Reproduction and development of the weakly electric fish, Pollimyrus isidori (Mormyridae, Teleostei) in captivity

Abstract

SynopsisNo clear sexual dimorphism occurs in Pollimyrus isidori. Females usually grew slightly larger than males. The anal-fin reflex, however, makes it easy to discriminate between males and females. Spawning took place during the first six hours of the dark phase, in the territory of the male. During each spawning act 2-4 eggs were laid. The male put the eggs into a well-hidden nest previously constructed from plant material. The eggs, free embryos, and larvae of several spawnings were guarded by the male for several weeks. The females laid on average 120 eggs per fractional spawning. The eggs were 2 mm in diameter, not adhesive and very yolky. Th!ere was a relationship between pH and viability (% embryos hatched). Hatching occurred on the third day after spawning. The respiratory network of segmental origin in the median fin fold was well developed in the free embryos. Transition to exogenous feeding occurred on day 14. The larval period ended when the fish .were 15 mm long and 40-50 days old. First gonadal recrudescence occurred at a total length of 6cm (about 200 days old). The environmental factors decreasing conductivity and pH, increasing water level, and imitation of rain led to gonadal recrudescence, but pH did not act as a cue. Recrudescence was triggered by a decrease of conductivity, but absolute values or ionic composition of the water were not important. Maturation was completed after about fifty days. Mature and spawning fish no longer required any variation of these environmental factors to maintain mature gonads. A steady and considerable increase in conductivity led to gonadal regression. The fish did not show postbreeding refractoriness.

Patterns of the electric organ discharge during courtship and spawning in the mormyrid fish, Pollimyrus isidori

“…Males cared for the eggs or young from different spawnings in the same nest or maintained two nests simultaneously. For the development of the fry, their larval discharge, and the male's brood caring behavior, see Kirschbaum (1977), Kirschbaum (1977, 1978), and Kirschbaum (1987); behavioral and some developmental aspects confirmed by our own observations. Over a period of 384 days the mean spawning cycle was 24 + 20 days with periods ranging from 6-71 days between spawning in tank I (tank III: 9.8 + 3.4 days in 8 spawnings).…”

“…13). Kirschbaum (1984Kirschbaum ( , 1987 has not studied electrical signalling, while Crawford et al (1986) could not find any courtship-specific electrical behavior except a '~ cessation of the electric organ during vocalization" One pair of our fish spawned 21 times in a 120 1 breeding tank (tank I) of which 17 spawnings were recorded (EOD, song and behavior). In a second tank (tank II; 360 1) a male failed to spawn with 2 females but later succeeded with two different females (5 spawnings).…”

“…4g), fertilized and dropped to the bottom of the tank (1 to 3 s for Oviposition). Close up video recordings did not permit the observation of an anal fin reflex of the male during Vent-toVent behavior as reported by Kirschbaum (1987).…”

“…Among the methods of bringing P. isidori into breeding condition (Kirschbaum 1984(Kirschbaum , 1987, primarily slowly decreasing water conductivity and imitation of rain were successful. Altering levels of water was not necessary in our fish, except for replacement from evaporation.…”

“…Spawning behavior has already briefly been described by Crawford et al (1986) with an emphasis on acoustic signalling, and Kirschbaum (1987) with an emphasis on reproductive biology. A more detailed analysis of the behavioral context in which electrical displays occur before and during courtship and spawning was needed for our study (summary time table, Fig.…”