Taxonomy & naming
Steatogenys ocellatus was described by Crampton, Thorsen & Albert in 2004 from the lowland Amazon basin, with type material collected near Tefé, Brazil, and the Río Itaya near Iquitos, Peru. It belongs to the order Gymnotiformes, the New-World electric knifefishes, and the genus Steatogenys sits within the sand- and bluntnose-knifefish lineage historically grouped with Hypopomidae and treated by some authors within Rhamphichthyoidea; sources cited here place the genus in Rhamphichthyidae. As in every gymnotiform, the defining trait is not appearance but physiology: the fish produces a weak electric organ discharge (EOD) used for electrolocation in the dark and for communication with other knifefish, a system built around a greatly elongated anal fin and the near-total loss of the tail and dorsal fin.
The species epithet ocellatus is Latin for "eyelike", referring to the round or oval, eye-like spot at the base of the pectoral fin that distinguishes this species from its congeners, along with mottled dark blotches across the pectoral-fin membrane. Despite the shared common name "knifefish", Steatogenys and its Gymnotiform relatives are not related to the Asian featherback and clown knifefishes (Chitala, Notopterus); the resemblance is convergent body shape only. The Catalog of Fishes is the authority for the valid name.
Morphology
Steatogenys ocellatus is a slender, elongate, laterally compressed fish, scaleless like all gymnotiforms, reaching a relatively large size for the genus — up to about 16 in in total length, larger than most of its close relatives. FishBase records 167 to 195 soft rays in the long anal fin that runs beneath most of the body and provides essentially all of the fish's locomotion, rippling in travelling waves to drive it forward or backward with equal facility; the pectoral fins carry around 15 rays and are used for fine steering and braking. There is no dorsal fin and no distinct tail fin of the ordinary kind.
The diagnostic feature is the pectoral-fin base marking: a round or oval, eye-like (ocellate) spot, paired with mottled dark blotches across the pectoral fin itself, which separates S. ocellatus from other Steatogenys species. Along the body runs the electric organ, generating a pulse-type discharge, with electroreceptive pits concentrated around the snout and mouth that read the field's distortions. Sexual maturity is reached at around 9.5 in total length; sexes are not reliably distinguished by external colour or shape.
Habitat
The species is known from the lowland Amazon basin, including localities around Lago Tefé in Brazil and the Río Itaya near Iquitos in Peru, and type material was collected in seasonally flooded forest. It is a benthopelagic fish of static or slow-flowing blackwater habitats, characteristically low in conductivity, among submerged roots, branches and leaf litter rather than in open, fast-flowing channels.
This is the classic Amazonian blackwater setting: warm, soft, acidic, tannin-stained water that transmits almost no useful light, which is precisely the environment electrolocation is built for. Like other bluntnose and sand knifefishes, S. ocellatus is thought to be cryptic and largely nocturnal, sheltering by day among root tangles and leaf litter and becoming active after dark; detailed field observations of its daily behaviour are sparse in the available literature.
Feeding
Steatogenys ocellatus is presumed to feed as its relatives in the bluntnose and sand knifefish group do: on small invertebrates living in and around leaf litter and submerged wood, located largely through electrolocation rather than sight in its dark blackwater habitat. Aquarium-trade sources describing the broader Gymnotiformes group note mosquito larvae, small crustaceans and worms as typical prey for similar-sized relatives, and a trophic level around 3.2 has been calculated for this species, consistent with a diet of small invertebrates and occasional smaller prey rather than fish.
Hunting is likely nocturnal and probing, working over submerged roots, branches and leaf litter after dark, guided by the electric sense reading the electrical signatures of buried or hidden prey. Specific dietary studies for this species are not available in the sources consulted here, so this account should be read as inference from the group rather than direct observation of S. ocellatus.
Mating
Courtship and pair formation in the bluntnose and sand knifefishes is understood mainly through the electric organ discharge itself: the pulse-type signal each fish generates functions as a form of communication as well as electrolocation, and in gymnotiforms generally, discharges can carry information about sex, size and identity that plausibly plays a role in finding and assessing mates in the dark. Specific courtship behaviour has not been documented for Steatogenys ocellatus in the sources reviewed here.
Given the poorly studied natural history of this species and its relatives, any account of courtship beyond the general electric-signalling framework common to the order would be speculative; this is a case where the honest position is that mating behaviour in the wild remains data sparse.
Breeding
As with most Hypopomidae and Rhamphichthyidae, breeding in Steatogenys ocellatus is poorly documented. Where reproduction has been studied in related small pulse-type knifefish, it typically involves egg-scattering among leaf litter, roots or sand rather than nest-building or mouthbrooding, generally timed to seasonal flooding, with little or no parental care afterwards. No breeding record — captive or field — specific to S. ocellatus appears in the sources consulted here.
The species is essentially unknown in aquarium culture, and there is no established breeding protocol to report. The honest summary is that reproduction in this species is rarely observed and data sparse, consistent with the pattern across its family.
In the aquarium
Steatogenys ocellatus is not an established aquarium species; it does not appear to be traded with any regularity, and the aquarium-hobby sources available describe the broader Gymnotiformes group rather than this species specifically. Where bluntnose and sand knifefishes do appear in the trade, the same care principles apply: because they are scaleless, they are unusually sensitive to ammonia and nitrate, and to copper-based or harsh medications, which should be avoided or dosed with care. A dim, quiet tank with soft, warm, blackwater-style conditions and plenty of fine cover — leaf litter, root tangles, sand for burrowing species in the group — suits their nocturnal, electrolocating lifestyle far better than a brightly lit community setup.
Given its size (to around 15.5 in) and predatory, nocturnal habits, any specimen that does appear in the trade should not be trusted with small or bite-size tankmates, which risk being hunted after dark. Prospective keepers should treat this as a specialist fish for very soft, acidic water rather than a general community species, and should expect very little published husbandry guidance to draw on.
Conservation
Steatogenys ocellatus is assessed as Least Concern on the IUCN Red List, an assessment from September 2020, reflecting its distribution across a substantial portion of the lowland Amazon basin and the absence of evidence for significant population decline. It is not currently considered a species of conservation concern.
As with many Amazon blackwater specialists, the main long-term pressures on its habitat are the general ones facing the basin: deforestation, altered flood regimes and water-quality degradation in the flooded forests and blackwater creeks it depends on. No species-specific threats or targeted conservation measures are documented in the sources reviewed here.