Reproduction of the weakly electric fish Eigenmannia virescens (Rhamphichtyidae, Teleostei) in captivity

Summary

• 1. The paper describes the effect of four environmental factors, conductivity, pH, water level, and rain, on gonadal recrudescence and regression in the tropical gymnotoid fish Eigenmannia virescens. The developmental state of the gonads was either assessed quantitatively (maturity coefficient, MC) or judged by visual inspection in the transparent fish (Fig. 1). Spawning indicates that fish are mature.

• 2. Decreasing conductivity and pH, increasing water level, and imitation of rain (‘rainy season’) led to gonadal recrudescence (Figs. 2, 4, and 5) in fish which had entirely regressed gonads at the beginning of the experiment. Increase of conductivity and pH and missing rain, when increase in water level continued (‘dry season’), provoked gonadal regression (Figs. 2, 4, and 5).

• 3. When the four exogenous factors were held constant for 102 days (Fig. 6), this had no effect on ovary weight; they remained regressed. In males, an increase to full maturity after 62 days was found.

• 4. Eigenmannia, imported from South America, reached maturity in the laboratory, but did not spawn at all or only very irregularly, whereas fishes which were born and raised in captivity spawned very regularly when mature (Table 1), under optimal conditions every third to fourth day (Fig. 7). Females about 15-cm long spawned 100–200 eggs each time.

• 5. Over a period of one year, ovary recrudescence could be induced four times and regression three times with a group of three females and four males (Fig. 8). Neither Eigenmannia females nor males showed post-breeding refractoriness. Ovarian recrudescence and regression could be stopped experimentally at apparently any level; maturation was always possible afterwards (Figs. 9 d and 10 d).

• 6. Experiments were designed to test the significance of the four factors as ‘zeitgebers.’ Gonad maturation occurred at decreasing, increasing, variable, and at constant pH values (Fig. 9); pH apparently is not a zeitgeber. Conductivity, water level, and rain are presumably all zeitgebers which together induce ovary maturation and spawning (Fig. 10e). Decreasing conductivity alone (Fig. 10a), rain plus rising water level (Fig. 10b), decreasing conductivity plus rain (Fig. 10c), and decreasing conductivity plus rising water level (Fig. 10d) provoked ovary growth, but did not lead to mature gonads. Decreasing conductivity alone and rain plus rising water level led to mature testes, but maturation seemed to be less complete as judged from the number of spermatozoa present compaired with the result of the experiment where all three effective factors acted together (Fig. 2).

• 7. Decreasing conductivity was one of the exogenous factors which induced gonad maturation. The fish also reached maturity when, at decreasing conductivity, the concentration of some of the ions (carbonate and total hardness) increased (Fig. 11).

• 8. Regression of ovaries could be provoked by continuous increase of conductivity alone (Fig. 7).

• 9. Mature fish no longer required the variation of the environmental factors, which had earlier provoked gonadal recrudescence, in order to continue spawning (Fig. 12).

• 10. A ‘rainy season’ which was interrupted by a very short ‘dry season’ (sudden, large increase of conductivity) led to irregular spawnings (Fig. 13) for two months in fish which were previously spawning regularly.