Taxonomy & naming
Alestes macrophthalmus was described by the British ichthyologist Albert Günther in 1867, and the name has been treated as valid ever since. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & van der Laan) records the valid combination as Alestes macrophthalmus Günther, 1867; the species remains in the genus Alestes, so no parenthetical authority is used.
The genus Alestes belongs to the family Alestidae, the African tetras, a grouping of characiform fishes endemic to sub-Saharan Africa. Alestidae encompasses a range of ecologies — from small schooling ornamentals such as the Congo tetra (Phenacogrammus interruptus) to large, wide-ranging piscivores like A. macrophthalmus itself. Within the genus, A. macrophthalmus is distinguished from its relatives principally by its very large, prominent adipose eyelid and by its relatively high branched anal-fin ray count of 18 to 20.
Morphology
Alestes macrophthalmus is one of the larger members of its family, reportedly reaching 23.5 in in standard length according to FishBase, though most adults encountered in fisheries catch studies are considerably smaller. The body is elongate and streamlined — torpedo-shaped, as the French common name "Torpedo Robber" reflects — with the lateral compression typical of characins.
The species' most immediately recognisable feature is the greatly developed adipose eyelid, a transparent or semitransparent fatty membrane that covers a large portion of the eye. This structure, which gives the fish both its scientific name (macrophthalmus, from the Greek for "large-eyed") and its English name Bigeye tetra, is well-developed in several Alestes relatives but is particularly conspicuous in this species. The anal fin comprises 18 to 20 branched rays, a count that aids in distinguishing it from congeners. Sexual dimorphism is reported in anal-fin shape, though details are sparse in the available literature.
Habitat
Alestes macrophthalmus inhabits freshwater rivers and associated waterbodies across a broad swath of west-central Africa. Its core range encompasses the Congo River basin from Pool Malebo downstream to the upper Lualaba and the Luapula River, with additional populations in the Malagarazi and Rusizi rivers feeding into Lake Tanganyika. The species also occurs in coastal river systems of Cameroon, Gabon, the Republic of Congo, the Democratic Republic of Congo, the Central African Republic, Angola, and Zambia.
FishBase characterises it as a pelagic species — occupying the open water column rather than benthic or littoral habitats — in freshwater tropical rivers and lakes. The Congo basin rivers it inhabits encompass an enormous range of current speeds, depths, and water chemistries, and the species presumably tolerates considerable environmental variation given its broad distribution. It is regarded as a tropical freshwater fish with no particular specialisation to still or flowing water.
Feeding
Alestes macrophthalmus is omnivorous, with a diet that shifts substantially with age and size. Juveniles are reported to be primarily insectivorous, feeding on insect larvae and other small invertebrates, while adults become increasingly piscivorous — active hunters of smaller fish — as they grow into their large adult body size. Beetles and weed have also been recorded among stomach contents.
This dietary ontogeny — insect-eater as a juvenile, fish-eater as an adult — is consistent with the pattern seen in many large characins that begin life in a small-bodied niche and shift to piscivory only once they attain sufficient size to pursue and handle fish prey. The pelagic lifestyle suits an active pursuit predator, and the streamlined body plan is consistent with open-water hunting.
Mating
Specific mating behaviour in Alestes macrophthalmus has not been well documented in the available literature, and the species is not kept or bred in captivity with any regularity. Based on what is known for the genus and family Alestidae more broadly, A. macrophthalmus is expected to be an open-water egg-scatterer with no parental care — the reproductive mode typical of Alestes and allied African characins.
Sexual dimorphism in anal-fin shape is reported, suggesting that the sexes can be distinguished externally during the breeding season, though the specific nature of this dimorphism (hooks, elongation, or altered fin profile) is not detailed in the sources consulted. Spawning is presumed to be seasonal in large African river systems, likely timed to river-level and temperature cues associated with the wet season, as is characteristic of many large riverine characins.
Breeding
No captive-breeding records for Alestes macrophthalmus are documented in the hobbyist or scientific literature reviewed here; the species' large adult size and primarily commercial-fishery context mean it has not attracted aquacultural or hobbyist breeding attention. By analogy with related Alestes species and with large African characins generally, the species is presumed to deposit adhesive or semi-adhesive eggs among submerged vegetation or to release non-adhesive eggs into open water, with neither parent guarding the clutch or caring for the larvae.
Fertile adults are likely recruited into spawning aggregations in river reaches with suitable conditions — temperature, flow regime, and availability of spawning substrate — during the appropriate season. Fry are expected to be initially planktonic or drift-feeding before transitioning to the insectivorous juvenile diet described above.
In the aquarium
Alestes macrophthalmus is not a practical aquarium fish. At a reported maximum of 23.5 in standard length, even an average adult individual would require a system far beyond what home or most public aquaria can provide. The species is documented primarily in the context of commercial and subsistence riverine fisheries in the Congo basin and adjacent drainages, and no established hobby trade in the species has been noted.
For aquarists interested in large African characins or Alestidae, the family includes more tractable species — notably the Congo tetra (Phenacogrammus interruptus) and various smaller Alestes relatives — that share some of the family's biological characteristics at a manageable scale. Alestes macrophthalmus is included here for completeness as a biologically significant member of the family present across a wide and ichthyologically important drainage system.
Conservation
Alestes macrophthalmus is assessed as Least Concern (LC) on the IUCN Red List, with the most recent assessment published on 10 May 2019. The Least Concern designation reflects the species' broad geographic range across the Congo River basin and multiple additional drainages, its apparent tolerance of varied freshwater conditions, and the absence of evidence of rapid population decline.
The species is subject to commercial and subsistence fishing pressure throughout its range and is recorded as a component of commercial riverine fisheries in the Congo basin. FishBase rates its fishing vulnerability as low to moderate. While industrial and artisanal fishing pressure on Congo basin fish communities is a recognized concern at the community level, A. macrophthalmus's wide distribution and Least Concern status indicate it is not currently at elevated extinction risk.