Taxonomy & naming
Sternopygus arenatus was described by Eydoux and Souleyet in 1850 and belongs to the family Sternopygidae — the glass and rattail knifefishes — within the order Gymnotiformes, the New World electric knifefishes. Gymnotiformes are bony fish but an unusual lineage, set apart from every other aquarium fish by an electric organ and by a body plan built around a single elongated anal fin rather than a conventional set of median fins.
The genus Sternopygus is generally the largest-bodied group within the family, in contrast to the smaller, semi-transparent Eigenmannia (the true glass knifefishes) and the deep-channel, poorly known Rhabdolichops and Distocyclus. Despite the shared common name, gymnotiforms are not related to the Asian featherback and clown knifefishes (Chitala, Notopterus); the resemblance is convergent body shape, not kinship. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the authority for the valid name Sternopygus arenatus.
Morphology
Like other rattail knifefishes, Sternopygus arenatus has an elongated, laterally compressed body that tapers gradually into a long, whip-like tail, with no dorsal fin and no distinct caudal fin. Locomotion comes almost entirely from an extremely long anal fin — FishBase records 180 to 225 soft rays and no anal spines — thrown into travelling waves along its length, which lets the fish glide forward or reverse smoothly without needing to turn. FishBase lists a maximum total length of 22 in, making it one of the larger species in its family.
Running beneath the skin is the electric organ, paired with electroreceptor pores scattered over the body that read the field it generates. Genetic work on the family's mitochondrial genome confirms that Sternopygidae produce a wave-type electric organ discharge — a continuous, near-sinusoidal signal, in contrast to the intermittent pulses of families such as Gymnotidae — used both for electrolocation and for species- and sex-specific communication. The species is scaleless, a trait shared across the order that has direct consequences for how sensitive fish of this kind are to water quality and medication.
Habitat
Sternopygus arenatus is known from the Guayaquil river basin on the Pacific slope of Ecuador, a trans-Andean drainage rather than the Amazon or Orinoco systems that hold most of its relatives — FishBase and biogeographic surveys of the genus describe it as effectively a single-basin endemic. It is a benthopelagic freshwater fish, living in the water column near the bottom rather than buried in sediment.
Congeneric Sternopygus, and the family more broadly, occupy a range of moving and still water within their basins — fast-flowing river channels, slower floodplain reaches, and static floodplain pools depending on season — typically over sand or among submerged roots and leaf litter. Estimated conditions for the genus run warm, around 75–82 °F, with a mildly acidic to near-neutral pH in the 6.0–7.2 range. As with other gymnotiforms, dim or turbid water suits a fish that reads its surroundings electrically rather than visually.
Feeding
Detailed dietary data for Sternopygus arenatus specifically is sparse, but the genus and family are understood to be nocturnal micro-predators, foraging after dark for small invertebrates — aquatic insect larvae, worms and small crustaceans — located largely by electrolocation rather than sight. The wave-type discharge that plays constantly around the fish distorts subtly around prey items, obstacles and other animals, and the fish reads those distortions through pores along its flanks.
In common with its relatives it is likely an active, probing feeder that works over the substrate and among cover rather than an ambush predator, resting up by day and becoming active as light fades. No aquarium feeding record exists for this species given how rarely, if ever, it appears in the trade.
Mating
Courtship in Sternopygus, as in other wave-type gymnotiforms, is understood to run substantially through electric signalling. Males hold territory and use their electric organ discharge to advertise and to signal to potential mates, with discharge characteristics differing between males and females — a pattern documented in the well-studied congener S. macrurus and consistent with the wave-type electrolocation system confirmed for the family by mitochondrial genome studies. The approach and pairing that precedes spawning appears to combine this electrical signalling with direct physical approach in cover.
Breeding is tied to season: congeneric data indicates spawning is timed to, or just ahead of, the rainy season, when rising water opens up flooded vegetation as spawning habitat. Specific courtship behaviour has not been documented for S. arenatus itself, and what is known is drawn from the better-studied S. macrurus.
Breeding
Sternopygus is an egg-laying genus. In the congener S. macrurus, adhesive eggs are scattered among submerged roots and floating vegetation rather than deposited in a nest, spawning is associated with the wet season, and fecundity can reach roughly 6,500 eggs per female; eggs hatch after about four days and larvae begin feeding around day eleven, with no parental care observed afterward. There is no specific breeding record for Sternopygus arenatus, and given how rarely the species is seen in the hobby, it should be treated as unbred in the aquarium and poorly documented in the wild alike.
Parental investment in the family is minimal: eggs are left among vegetation to develop unguarded, a pattern consistent across the Sternopygidae rather than something specific to this species.
In the aquarium
Sternopygus arenatus is not an established aquarium fish — it is known from ichthyological survey material rather than the trade, and there is no meaningful hobbyist keeping record for this species. What follows is inferred from the needs of the genus and family. Like all gymnotiforms it is scaleless and correspondingly sensitive to ammonia, nitrite and nitrate, and to copper-based or otherwise harsh medications, which typically need to be dosed at reduced strength or avoided; mature filtration and warm, soft, gently moving, near-neutral to slightly acidic water would be the baseline to provide.
As a large-bodied (to 22 in), nocturnal, electrolocating predator it would need generous swimming space, dim lighting, and secure cover such as pipes or root tangles to retreat into by day, along with tankmates too large to be treated as prey after dark. Given the total absence of a keeping record and its restricted, single-basin distribution, this is a species better left to specialist or research contexts than pursued for a home tank; if it were ever offered, captive-bred or responsibly sourced stock would be the only defensible choice.
Conservation
Sternopygus arenatus is assessed as Least Concern on the IUCN Red List, based on an assessment dated 2 October 2020. As a species confined to a single Pacific-slope drainage — the Guayaquil river basin of Ecuador — rather than one of the continent-spanning Amazon or Orinoco systems, its overall range is inherently narrow even where the current status is not one of elevated concern.
Single-basin endemics of this kind are the members of the genus most exposed to local pressures — habitat alteration, water pollution and land-use change within their one drainage — even when a Least Concern rating reflects no current population decline. No species-specific threats are documented in the sources reviewed here; the appropriate conservation posture is to note the narrow range rather than to assert active decline that is not evidenced.