The Effect of Intruder Density on Territoriality and Dominance in Male Swimming Crab ( Portunus trituberculatus )

Territorial behavior of animals is affected by numerous factors, one being the number of intruders. The swimming crab ( ), an important commercial and ecological species on the continental shelf of Asia, usually needs to defend its territory from intrusion by other crabs, especially in habitats with high densities of conspecifics. To clarify the underlying patterns of how protects its territory, we assessed the territorial behavior of occupant crabs (territory holders) when presented with different numbers of intruders using an indoor observation system. We calculated the territory size of the occupants and quantified their behavioral responses to intruders. With an increased number of intruders, the territory size the occupants owned significantly decreased, and their behavior adjusted accordingly. Besides, the territorial behavior score, reflecting the territoriality of crab, decreased significantly. Furthermore, in a high density group that had seven intruders, the occupants showed a higher dominance hierarchy than the intruders, indicating the ascendancy of occupants in territorial competition with intruders. These results revealed that as the number of intruders increased, the territory size of shrunk because the fight for territory became more intense.