Eigenmannia virescens – Glass Knifefish

Eigenmannia virescens or Glass Knifefish uses electric pulses to communicate with its own kind and to locate its prey.


Eigenmannia virescens was first described by Valenciennes in 1836. Originally this species received the name Sternachus virescens, this name is currently seen as a synonym. Other synonyms are: Sternopygus tumifrons and Sternopygus lineatus. The name of the genus Eigenmannia refers to the scientist Eigenmann.


The body of Eigenmannia virescens is green but transparent. This makes it possible to see internal organs and the spine of the fish. At the lower half of the body, two dark, horizontal lines can be seen. In addition does this species lack a caudal fin. The shape of the body make it look similar to a knife. The name of the family to which this species belongs refers to this shape, knifefish.
Differences between the sexes can be seen on basis of the size of the fish. Males can grow to become twice the size of a female.

Distribution and Habitat

Eigenmannia virescens has a rather large distribution throughout South-America. It can be found East of the Andes. Here it can be found in waters throughout the northern part of the continent.
Its large distribution has the consequence that it can be found in different biotopes. They occur in both, black and white water. Despite this, the fish mainly inhibits still deep waters on a substrate of plant debris.


These knifefish are carnivorous. Therefore, their diet can consist of: Tubifex, earthworms, snails and small fish. Eventually the fish can be learned to eat frozen or fabricated foods.
When hunting these fish actively search for their prey. When doing so they use electric signals to locate their food. When a victim is located, the fish will swim backwards to their prey. When doing so the fish “scan” their food until to pinpoint its exact location. When their target is close to their mouth, Eigenmannia virescens will swallow it with one, swift movement.

The aquarium

Eigenmannia virescens needs a sizable aquarium. The tank should be densely planted. In addition floating plants need to be added to dim the light. Sufficient hiding spaces need to be created by rocks and wood. The current needs to be sluggish to not present at all. However, due to its sensitivity to pollution and changing water conditions, a powerful filter should be provided.

Due to its carnivorous diet and its large size, the fish will need to be kept with larger species. Small fish will be seen as food. The fish themselves are peaceful towards each other and other species. However, due to a social hierarchy within the shoal, fights can occur between the fish. These fights will, fortunately, almost never lead to injuries. The fish maintain complex relationships with each other. They communicate with each other by sending electronic pulses through the water. Noticeably, the pulses of females are stronger than males. Electronic pulses are, therefore, not only used for locating objects but also for communication.


Reproducing Eigenmannia virescens can be difficult. The fish should be kept in a special aquarium. To condition the fish for breeding the water should be soft (2-4 DH). By simulating the rainy season, spawning will be triggered. This simulation can be done by pouring slightly cooler water in the aquarium over the course of a few hours. Spawning will occur in the morning. When spawning, the fish will mate several times during 3 to 6 hours. This will eventually result in 100 to 200 eggs per female. The eggs, whom are laid in the roots of floating plants, can be removed after breeding. The fry can be fed with Artemia nauplia.


Eigenmannia virescens summary page. (n.d.). Retrieved July 15, 2017, from http://www.fishbase.org/summary/12221

Hopkins, C. D. (1974). Electric communication: functions in the social behavior of Eigenmannia virescens. Behaviour, 50(3), 270-304.

Kramer, B., & Otto, B. (1988). Female discharges are more electrifying: spontaneous preference in the electric fish, Eigenmannia (Gymnotiformes, Teleostei). Behavioral Ecology and Sociobiology, 23(1), 55-60.

M. (n.d.). Glass Knife Fish | Eigenmannia virescens. Retrieved July 15, 2017, from http://fish.mongabay.com/species/Eigenmannia_virescens.html

Nanjappa, P., Brand, L., & Lannoo, M. J. (2000). Swimming patterns associated with foraging in phylogenetically and ecologically diverse American weakly electric teleosts (Gymnotiformes). Environmental Biology of Fishes, 58(1), 97-104.


R. Eltingh

Copyright foto’s

J. de Lange

Last Updated on 27 September 2020 by John

Additional information






Eigenmannia lineata, Sternachus virescens, Sternopygus lineatus, Sternopygus tumifrons

Common names

Glass Knifefish


Social Behaviour

Breeding behaviour





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