Fertilization strategies for winter flounder: effects of spermatozoa density and the duration of gamete receptivity

Winter flounder is one of the most commonly used models for studying fish biology in North America; however little is known about their reproductive ecology, especially during the spawning event. The objectives of this research were to determine the optimal number of spermatozoa required to fertilize eggs and to explore how long spermatozoa (30 to 240 s post-activation) and eggs (30 to 7680 s post-activation) are receptive to fertilization after exposure to seawater. We conducted experiments using gametes from wild-caught fish and measured fertilization success by examining eggs at 5 to 6 d post-fertilization. On average 34 038 sperm cells per egg were required to fertilize 81.3% of the eggs. Duration after spermatozoa activation had an effect on the proportion of eggs fertilized (F-3,F-6.69 = 338.38; p <0.0001; mixed-model ANOVA). At 30 s post-spermatozoa activation, 98% of the eggs were fertilized. After 60 s, a significant decrease in fertilization success was detected. Duration after egg exposure to seawater had an effect on the proportion of eggs fertilized (F-8,F-16 = 19.89; p <0.0001; mixed-model ANOVA). For all trials (30 to 1920 s), the percentage of eggs fertilized ranged from 61 to 90%. A significant decrease to 11% occurred at 3840 s after egg exposure. This area of research has particular importance for our understanding of reproductive strategies, evolutionary challenges, reproductive potential and recruitment. In addition, examining sperm-egg interactions provides information important to management of living and frozen-thawed gene banks.