Pre‐oviposition Ejaculation in Externally Fertilizing Fish: How Sneaker Male Rose Bitterlings Contrive to Mate

Abstract Pre‐oviposition ejaculation as a mating tactic of sneaker males in the rose bitterling, Rhodeus ocellatus , was studied under natural and artificial conditions. In a small pond in Yao city, Osaka, Japan, the operational sex ratio of males and females was found to be approximately 3.5:1 and the proportion of males to the mussels, which serve as spawning beds for the rose bitterling, was approximately 2:1. Competitively subordinate rose bitterling males which spawned into the mussels participated in mating by sneaking, because not all males could occupy territories around the mussels. The sneaker males often released sperm not only after but also before egg‐laying (this ejaculation movement by the male before egg‐laying is termed ‘pre‐oviposition ejaculation’). In pair spawning with sneaker, the sneakers frequently performed pre‐oviposition ejaculations, which territorial males never performed. In the field, pre‐oviposition ejaculations by sneakers coincided with the leading of females by territorial males. Under artificial conditions, I demonstrated by using electrophoretic paternal analyses that the pre‐oviposition ejaculations by the sneakers were more effective than the post‐oviposition ejaculations by the territorial males. In addition, there were negative size‐dependences in ejaculation achievement rate and fertilization success of the sneaker males.