The influence of male size on reproductive success in the porcellanid crab Petrolisthes japonicus

Sexual selection theory predicts that males with larger bodies and chelipeds achieve higher reproductive success in porcellanid crabs (Decapoda Anomura), which exhibit sexual size dimorphism. However, the relationship between male size and reproductive success remains unexplored. This study tested whether male reproductive success is size-dependent in porcellanid crabs using Petrolisthes japonicus as a model species. We conducted 108 mating trials under laboratory conditions, each involving a female and two males of different sizes. During the trials, we observed mating behaviors and assessed spawning, fecundity, and offspring size of the females, all of which influence male reproductive success. Copulation occurred in 64 trials, with agonistic interactions occasionally observed between males. Larger males initiated more frequent contests against smaller males, suggesting a competitive advantage. In 59 trials, females copulated once with either a large male or a small male, while double matings occurred in five trials. Females copulated exclusively with large males in 44 trials and with small males in 18 trials, demonstrating a significant bias toward larger males. Copulation probability with the large male increased significantly when the small male was much smaller than the female. Mating approaches also varied: when mating with large males, approach patterns were equally distributed among male, female, and mutual approaches, while mating with small males occurred primarily via male-initiated approaches. Male size did not affect female spawning, fecundity, or offspring size, indicating that male reproductive success primarily depends on the number of females they copulate with and the fecundity of those females. Our results highlight the large-male mating advantage in P. japonicus, with both male-male competition and female mate choice likely contributing to this pattern. Further experimental studies are needed to clarify the relative importance of these mechanisms in shaping reproductive success in this species. (1) Larger males had a significant mating advantage over smaller males. (2) Females preferentially mated with larger males, especially when the small male was substantially smaller than the female. (3) Both male-male competition and female mate choice influence reproductive success.