Fertilization in a sea urchin is not only a water column process: Effects of water flow on fertilization near a spawning female

Fertilization efficiency in free-spawning invertebrates in the marine environment depends on the complex interaction between biological and physical factors. Experimental evidence indicates that in some taxa, a considerable amount of fertilization may take place on the substrate and within flow structures in close proximity to a spawning female. Gametes can be retained on a spawning animal, resulting in their slow release over relatively long periods of time, and retained eggs can be fertilized before they are released into the water column. Hydrodynamic conditions are likely to influence both the retention of gametes and their subsequent mixing in the water column as well as the relative importance of fertilization in the water column and in other locations near spawning animals. Here, fertilization in the broadcast-spawning sea urchin Strongylocentrotus droebachiensis was explored over a range of flow velocities (ū = 2 to 15 cm s-1) to determine the effect of velocity on fertilization and the relative contribution of different locations (aboral surface of the female, water column, wake, and the substrate behind the female) to overall fertilization. As velocity increased, the percentage of eggs fertilized declined in all locations. At all velocities, more eggs were fertilized on the aboral surface than in the water column. Further, as velocity increased, the relative contribution of the aboral surface to overall fertilization increased. These results highlight the importance of considering the interaction of hydrodynamics, organism morphology, and gamete properties in studies of fertilization in broadcast-spawning invertebrates.