Does the rainbow trout ovarian fluid guide the spermatozoon on its way to the egg?

Fertilization of freshwater fish occurs in an environment that may negatively affect the gametes; therefore, the specific mechanisms triggering the encounters of gametes would be highly expedient. The egg and ovarian fluid (OF) are likely the primary sources of these triggers in fish that we confirmed here for rainbow trout Oncorhynchus mykiss . The ovarian fluid significantly affected the spermatozoa performance: it supported high velocity for a more extended period and changed the motility pattern from tumbling in water to straightforward moving in the ovarian fluid. Rainbow trout OF induced a trapping chemotaxis-like effect on activated male gametes, and this effect depended on the properties of the activating media. The interaction of the spermatozoa with the attracting agents was accompanied by their “turn-and-run” behavior involving asymmetric flagellar beating and Ca 2+ concentration bursts in the bent flagella segment, characteristic of the chemotactic response. Collectively, the ovarian fluid creates the optimal environment for rainbow trout spermatozoa performance, being an effective promoter of fertilization. The individual peculiarities of the egg (ovarian fluid) – sperm interaction in rainbow trout reflect the specific features of the spawning process in this species.