Tactic‐specific sperm traits in the dusky frillgoby ( Bathygobius fuscus )

Sperm competition between males with different reproductive tactics can be a strong selective force that shapes the evolution of tactic‐specific reproductive traits. In fishes, sneak fertilization of the female's eggs is the most widespread male alternative reproductive tactic and sneaker males generally exhibit enlarged testes as a tactic‐specific trait due to the higher risk of sperm competition they face. In addition, traits of the sperm themselves, such as size, density, velocity and longevity, vary with reproductive tactic in some species. Here, we investigated differences in the sperm traits between males of dusky frillgoby ( Bathygobius fuscus ) adopting nest‐holding tactics and sneaking tactics. In this species, sneak nest intrusion lasts only a few seconds to a minute, whereas females lay eggs over several hours. A unique feature of this species is that both types of males deposit sperm‐containing mucus on the nests, from which the sperm are gradually released into the water. We found that the sperm of sneaker males are present at a higher concentration in the testes, are longer lived and decrease in velocity more gradually than the sperm of nest‐holding males, despite no differences in sperm size being detected. These tactic‐specific sperm traits of sneaker males appear to contribute enhancing their fertilization success under the limited opportunity for sneaking in this species.