A proposed mechanism for turbulent enhancement of broadcast spawning efficiency

The broadcast spawning reproductive strategy relies on turbulent-stirring processes in the flow to bring together gametes previously released by adult males and females. The subsequent fertilization rate depends on the product of co-occurring concentrations of egg and sperm. Turbulent mixing produces a strong average dilution in these concentrations, suggesting that an increase in turbulence would reduce the average local fertilization rate. However, turbulent dilution occurs at time scales that may be long compared to those associated with fertilization. Therefore, the instantaneous structure of egg and sperm filaments at shorter time scales must be considered. In this paper, a mechanism is proposed whereby coherent turbulent structures in the velocity field cause coalescence between high-concentration filaments of egg and sperm, significantly enhancing the average fertilization rate. Simple analytical and numerical models are used to demonstrate how the mechanism works, and to make qualitative estimates of its effect on the resulting fertilization rate. The results suggest that the efficiency of broadcast spawning is a consequence of features in the instantaneous turbulent field, and that this efficiency is not captured by models that consider only time-averaged features of the flow.