Size-Dependent Agonistic Interaction Patterns in Juvenile Male Swimming Crabs (Portunus trituberculatus)

Body size is a key determinant of agonistic interactions in swimming crabs. This study quantified agonistic interactions during pairwise contests among different size classes. Four size classes of male juveniles, Portunus trituberculatus, were examined (extra-large: 70.16 ± 1.12 g; large: 45.07 ± 1.15 g; medium: 25.30 ± 1.19 g; small: 15.08 ± 1.73 g; n = 12 per size class). The frequency and duration of agonistic behaviors and fighting intensity were recorded by a video recording system and analyzed. Larger crabs initiated more frequent and intense aggressive interactions, whereas smaller crabs exhibited mainly avoidance and submissive responses, particularly against larger opponents. The most intense and prolonged contests occurred between opponents with relatively small size differences. Conversely, highly size-mismatched pairs exhibited shorter, less intense interactions. These agonistic interactions were strongly size-dependent, consistent with Resource Holding Potential theory and size-contest dynamics. These behavioral patterns provide insights into social dynamics and inform aquaculture management practices.