Reproductive investment in the knifefish Brachyhypopomus occidentalis

An organism's ability to produce viable offspring, or reproductive fitness, often depends on several environmental and ecological factors such as temperature, food availability and predation. Changes in these factors can act as stressors, forcing organisms to optimize energy allocation, resulting in trade-offs. In this study, we investigate the reproductive biology of the South American electric fish Brachyhypopomus occidentalis, examining whether perceived predation risk can influence reproductive output. We sampled four populations during the reproductive season at two pairs of sites with differing levels of predation risk across independent river drainages. We assessed several traits including sexual dimorphism and reproductive traits such as gonad mass, fecundity, and gamete morphology parameters. Our findings reveal physical sexual dimorphism, with males being larger and heavier than females. Individuals from low predation risk sites were significantly larger, had heavier gonads, and exhibited higher gonadosomatic indices. Females from low predation risk sites invested more in gonad development, showing increased relative and absolute fecundities and higher oocyte counts at each maturity stage. Collectively, our results strongly suggest that predation risk plays a significant role in shaping reproductive strategies in electric fishes and also underscore how ecological pressures can drive variation in reproductive investment across fish populations.