relative to body depth, with a wide gape (the character that prompted the species name) and a lower jaw that slightly protrudes. The dorsal fin begins well behind the pectoral fins, which is characteristic of the longfin Anguilla species group.
Sex and size dimorphism is pronounced: FishBase records males reaching approximately 39.5 in total length, while females can reach 65 in standard length and a weight of around 9 kg. Females are therefore substantially larger than males, as is typical across the genus. Coloration is generally brownish to olive-green on the dorsal surface, becoming paler or yellowish on the flanks and belly; the skin has a characteristic iridescent sheen. Fin membranes are typically darker than the body. Age can be considerable — large female eels of this size in temperate congeners may be 20–30 years old, and similar longevity is likely in A. megastoma, though the data are sparse.
Habitat
Anguilla megastoma is distributed across tropical Pacific island systems from Sulawesi and eastern Indonesia east through Micronesia, Melanesia and Polynesia to the Society Islands and Pitcairn, roughly between 1°S and 25°S latitude. FishBase records it across this broad Pacific range as a catadromous species that uses freshwater rivers, streams and lakes during its continental growth phase, moving into estuaries and brackish-water zones as it matures toward the downstream silver-eel migration.
On oceanic islands, where rivers are often short and fast, the eel must adapt to high-gradient streams running from volcanic interiors to the coast. In the Society Islands, glass eels (leptocephalus larvae that have metamorphosed and begun entering fresh water) have been documented arriving with a mean pelagic larval duration of 118 ± 11 days (range 96–150 days), reflecting the length of the oceanic drift. Temperature data for the species' freshwater habitat are recorded by FishBase as 39–49 °F (mean 41 °F), though it should be noted that such figures may reflect the upper-elevation stream habitats of some surveyed islands rather than the species' full thermal tolerance — Pacific island rivers can be substantially warmer at low elevation. The eel tolerates both fresh and brackish conditions during its growth phase.
Feeding
Anguilla megastoma is a generalist nocturnal predator during its freshwater growth phase. Like other large Anguilla species, it likely preys upon fish, invertebrates, crustaceans, and whatever benthic and littoral prey is available in the rivers, lakes and estuaries it inhabits. Detailed dietary studies specific to this species are sparse in the published literature; the broad trophic picture is inferred from congeners and from the general Anguillidae feeding strategy.
As with other true eels, A. megastoma presumably stops feeding during the oceanic spawning migration, when physiological resources are redirected toward gonadal maturation. The energy reserves accumulated over years or decades in fresh water fuel the entire spawning run — the fish eats nothing after leaving fresh water and dies after a single spawn at sea.
Mating
All members of the genus Anguilla are semelparous, reproducing once in their lifetime. Anguilla megastoma does not pair-bond, nest, or exhibit any form of mate choice in the conventional sense: sexually mature adults ('silver eels') converge on deep-ocean spawning grounds after an extended downstream and oceanic migration, release eggs and sperm into open water, and die. The exact location of the A. megastoma spawning aggregation in the Pacific has not been confirmed, in contrast to A. anguilla (Sargasso Sea) and A. japonica (near the West Mariana Ridge), which have been traced by tagging and larval studies.
There is no courtship, no territorial behaviour, and no pair formation. The spawning grounds are presumed to lie in deep, warm oceanic water to the west or north of the species' Pacific island range, but direct evidence is lacking. This remains one of the most significant gaps in the species' biology.
Breeding
Anguilla megastoma cannot be bred in captivity. This is not a practical limitation but a biological absolute: like all Anguilla species, it is catadromous and semelparous. After years or decades of growth in fresh and brackish water, the maturing eel undergoes dramatic physiological changes — the gut degenerates, the eyes enlarge, the flanks become silvery — and it migrates downstream and out to sea on a one-way journey. It spawns once in deep oceanic water and dies; no mature adult returns to fresh water. The leptocephalus (leaf-shaped) larvae that hatch from the eggs drift on oceanic currents, sometimes for months, before metamorphosing into glass eels and entering freshwater systems. Glass eels on the Society Islands have been recorded with a pelagic duration of approximately 96–150 days before river entry.
Commercial eel 'farming' for this and other Anguilla species depends entirely on catching wild glass eels and raising them in ponds — there is no captive breeding programme for any Anguilla species. Any hobbyist-trade animal labelled as this species would be a wild-caught juvenile or subadult.
In the aquarium
Anguilla megastoma is not a mainstream aquarium fish and is not suited to the typical home set-up. It grows to well over a metre in the case of females (records to 65 in), is a powerful predator, and has a life history rooted in oceanic migration — it is fundamentally a wild species of Pacific river systems rather than a domesticated aquarium animal. In countries across its range it is a culturally and commercially important food fish harvested by indigenous communities, and local regulations may restrict collection or sale.
Should any specimen be encountered in the trade, the considerations common to all large true eels apply: a very large aquarium (an adult female would require several metres of length), a sealed lid — true eels are escape artists capable of climbing vertical surfaces and squeezing through small gaps — a soft sand substrate, low lighting, and cave-like retreats. The species is sensitive to copper-based medications. It would likely be incompatible with any fish small enough to swallow. The honest assessment, however, is that this is not an aquarium fish: it belongs in the rivers and lakes of Pacific islands, and hobbyist interest should stop at observation rather than acquisition.
Conservation
Anguilla megastoma is assessed as Data Deficient (DD) on the IUCN Red List, assessed in November 2018. The DD listing reflects a genuine shortage of population-level data rather than security: the species is assessed as having a very high vulnerability to fishing pressure (scored 90/100 on FishBase's vulnerability index) and a minimum population doubling time exceeding 14 years — biological parameters that resemble those of the threatened temperate congeners. The combination of slow maturation, a semelparous life history, and dependence on both freshwater growth habitat and open-ocean spawning conditions makes any catadromous Anguilla inherently vulnerable to overharvest, habitat degradation, and migratory barriers.
Across the Pacific, A. megastoma is harvested as a food source and has cultural significance in many island communities. On small, geographically isolated islands, localized overfishing or stream degradation could have disproportionate effects on recruitment because there is no source-sink connectivity with large continental river systems. The glass-eel colonization data from the Society Islands suggest some connectivity across the Pacific, but the degree to which island subpopulations are self-seeding versus maintained by oceanic larval supply is unknown. Until adequate population surveys are conducted, its true conservation status cannot be determined — a gap that urgent given the broader crisis facing the Anguillidae worldwide.