Taxonomy & naming
Rhabdolichops stewarti was described by Lundberg & Mago-Leccia in 1986, one of several species in the genus Rhabdolichops, placed in 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, defined by its electric organ and by a body plan built around a single long anal fin rather than the paired fins and separate dorsal and tail fins of most teleosts.
The species epithet honours Donald J. Stewart, an ichthyologist at the Museum of Zoology, University of Michigan, who brought this knifefish to the describers' attention — a friend-and-colleague dedication of the kind common in gymnotiform taxonomy. Rhabdolichops sits close to Eigenmannia within Sternopygidae, and later taxonomic work on the genus (a 2006 Copeia revision describing three further new species) notes that some Rhabdolichops retain transitional characters between the two genera, underscoring how recently this part of the family tree has been worked out. As with all valid names on this site, the Catalog of Fishes (Eschmeyer, CAS) is treated as the authority. The New-World gymnotiform knifefishes, including this one, are entirely unrelated to the Asian featherback and clown knifefishes (Chitala, Notopterus); the similar silhouette is convergence, not kinship.
Morphology
Rhabdolichops stewarti has the classic glass-knifefish build: a long, slender, laterally compressed body tapering to a fine point at the tail, with no dorsal fin and no distinct caudal fin. Locomotion comes from a single elongated anal fin running along most of the underside, thrown into travelling waves that drive the fish forward or backward without needing to bend its body — the same mechanism used across the family, and the reason gymnotiforms can hover, reverse and thread through cover as easily as they swim ahead. Published notes on the genus describe large, translucent electrogenic tissue visible toward the tail region in several species, a feature tied to the electric organ itself.
Adults reach roughly 12 in, making this one of the larger, more elongate glass knifefishes rather than a small nano species. As in its relatives, the skin is scaleless and the body carries the sensory pores that read the electric field the fish projects around itself — its primary means of sensing prey, obstacles and other fish in the dark, sediment-stained channels it inhabits. Some congeners show sexual dimorphism in features such as caudal-appendage length, but nothing of that kind has been specifically documented for R. stewarti; treat any external sexing as unreliable pending further study.
Habitat
The species is recorded from the deep water channels of major rivers in the Amazon and Orinoco basins of tropical South America — open, flowing main-channel habitat rather than shallow marginal vegetation, which is where several of its Rhabdolichops relatives are typically found. Within the genus, some species favour low-conductivity blackwater systems while others occupy higher-conductivity whitewater channels, reflecting real ecological differentiation across what looks superficially like a uniform group of slender, transparent fish.
As with every gymnotiform, darkness and turbidity are central to how this fish lives: eyesight counts for little in deep, sediment-laden or tannin-stained river water, and the electric sense does the work instead, letting the fish detect prey and navigate obstacles it cannot see. Little else about this particular species' preferred microhabitat has been documented in the literature available; much of what can be said with confidence is inferred from the genus as a whole rather than confirmed for R. stewarti specifically.
Feeding
Rhabdolichops stewarti has not been the subject of dedicated dietary study, but its biology places it among the nocturnal micro-predators typical of Sternopygidae: an active hunter of small invertebrates — aquatic insect larvae, small crustaceans and similar prey — located in the dark largely through electrolocation rather than sight. Like its relatives it is a probing forager over open water and structure rather than a sit-and-wait ambush predator.
In aquarium contexts, care guides for glass knifefishes in general (most directly documented for the more commonly kept Eigenmannia, but broadly applicable to the group) recommend meaty live and frozen foods — bloodworm, brine shrimp, daphnia and other small crustaceans — offered after dark when these fish become active. Data on R. stewarti's specific prey preferences remain sparse.
Mating
As in other Sternopygidae, the electric organ discharge in Rhabdolichops is understood to carry information beyond simple electrolocation — a continuous, wave-type signal whose frequency and character can vary between individuals and sexes, functioning as a channel for courtship and rivalry in the dark. Eigenmannia, the best-studied genus in the family, shows sex-specific discharge frequencies and a well-documented jamming-avoidance response when two fish's signals interfere, and the same broad wave-type signalling toolkit is shared across the family, Rhabdolichops included.
No courtship behaviour specific to R. stewarti has been documented in the sources available for this species; what can be said follows from its placement within a family where electric signalling, rather than visual or elaborate physical display, is the primary courtship channel.
Breeding
Rhabdolichops stewarti has no documented spawning record in the literature reviewed here. Sternopygidae as a family are egg-layers that scatter adhesive eggs among plant roots and floating vegetation, typically triggered by warm, soft, rising-water conditions during the wet season, with little to no parental care given afterward — the eggs are simply left among cover. Eigenmannia is bred in captivity somewhat more readily than most gymnotiforms, but that ease has not been demonstrated for Rhabdolichops.
Given the complete absence of species-specific breeding data, this is best treated as an unbred or essentially unrecorded species in aquarium terms; any protocol beyond the family-level pattern above would be speculation rather than fact.
In the aquarium
Rhabdolichops stewarti is not an established aquarium fish, and it is rarely if ever seen in the trade; general care guidance for glass knifefishes (most concretely documented for Eigenmannia, but reasonably extended to Rhabdolichops) points to warm water around 75–82 °F, soft and mildly acidic to near-neutral chemistry (roughly pH 6.0–7.5), dim lighting, dense cover such as driftwood, PVC tubing or fine-leaved planting, and a sandy substrate. Like all gymnotiforms it is scaleless, so ammonia, nitrite and nitrate must be tightly controlled, and copper-based or otherwise harsh medications should be avoided or used at reduced dose.
Because it is a nocturnal, electrolocating predator, tankmates should be too large to be eaten and not prone to nipping the long anal fin; anything shrimp- or small-fish-sized is at risk after dark. At a reported adult size around 12 in this is not a small fish, and any aquarium attempting to house it would need to reflect that. In practice, this species belongs more to museum collections and ichthyological survey records than to the hobby, and captive-bred stock does not exist to draw on.
Conservation
Rhabdolichops stewarti is assessed on the IUCN Red List as Least Concern. It is recognised in official U.S. taxonomic reference (ITIS TSN 640313), though that record carries no conservation designation of its own.
As a species of the deep river channels of the Amazon and Orinoco basins, it shares the general pressures facing Neotropical freshwater fish more broadly — habitat alteration, damming, deforestation and pollution across those basins — but no assessment ties a specific population trend or threat to this species.