Abstract

Many organisms reproduce by releasing gametes into the environment. However, very little is known about what proportion of released eggs become fertilized. We examined the influence of spawning group size, degree of aggregation, position within an aggregation, and water flow, on in situ fertilization in the sea urchin Strongylocentrotus franciscanus. This study was conducted at a depth of 9 m on the west coast of Vancouver Island, British Columbia, Canada. Males were simulated by syringes filled with sperm; females were simulated by sperm—permeable containers filled with eggs. Individuals were placed 0.5 or 2.0 m apart within a 2 x 2 or 4 x 4 (group size of 4 or 16 individuals) experimental array. The results indicate that group size, degree of aggregation, position within a spawning group, and water flow all affect fertilization success. Fertilization success. Fertilization success ranged from 0 to 82%. Increases in group size and aggregation, decreases in flow velocity, and central and downstream positions within an aggregation all lead to increase in fertilization success. Thus, individual reproductive performance is dependent on, and highly sensitive to, population parameters and environmental conditions.