Linear Dominance Hierarchies in Female Grass Shrimp Palaemon pugio

AbstractThe grass shrimp Palaemon pugio is an abundant and ecologically important species in estuarine habitats in the northwest Atlantic and is commonly used as an indicator species for environmental contamination, but little is known about its behavior. We examined aggression and dominance in P. pugio by using experimental arenas and groups of five shrimp that were observed an hour at a time for three days in the laboratory. Female shrimp showed high levels of initial aggression that quickly decreased over time, suggesting the formation of a dominance hierarchy. The dominance hierarchy between groups of females appeared linear, with shrimp at five distinct ranks in most replicates and higher-ranked individuals dominating lower-ranked ones. Dominance was significantly correlated with cheliped size but not body size, suggesting that larger chelipeds in female P. pugio may have evolved as a tool to establish dominance, perhaps to gain access to shelter in salt marshes, seagrass beds, or oyster reefs. Grass shrimp are preyed upon by a wide range of estuarine fish, and females are especially vulnerable; so securing access to shelter may provide a major selective advantage, with the dominance hierarchy evolving to save energy through a long-term reduction in fighting over resources.