Sperm limitation in the sea.

Because sperm outnumber eggs, it is often assumed that variation in female reproductive success has little to do with male or sperm availability. Similarly for males, access to viable eggs and sperm competition are thought to drive variation in male fertilization success. These assumptions result from empirical studies on organisms with internal fertilization. However, recent evidence from free-spawning organisms suggests that sperm can often be limiting. This finding may alter our perspective on mating-system evolution, especially in externally fertilizing organisms.