Glass Knifefish

The glass knifefish (Eigenmannia virescens) is a weakly electric fish in the family Sternopygidae, order Gymnotiformes, native to freshwater habitats across tropical South America including the Amazon, Orinoco, and Paraguay-Parana river systems. Like its relatives the banded and black ghost knifefish, it lacks a dorsal fin and instead propels itself with an elongated, wave-like anal fin, allowing forward and backward swimming with equal ease. Its near-transparent body, through which internal organs are sometimes faintly visible, gives the species its common name. Glass knifefish are widely kept in aquariums and studied by scientists for their electric-organ discharges, used for both navigation and social signaling among individuals.

The glass knifefish is identified by its slender, laterally compressed, largely translucent body.

• Pale yellow-green to olive sheen with a faint silvery, glassy transparency, especially toward the tail
• Long, ribbon-like anal fin extends almost the full length of the underside and undulates continuously
• No dorsal fin and no pelvic fins
• Pointed tail and small, forward-set head with relatively large eyes
• Typically 10-20 cm, notably smaller and more slender than the similar-shaped but larger, opaque banded knifefish

Unlike the banded knifefish, it lacks dark bands and instead appears nearly uniform and see-through, the clearest way to distinguish the two at a glance.

Glass knifefish inhabit slow-moving freshwater rivers, streams, and floodplain lakes throughout much of tropical South America, from the Orinoco basin in Venezuela and Colombia south through the Amazon and into the Rio de la Plata drainage. They favor warm water around 24-28C with moderate to dense aquatic vegetation, submerged roots, and driftwood, which provide shelter during the day. They often form loose aggregations in calm backwaters, shaded banks, and areas with soft sediment. Being weakly electric and largely nocturnal, they are well suited to turbid or dimly lit water where vision offers little advantage for navigation or finding prey.

Glass knifefish are gregarious and often gather in loose groups, unusual among electric knifefishes, using their electric organ discharges not only for electrolocation but also for social communication between individuals within the group. They are most active at dusk and during the night, hiding among vegetation or submerged debris by day. Their long undulating anal fin allows precise forward and backward swimming, useful for maneuvering through dense cover while hunting small invertebrates such as insect larvae and crustaceans. Electric signal frequency can shift when individuals interact, a phenomenon known as the jamming avoidance response, which prevents their electric fields from interfering with one another. Spawning is linked to seasonal rains, with eggs deposited among plants.