Size-advantage in nest-holding tactics in the dusky frillgoby Bathygobius fuscus males: mating success and parental care success but not fertilization success

Body size is a critical trait influencing reproductive success through mate competition, female choice, and parental care ability. In fish species with alternative reproductive tactics, large males typically adopt dominant tactics, while smaller males employ parasitic tactics, such as sneaking tactics. However, the reproductive advantage of larger males can be compromised by intense sperm competition with small males. We investigated the effects of body size on the reproductive success of nest-holding males in the dusky frillgoby Bathygobius fuscus , a species where small males frequently engage in sneaking tactics. Field investigations using artificial nests revealed that larger nest-holding males had significantly higher mating success (number of eggs acquired) and parental care success (egg survival rate). In contrast, DNA paternity analysis showed that fertilization success (paternity rate) was not correlated with male body size. This is because of the presence of males with extremely low paternity rate, including two cases of zero paternity, suggesting high sneaking pressure and replacement of nest-holding males (i.e., alloparental care). Consequently, the cumulative reproductive success of nest-holding males showed no significant relationship with body size. These results showed that the size-advantages of nest-holding males in attracting females and tending eggs are offset by the sperm competition with sneaker males. The lack of a clear size advantage in reproductive success may explain the plastic nature of reproductive tactics in this species, where large males may occasionally adopt sneaking tactics or abandon nests.