In externally fertilising animals, eggs are released together with maternal reproductive fluids, creating a local microenvironment in which fertilisation occurs. Despite their ubiquity, the functional roles of these fluids in shaping sperm behaviour and fertilisation success remain poorly understood. Here we show that ovarian fluid modulates sperm behaviour and fertilisation outcomes in three freshwater fish species with contrasting spawning strategies: rainbow trout (Oncorhynchus mykiss), common carp (Cyprinus carpio), and sterlet (Acipenser ruthenus). Using analyses of sperm movement, chemoattraction, and controlled fertilisation experiments, we demonstrate that ovarian fluid exerts strong but species-specific effects on sperm motility patterns, longevity, and responses to eggs. These effects translate into pronounced differences in fertilisation dynamics, particularly under time-limited sperm–egg contact conditions. Together, our results establish a comparative experimental framework for understanding how ovarian fluid shapes sperm performance in externally fertilising vertebrates and highlight its role as an active component of the fertilisation environment.
Kholodnyy et al. (Wed,) studied this question.