No abstract

Japanese medaka (Oryzias latipes) are a teleost fish and an emerging vertebrate model for ecotoxicology, developmental, genetics, and physiology research. Medaka are also used extensively to investigate vertebrate reproduction, which is an essential biological function as it allows a species to perpetuate. Sperm quality is an important indicator of male fertility and thus reproduction success. Techniques for extracting sperm and sperm analysis are well documented for many species, including for teleost fish. Collecting semen is relatively simple in larger fish but can be more complicated in small model fish as they produce less sperm and are more delicate. This article therefore describes two methods of sperm collection in the small model fish Japanese medaka: testes dissection and abdominal massage. We demonstrate that both approaches are viable for medaka and show that abdominal massage can be performed a repeated number of times as the fish quickly recover from the procedure. We also describe a protocol for computer-assisted sperm analysis in medaka to objectively assess several important indicators of medaka sperm quality (motility, progressivity, duration of motility, relative concentration). The use of these procedures combined with the other advantages of using this small teleost model will greatly improve the understanding of the environmental, physiological, and genetic factors influencing fertility in vertebrate males.

No abstract

Environmental light is perceived and anticipated by organisms to synchronize their biological cycles. Therefore, exposure to artificial light at night could disrupt diurnal and seasonal rhythmicity. Reproduction is a complex physiological process involving integration of environmental signals by the brain, and release of endocrine signals by the pituitary that regulate gametogenesis and spawning. In addition, males from many species form a dominance hierarchy that, through a combination of aggressive and protective behavior, influences their reproductive success. In this study, we investigated the effect of different light regimes, including light pollution at night and continuous light, on the fitness of male fish within a dominance hierarchy using a model fish, the Japanese medaka. In normal light/dark rhythm conditions, we observed that dominant males are more aggressive, remain closer to the female, and spend ten-fold more time spawning than subordinates. By using males with different genotypes, we determined the paternity of the progeny and found that even though subordinate males spend less time with the females, they are equally successful at fertilizing eggs in normal light conditions due to an efficient sneaking behavior. However, when exposed to light at night, dominant males fertilize more eggs. We indeed found that when exposed to nocturnal light pollution, dominant males produce higher quality sperm than subordinate males. Surprisingly, we did not find differences in circulating sex steroid levels, pituitary gonadotropin levels, or gonadosomatic index between dominant and subordinate males, neither in control nor night light condition. Continuous light was found to completely inhibit establishment of male hierarchy. This study is the first to report an effect of light pollution on sperm quality with an impact on male fertilization success in any vertebrate. It has broad implications for fish ecology in urban areas with potential impacts on the genetic diversity of these fish populations.