Taxonomy & naming
Sternopygus macrurus was described by Bloch & Schneider in 1801 and is the type species of Sternopygus, a genus within the family Sternopygidae — the glass and rattail knifefishes — in the order Gymnotiformes, the New-World electric knifefishes. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the authority for the valid name and its synonymy.
Gymnotiformes are bony fish but an unusual lineage among them: every genus except a handful produce a weak electric organ discharge (EOD) used for electrolocation and communication, and the group is defined as much by that electric organ and its long, undulating anal fin as by conventional anatomy. Sternopygidae are not related to the superficially similar Asian featherback and clown knifefishes (Chitala, Notopterus), which belong to an entirely different order and share only a convergent body shape. Within the Gymnotiformes, Sternopygidae are wave-type electric fish, producing a continuous, near-sinusoidal discharge rather than the intermittent pulses used by families such as Gymnotidae.
Morphology
The longtail knifefish has the classic Sternopygus build: a laterally compressed body that is deepest just behind the head and tapers gradually into an extremely long, thin tail — hence "rattail". There is no dorsal fin and no distinct caudal fin; locomotion comes almost entirely from a very long anal fin running along most of the ventral length of the body, thrown into travelling waves that drive the fish forward or backward with equal ease. The skin is scaleless, a trait shared across the order that makes these fish notably sensitive to poor water quality and to certain medications.
FishBase records a maximum total length of 55.5 in, making S. macrurus one of the largest species in its family — considerably bigger than aquarium-trade staples like the glass knifefish (Eigenmannia). The electric organ runs along the body and is paired with rows of sensory pores (the electroreceptors) distributed over the skin that read the self-generated field. The species produces a wave-type EOD with a low, dominant frequency in the 50–150 Hz range; the discharge frequency differs between the sexes and shifts seasonally, and males use it to defend territory.
Habitat
Sternopygus macrurus is broadly distributed across tropical South America, recorded from the Amazon and Orinoco basins, the Guianas, the Magdalena system, and Atlantic-draining rivers of Brazil. It is a benthopelagic fish of swamps, creeks, rivers and other lentic (still or slow) waters rather than fast, open channels, often sheltering among submerged roots, leaf litter and dense vegetation.
FishBase gives typical conditions of 72–82 °F, pH 6.0–6.5, and soft to moderately mineralised water (roughly 4–15 dH) — warm, soft, acidic-to-near-neutral South American blackwater and whitewater habitat. As with other gymnotiforms, these waters are often turbid or tannin-stained, conditions in which vision counts for little and the electric sense does the work of finding food, avoiding obstacles and detecting other fish.
Feeding
Sternopygus macrurus is a nocturnal micro-predator, feeding on small invertebrates and aquatic insect larvae that it locates largely by electrolocation rather than sight — its continuous wave-type discharge lets it detect the electrical signature of prey hidden in sediment or tucked among roots and leaf litter, day or night, but it is most active after dark.
In the aquarium it takes standard meaty foods for electric knifefishes — bloodworm, blackworm, chopped earthworm and brine shrimp, along with sinking frozen or prepared foods of a suitable size — and, like its relatives, feeds best once the lights are down. Its mouth and feeding habits are those of a deliberate forager rather than an ambush hunter, working over the substrate and cover rather than chasing prey in open water.
Mating
Courtship in Sternopygus macrurus is closely tied to its electric sense. Males establish territories and use their electric organ discharge to signal presence and readiness, and to attract females; the EOD carries information about sex and individual identity, and its frequency is known to vary between males and females and across the season, likely tied to reproductive state. This wave-type electrical signalling — the same general mechanism that underlies the famous jamming-avoidance response studied in the related glass knifefish Eigenmannia — appears to be the primary channel of communication between prospective mates in the dim, often turbid water these fish inhabit.
Beyond the electrical courtship, specific pairing and spawning behaviour is not well documented for this species; much of what is known comes from studies of the electric signal itself rather than direct observation of spawning.
Breeding
Sternopygus macrurus is an egg-layer. Females produce large clutches — on the order of several thousand adhesive eggs per spawning — released among aquatic plants and other submerged vegetation, and the eggs hatch into larvae after roughly four days. As with most Sternopygidae, parental care is minimal: eggs are left among the vegetation rather than guarded.
Breeding in the home aquarium is rarely documented for this species, and most individuals in the trade are wild-caught. Its relative Eigenmannia (the glass knifefish) is bred in captivity more readily than most gymnotiforms, but S. macrurus itself is data sparse on this point — the aquarist should expect to keep rather than breed it, and treat any successful captive spawning as noteworthy rather than routine.
In the aquarium
This is a large, long-lived fish that needs room to match: adults can approach the 55.5 in maximum recorded for the species, so a tank intended to hold a mature longtail knifefish long-term must be sized accordingly, with plenty of open floor space and secure cover — driftwood, root tangles, wide pipes or dense planting — for it to retreat into by day. Subdued lighting suits it better than a brightly lit display, echoing the dim, tannin-stained waters it comes from.
Being scaleless, S. macrurus is sensitive to ammonia and nitrate and to many common medications, particularly copper-based treatments, which should be avoided or used at reduced dose. It is a nocturnal predator and, while generally peaceful toward fish too large to be seen as prey, is not safe with small or bite-size tankmates and shrimp, which it will hunt after dark by electrolocation. Because reliable captive-bred stock is uncommon for this species, prospective keepers should expect wild-caught origin and budget for the eventual size the fish will reach.
Conservation
Sternopygus macrurus is assessed as Least Concern, reflecting its wide distribution across multiple major South American river systems — the Amazon, Orinoco, Magdalena, the Guianas and Atlantic Brazilian drainages — and its tolerance of a range of still and slow-flowing habitats. FishBase records no elevated conservation concern for the species, and it continues to be taken by commercial fisheries as well as the aquarium trade.
That broad security does not remove all risk to the populations it depends on: habitat degradation, pollution and hydrological change affecting South American freshwater swamps, creeks and rivers remain relevant pressures across its range, as they are for the wider Gymnotiformes. As with other wild-caught electric knifefishes, favouring known-provenance stock and avoiding unnecessary collection pressure is the most useful stance available to the hobby.