BADGLEY, BD*; THOMAS, FIM; YUND, PO; University of South Florida; University of South Florida; University of New England: The effects of hydrodynamics on fertilization success in the green sea urchin, Strongylocentrotus droebachiensis.

Among free spawning invertebrates such as sea urchins, it is often assumed that sperm and eggs are advected away from the animals in a plume and rapidly dilute in the water column to ecologically irrelevant concentrations. However, urchin gametes are highly viscous and resist mixing. During spawning, gametes can be retained on or near the urchin and strands or clumps of gametes can remain intact for an appreciable length of time in the water column. Furthermore, local hydrodynamic conditions play a key role in determining rates at which eggs and sperm are advected away from the animals and mixed into the water column. The combination of these effects can greatly alter the amount of time that sperm and eggs interact in relatively high concentrations, thereby altering overall fertilization success. In this study, we quantified the percentage of eggs fertilized on or near the urchins and in the water column as a function of water velocity and bottom roughness in the green urchin, Strongylocentrotus droebachiensis. At low flow speeds, a significant number of eggs and sperm spent time entrained in local eddies or deposited on the substrate prior to being advected away from the animals. As flow speed increased, eggs were quickly mixed into the water column and there was little retention of eggs near the spawning animal. As a result, there was a transition from fertilization being a local process at lower flow speeds to being a water column process at higher flow speeds and overall more eggs were fertilized at slow relative to high flow speeds.