Sneak early, as females lay eggs fast: sperm competition in a fish with long spawning duration

In species with external fertilisation, synchronizing sperm release by males with egg release by females is generally considered crucial, yet little is known about sperm release strategies when females lay eggs over extended periods. In the intertidal goby Bathygobius fuscus, smaller males intrude into the nest during pair spawning, which lasts several hours, and release sperm. Here, we investigated how the timing of sneaker intrusion affects the paternity of sneaker males, using controlled aquarium experiments combined with genetic paternity analysis. Our results showed that early nest intrusion after the onset of egg-laying (within approximately 1 hour) significantly increased sneaker male paternity. This may be partly because early released sperm remain viable for several hours and can fertilise eggs laid later. In addition to the increased fertilization rate, another possible reason why sneaker males attempt early intrusion is that females lay more eggs during the early stages of egg-laying (approximately 70% of their total eggs during the first hour of spawning), giving the males the opportunity to fertilize a greater number of eggs. These findings suggest that early sneaking increases fertilization opportunities for sneaker males, which is thought to be favorable given the traits of long egg-laying period and long sperm longevity in this species. This study proposes the importance of considering the timing and sequence of sperm release in understanding the consequences of sperm competition in externally fertilising species with a time gap between sperm release and fertilization, similar to that of species with internal fertilisation.