Variation in sperm swimming behaviour and its effect on fertilization success in the serpulid polychaete Galeolaria caespitosa

Established models of fertilization kinetics in free-spawning marine invertebrates predict that fertilization success is dependent upon sperm swimming velocity. Despite the prevalence of these models, there are very few published tests of this assumption. To test this, the effects of sperm swimming characteristics on fertilization success were studied in Galeolaria caespitosa (Polychaeta, Serpulidae). Both sperm activity (% motility) and sperm swimming velocities were highly variable within this species. Sperm were motile for up to 6-7 h after activation; however, mean motility, swimming velocity and fertilization success decreased after 4 h. Eggs of G. caespitosa were fertilizable up to 10 h after spawning; however, the number of embryos resulting from fertilization by fresh sperm also decreased after 4 h. Sperm motility and velocity were not affected by water-soluble egg extracts. When gamete concentration, contact time, and age were held constant, fertilization success in G. caespitosa remained highly variable. Some of this variability was attributable to observed variability in sperm motility and velocity. Fertilization success was positively related to sperm velocity (P = 0.014), but there was no correlation between percent sperm motility and fertilization success (P = 0.85). Cross-fertilization experiments indicated that individual egg fertilizability and male-female compatibility also play an important role in determining the fertilization success and thus may obscure any effects of sperm motility.