Breeding the Glass Knifefish (Eigenmannia virescens)

Eigenmannia virescens is a fractional spawner that lays 100-200 adhesive eggs on plants; male electric courtship triggers spawning, and it breeds in captivity.

Overview

The Glass Knifefish (Eigenmannia virescens) is a weakly electric, gregarious South American gymnotiform widely distributed east of the Andes from the Orinoco to the La Plata basins, reaching up to 44 cm SL. It inhabits deep still waters and creeks rich in plant debris and is nocturnal (FishBase). Unlike the territorial ghost knifefish, it is social and is one of the few knifefish reproduced in aquaria.

Sexing

Direct external sexing is unreliable; sexual recognition is electric. Males display increased electric-discharge amplitude during nighttime breeding competition, and the male's electric courtship signal is what identifies and stimulates a ripe female (FishBase).

Conditioning

FishBase records a fractional spawner with fecundity around 905 eggs released in successive batches. Conditioning relies on a stable group with steady meaty feeding; the species tolerates 18-30 C, so a mature, well-fed school provides the base for spawning.

Breeding Setup

Because eggs adhere to the leaves of aquatic plants (FishBase), the tank needs dense fine-leaved plants and, in practice, the roots of floating plants as spawning sites. Soft, slightly acidic water and dim, nocturnal conditions match the natural habitat.

Spawning Behavior & Trigger

The species lays 100-200 eggs that adhere to plant leaves (FishBase). Spawning is triggered by simulating the rainy season - softening and cooling the water to lower conductivity and pH while raising the level; FishBase notes that playback of a conspecific male's electric courtship display elicited spawning in ripe females, confirming the electric signal as the proximate cue.

Egg & Fry Care

Eggs are deposited among plant leaves and roots and develop there (FishBase). As a fractional spawner the female releases eggs over repeated pairings, so plant cover must be ample; captive-raised fish are reported to spawn regularly once mature under good conditions.

Common Challenges

The main difficulties are recognising a ripe pair without electrical recording and reproducing the rainy-season trigger reliably; predation of the adhesive eggs by the school itself also reduces yield, so dense planting or egg transfer helps.