Taxonomy & naming
Sternopygus obtusirostris was described by Franz Steindachner in 1881 and belongs to the family Sternopygidae — the glass and rattail knifefishes — within the order Gymnotiformes, the New-World electric knifefishes. Like all gymnotiforms it is a bony fish but an unusual one: this order is defined by a body-length electric organ and a single long anal fin used for propulsion, with no dorsal fin and no proper tail fin. Sternopygidae are the wave-type "hummers" of the order, producing a continuous, high-duty-cycle electric organ discharge rather than the intermittent pulses of families such as Gymnotidae.
For many years this species was treated as a junior synonym of the broadly distributed Sternopygus macrurus, but it was subsequently redescribed as a valid, distinct species by Crampton, Hulen and Albert on the basis of morphological and distributional differences. The Catalog of Fishes (Eschmeyer, CAS) is the authority for the currently valid name. As with other Gymnotiformes, this fish shares nothing beyond superficial body shape with the Asian featherback and clown knifefishes (Chitala, Notopterus) — those are bonytongues from an entirely different order, and the resemblance is convergent, not a sign of relatedness.
Morphology
Sternopygus obtusirostris has the classic rattail-knifefish build: a long, laterally compressed body that tapers gradually into a slender, whip-like tail, with an extended anal fin running most of the length of the underside providing the fish's only real means of propulsion. There is no dorsal fin and no separate tail fin in the conventional sense. Total length reaches 23.5–31.5 in, making this one of the larger species in its genus — comparable in scale to its well-studied congener S. macrurus, which is reported to reach up to 39.5 in TL.
As in other gymnotiforms, the body is scaleless, and an electric organ runs along much of its length, paired with electroreceptor pores scattered over the skin that read the field the organ generates. The discharge is wave-type: a continuous, quasi-sinusoidal signal rather than discrete pulses, and — as documented in the closely related S. macrurus — the frequency of that signal is sexually dimorphic, with females running a higher baseline frequency than males. Field studies note that S. obtusirostris can modulate its discharge intensity by roughly 40 percent, with peak activity and signalling after dark.
Habitat
This species is a blackwater specialist restricted to the central Amazon basin, known from the region around Tefé and the stretch of the Amazon mainstem down to Óbidos in Pará, along with the lower and middle Rio Negro. It is found in blackwater rivers and their seasonally inundated floodplain lakes and forests, and research on its distribution notes it is conspicuously absent from whitewater rivers, várzea (whitewater-fed) floodplains, and blackwater forest streams — a fairly specific ecological preference for a particular kind of blackwater-river habitat rather than blackwater environments generally.
Blackwater rivers in this region are warm, soft and acidic, stained tea-dark by dissolved tannins from decaying leaf litter, and correspondingly dim — conditions where vision is of limited use and an electric sense becomes the primary way to navigate, find prey and detect other fish. The species is nocturnal, consistent with the broader pattern in Sternopygidae of increased activity and electric signalling after dark.
Feeding
Detailed dietary data specific to Sternopygus obtusirostris is sparse, but the pattern in its well-studied congener S. macrurus is informative for a genus with a shared body plan and sensory system: a diet of small invertebrates, including aquatic insect larvae, located primarily by electrolocation rather than sight. As a nocturnal fish with a continuous wave-type electric discharge, S. obtusirostris is best understood as a night-active micro-predator that probes the water column and substrate for small prey once darkness falls, using its electric field to detect prey it cannot see.
In the aquarium, rattail knifefish of this genus are generally kept on meaty foods — bloodworm, chopped earthworm, small crustaceans and similar frozen or live fare — offered in the evening to match their natural activity pattern, though specific husbandry data for this species in captivity is limited.
Mating
As in other Sternopygidae, courtship in this genus is closely tied to the electric organ discharge. In the congener S. macrurus, territorial males use electric signals to attract mates, and the wave-type discharge frequency differs between the sexes — a sexually dimorphic signal that plausibly plays a role in species and sex recognition during the approach to spawning. No detailed courtship account exists specifically for S. obtusirostris, but given the shared physiology and family-wide pattern of electric communication, its mating behaviour is reasonably assumed to follow a similar template of electric signalling combined with physical approach in the dark blackwater habitats it occupies.
Spawning in the genus appears to be timed with the seasonal cycle: in S. macrurus, peak spawning activity occurs before and during the rainy season, when rising water levels open up flooded vegetation and floodplain habitat for egg deposition.
Breeding
Sternopygus obtusirostris is not documented as bred in the aquarium hobby, and detailed reproductive data for the species itself is sparse. Drawing on the better-studied congener S. macrurus, Sternopygidae as a family are egg-layers that scatter adhesive eggs among plant roots and floating or flooded vegetation rather than mouthbrooding or guarding a nest; S. macrurus is reported to produce several thousand eggs per spawn, with eggs hatching in around four days and larvae beginning to feed roughly a week after spawning. Parental care after spawning is minimal to absent.
For the aquarist, the honest summary is that this species is essentially unbred and unknown in captivity — its large adult size, blackwater habitat requirements and narrow natural range make it a poor candidate for a home breeding project, and any account of its breeding must lean on inference from its genus rather than direct observation of S. obtusirostris itself.
In the aquarium
Sternopygus obtusirostris is rarely, if ever, seen in the aquarium trade, and there is little practical husbandry information specific to the species. What can be said follows from its biology and its genus: this is a large fish — 23.5–31.5 in total length as an adult — that would need a very large, long aquarium with generous floor space, not a typical community setup. Being scaleless, like all gymnotiforms, it would require careful attention to water quality and medication dosing, with copper-based treatments avoided and other medications used at reduced strength.
As a nocturnal electroreceptive predator from dim blackwater habitat, it would need subdued lighting, secure cover such as pipes or root tangles to retreat into by day, and warm, soft, acidic water reflecting its native blackwater rivers (roughly pH 6.0–6.5 per data reported for the genus). It should not be kept with fish or invertebrates small enough to be treated as prey after dark. Given how seldom this species appears in the hobby, anyone encountering it should treat it as a specialist proposition, not an impulse purchase.
Conservation
Sternopygus obtusirostris is assessed by the IUCN as Least Concern. It occupies a defined but not minuscule range within the central Amazon blackwater system — from around Tefé to Óbidos along the Amazon mainstem, plus the lower and middle Rio Negro — and its ecological specialisation for blackwater river habitat, rather than the full range of Amazonian aquatic environments, makes it more range-restricted than the widespread S. macrurus with which it was long confused.
No species-specific threats are documented, but as with other blackwater-restricted Amazonian fish, plausible long-term pressures would include habitat alteration from deforestation, changes to natural flooding regimes, and water-quality degradation within its home range. The Least Concern status should be read against a comparatively narrow, well-defined distribution rather than the continent-spanning range typical of some of its relatives.