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Percent fertilization of eggs of the echinoid Strongylocentrotus droebachiensis (O. F. Müller) was determined both in laboratory and field experiments. In the laboratory, over 50% of the eggs were fertilized only in relatively dense sperm suspensions (&gt; 10 6 sperm/I); such suspensions retained their potency for less than 20 minutes. In the field, divers induced individual S. droebachiensis to spawn with KCl injections. Along five meter transects running directly downcurrent from spawning males, fixed volumes of seawater presumably containing sperm were drawn into syringes already containing eggs. Within 20 cm of spawning males 60-95% fertilization usually occurred; at distances greater than 20 cm less than 15% of the eggs were fertilized. Higher percentages of eggs were fertilized when current speeds were low (&lt;0.2 m/s); swifter currents quickly diluted sperm so that little fertilization occurred. When several males were induced to spawn synchronously, percent fertilization increased but was generally less than 40% at distances greater than 2 m downstream. These results indicate that production of zygotes could be much less than production of eggs. Life-tables based on estimates of egg production may then be in error, unless adults aggregate and spawn synchronously, countering dilution of sperm by currents.

THE ECOLOGY OF FERTILIZATION OF ECHINOID EGGS: THE CONSEQUENCES OF SPERM DILUTION, ADULT AGGREGATION, AND SYNCHRONOUS SPAWNING