Environmental control of cyclical reproduction of tropical freshwater fish: Evidence from comparative experimental data
In 3 neotropical gymnotiform fishes gonad maturation and spawning could be obtained by experimental variation of 3 environmental factors, 1) imitation of rain (R), 2) decreasing conductivity (C), and 3) increasing water level (WL), or only 2 factors (WL and C) (4 species) or just (C) (1 species). The basis of the regulation of cyclical reproduction is induction of gonad maturation (yolk vesicle formation, vitellogenesis) and all stages of spermatogenesis. In more than 10 African mormyrid species decrease of C alone was sufficient to induce gonad maturation, spawning could be obtained in 9 species. In 2 species of the family Schilbeidae (Pareutropius buffei, Parailia pellucida) and 2 species of the genus Synodontis (S. nigrita, S. nigriventris) (Mochokidae) variation of all 3 factors led to gonad maturation. Detailed experimental analysis in Pareutropius buffei showed that imitation of R was sufficient to induce complete gonad maturation. In the South-East Asian catfish Kryptopterus bicirrhis variation of all 3 environmental factors led to complete gonad maturation. In the two notopterid knifefishes Xenomystus nigri (Africa) and Notopterus notopterus (South-East Asia) and the African Polypterus senegalus no clear correlation between variation of environmental factors and gonad development or sex hormone titers could be found.