How do freshwater fish sperm find the egg? The physicochemical factors guiding the gamete encounters of externally fertilizing freshwater fish

Abstract The lifespan of spermatozoa from externally fertilizing freshwater fish ranges from a few seconds to several minutes, depending on the species. External factors, such as temperature, background flows and ion composition, play an important role in fertilization success. Specific mechanisms guiding spermatozoa appear to be essential to maximize the sperm–egg encounter under these strenuous conditions. Although some existing data support the hypothesis that both the ovarian fluid and the eggs may release chemoattractants that significantly affect spermatozoa behaviour and the fertilization outcome, this hypothesis is still open to debate, as the existence of freshwater fish spermatozoa chemotaxis has yet to be demonstrated; in addition, specific mechanisms supporting spermatozoa guidance and gamete selection have not been elucidated. Is the natural selection of gametes determined by a combination of different physicochemical phenomena? Alternatively, is the natural selection of species‐specific gametes biased towards the species‐specific guidance mechanisms of their natural landscape? These questions have received more attention as new studies have revealed potential, distinct guidance mechanisms in freshwater fish reproduction. In this review, we discuss the empirical studies supporting different hypotheses about freshwater fish gamete guidance and highlight the synergistic combination of experiments and biomathematical modelling to explore these questions. Finally, we discuss the challenges in understanding the mechanisms behind sperm guidance in freshwater fish species, and we suppose that knowledge about the mechanisms that underlie spermatozoa selection and guidance in freshwater fish species may elucidate the impact of the traditional aquaculture practice of artificial fertilization on progeny quality and species sustainability.