When neighbors become family: the dear-enemy effect of swimming crab and the verification of the formation hypothesis

The neighbor-stranger response difference, which serves as a fundamental social network relationship, plays a crucial role in establishing and expanding complex social networks. However, the underlying causes of this phenomenon have received limited attention in controlled indoor settings. This study focused on Portunus trituberculatus, a representative crustacean species in the Western Pacific Ocean. We analyzed the behavioral responses of crabs towards intruders of varying familiarity and threat levels. Additionally, we quantified various parameters such as territory size, behavior quantification, time allocation, and territorial behavior score. The results showed that with increased familiarity, there was a decrease in the frequency of occupiers’ aggressive dash and aggressive wave. As the threat from the intruder decreased, occupiers displayed fewer territorial behaviors and engaged in fewer bouts, while the success rate in fights improved and the territorial behavior score declined. The results indicate a clear dear-enemy effect in the territorial defense of P. trituberculatus, and demonstrates that crabs adjusted the defense strategy according to the relative threat level of intruders. We verified the formation hypothesis of dear-enemy effect in crabs. In the future, we will expand the research from individuals to populations, analyze crustacean social networks from the perspective of group decision making, and provide more support for population dynamic analysis and decision evolution research of crustaceans. The dear-enemy effect is considered as a potential driving force in the formation and evolution of social structures. However, the reasons behind the dear enemy effect, have seldom been investigated in controlled indoor settings. This study focused on Portunus trituberculatus to explore their territorial behavior and neighbor-stranger response difference. By analyzing the behavioral responses of crabs towards intruders of varying familiarity and threat levels, we discovered a clear dear-enemy effect in the territorial defense of P. trituberculatus. The finding sheds light on the recognition and evaluation rules towards intruders and provides a behavioral foundation for understanding the social networks of crustaceans. Our research lays the groundwork for further exploration of crustacean social networks, from individual behaviors to population dynamics and decision-making processes.