Effect of flow structures on the transport of gametes in broadcast-spawning sea urchin

Broadcasted fertilization for the reproduction of invertebrates is accomplished through a complicated interaction between spawned gametes and the surrounding flows. Most gametes encounter each other in the vicinity of the sea urchin body where unique flow structures develop, and so analysis of local flow characteristics allows us to better understand the effect of flow on the fertilization process. This study applied a Lagrangian framework based on computational fluid dynamics to estimate the fertilization rate of eggs in a range of flow velocities (0.025-0.2 m/s) and the fertilization rate was the highest at U = 0.1 m/s, which is an intermediate flow speed. Among the four classified sub-zones, such as the aboral surface, wake, substrate, and water column, fertilization occurred most frequently in the wake and substrate regions. The relationship between fertilization rate and flow structures was investigated using three parameters: 1) standardized Morisita index to quantify the pattern of gamete distribution, 2) length of the recirculation zone to specify the areas where the eggs are most frequently fertilized, and 3) integral scale to estimate the dimension of vortex structures downstream. The results of this study show that the fertilization rate is higher inside the recirculation zone, especially when strong vortex structures are observed because the flow provides a favorable condition for the gametes to aggregate and collide with each other.