Species-sex-specific behavioral interaction profiles between invasive mosquitofish and native medaka

Introduced species can affect native species through direct behavioral interactions, but such effects are often summarized at the species level, potentially obscuring variation among sex classes and actor-receiver combinations. We examined species-sex-specific behavioral interaction profiles between invasive western mosquitofish (Gambusia affinis) and native Japanese medaka (Oryzias latipes) under controlled laboratory conditions. Behavioral observations were conducted in 274 trials across 27 species-sex composition treatments. We recorded tracking events, defined as cases in which an actor fish followed or chased a receiver, and fin-nipping following tracking, defined as cases in which the tracking actor pecked the receiver's fin immediately after tracking. Across all trials, 11,847 tracking events were recorded, of which 2,057 were accompanied by fin-nipping. Tracking frequency differed among actor-receiver combinations, with male mosquitofish showing the highest standardized tracking frequencies across multiple receiver classes. Tracking events accompanied by fin-nipping were also frequent when male mosquitofish acted as initiators. However, the probability that a tracking event was accompanied by fin-nipping differed significantly among actor classes, whereas no significant effects of receiver class or the actor × receiver interaction were detected. Survival analysis further showed that tracking events accompanied by fin-nipping occurred earlier when mosquitofish, especially males, acted as initiators. These results indicate that behavioral interactions between mosquitofish and medaka are not simply characterized by receiver-specific heterospecific aggression. Instead, interactions between mosquitofish and medaka followed an actor-centered pattern, in which male mosquitofish frequently initiated tracking and some of these tracking events were followed by fin-nipping.