Taxonomy & naming
Eigenmannia virescens was described by Valenciennes in 1836 and belongs to the family Sternopygidae, the glass and rattail knifefishes, within the order Gymnotiformes. The Sternopygidae are the "wave-type" knifefishes: their electric organ discharge is a continuous, near-sinusoidal wave rather than the discrete pulses of Gymnotus and the bluntnose knifefishes. The genus name honours the American ichthyologist Carl H. Eigenmann, who did much of the foundational work on South American fishes.
The "glass knifefish" sold in the trade is usually referred to this species, but E. virescens as currently understood is a wide-ranging complex, and populations across its enormous range may not all prove to be the same species as taxonomy is refined. As always the Catalog of Fishes is the authority for the valid name. Like all Gymnotiformes it is unrelated to the Asian featherback knifefishes despite the shared common name.
Morphology
The glass knifefish is a slim, tapering, laterally compressed fish, translucent enough in life that the vertebral column and body cavity show through the flanks — hence "glass". It has the standard gymnotiform build: no dorsal fin, no proper tail, and a long anal fin running most of the underside that provides all its propulsion, driving the fish forwards or backwards with equal control. It grows to around 17.5 in in total length, though many aquarium specimens are considerably smaller, and the trade names "glass" and "green" knifefish are both used.
The electric organ lies along the body, and the skin carries the electroreceptors that read the field it generates. As in all knifefish the body is scaleless. The species is not strongly marked, being pale and greenish-silver, and much of its visual interest lies precisely in its transparency and its endlessly fluid, fin-driven swimming.
Habitat
Eigenmannia virescens is one of the most widespread of all the knifefishes, ranging across South America east of the Andes from the Orinoco south to the La Plata basin, with populations in the Magdalena drainage as well. It is a fish of lowland floodplains, characteristically living beneath the floating meadows of aquatic plants that fringe river channels and cover backwaters, and among submerged roots and vegetation.
These are warm, soft to moderately mineralised, near-neutral waters, and like the rest of the order the glass knifefish is most active in low light. The tangled, shaded microhabitat under floating vegetation suits an electrolocating fish well — it is a world of little visibility where an electric sense pays off — and it offers both cover and a rich supply of small invertebrate prey. In the aquarium the species wants the same: shade, cover and clean, warm, gently-moving water.
Feeding
The glass knifefish is a nocturnal micropredator. It feeds on small invertebrates — aquatic insect larvae, small crustaceans and other zooplankton and benthic animals — hunted at night among vegetation and detected in part by its electric sense. It is a comparatively delicate feeder rather than a hunter of other fish, which makes it less of a threat to tankmates than the larger knifefishes, though very small fish and shrimp are still potential prey.
In the aquarium it takes small meaty foods — bloodworm, brine shrimp, blackworm, small frozen foods — offered in the evening when it becomes active. A group will feed more confidently than a lone fish. As with its relatives, patience with feeding times and a dimly-lit, secure tank do more for its health than any particular food.
Mating
Eigenmannia is the animal in which the electric side of gymnotiform courtship is best understood. Its continuous discharge has a characteristic frequency, and when two fish with similar frequencies meet, each shifts its own discharge away from the other's so that the two signals do not jam each other's electrolocation — the famous "jamming avoidance response", one of the most completely worked-out behaviours in all of neuroscience. Frequency and its modulation also carry social information: males and females discharge at different frequencies, and fish produce distinctive rises and interruptions during aggressive and courtship encounters.
Spawning itself takes place among plant roots and vegetation, typically in the warmer, higher-water season, with courtship playing out as a mixture of these electric exchanges and physical approach in the dark. The electric duet is not a curiosity bolted onto the fish's biology; in Eigenmannia it is much of the courtship.
Breeding
The glass knifefish is bred in captivity more readily than most knifefishes, and its reproduction has been studied both in the aquarium and in the laboratory. Spawning occurs among fine-leaved plants and root masses into which the adhesive eggs are laid, generally triggered by the warm, soft, higher-water conditions that mimic the onset of the wet season. A mature group in a well-planted, dimly-lit tank offers the best chance.
Parental investment is limited compared with the electric eel's guarded nest; the eggs are left among vegetation rather than tended through to independence. Even so, the relative willingness of Eigenmannia to spawn, and the ease of keeping it in groups, make it one of the more approachable knifefishes for an aquarist interested in breeding one of these electric fish — and a far more realistic proposition than the black ghost or the electric eel.
In the aquarium
The glass knifefish is among the easier gymnotiforms to keep, and unusually for the group it does well in a small shoal rather than alone — a point worth knowing, since a lone fish is often shy. It needs a well-planted, dimly-lit tank with plenty of cover and clean, warm, soft to moderately hard, near-neutral water. It is peaceful towards fish too large to eat, and its delicate feeding habits make it a reasonable companion for medium-sized, non-aggressive tankmates, though small fish and shrimp may still be taken at night.
Because it is scaleless it shares the family's sensitivity to poor water quality and to copper- and other harsh medications, so mature filtration, regular water changes and gentle treatment are important. Give it space — an adult group needs a sizeable tank — subdued lighting, and time to settle, and it becomes an active, confident fish whose ceaseless, transparent, fin-driven swimming is a pleasure to watch. It is one of the best knifefishes for an aquarist who wants the electric-fish experience without the size and hazard of a ghost knifefish or an eel.
Conservation
Eigenmannia virescens is assessed as Least Concern on the IUCN Red List, in an assessment reflecting its very broad distribution across much of tropical and subtropical South America and its tolerance of the widespread floodplain habitats it favours. It is common and not, at present, a species of conservation concern.
The main caveat is taxonomic rather than one of threat: because the "glass knifefish" spans such an enormous range and is likely a complex of more than one species, a Least Concern assessment for the group as a whole could mask narrower-range forms within it that are less secure. For the hobby the species is unproblematic — hardy, widespread, and breedable in captivity — and captive-bred or responsibly-collected stock kept in good conditions poses no conservation issue.