American lobsters, Homarus americanus, use vision for initial opponent evaluation and subsequent memory

Many animals, including crustaceans, engage in dominance fights and thus the risk of damage. This makes opponent assessment a critical component of social information gathering. American lobsters, Homarus americanus (H. Milne-Edwards, 1837), are well known for surprisingly complex social behavior. Males fight for dominance, and winners are preferred by females. Fighting success is correlated with body and claw size. To avoid injury, opponents assess each other before and during a fight. The loser later recognizes the winner based on urine odor and retreats early from subsequent fights. Most research has explored recognition in the context of chemical ecology. Here, we describe four experiments testing the largely unexplored visual communication of male lobsters. Two experiments address evaluation within unfamiliar pairs. The first shows that the visual image of a lobster's own reflection in a mirror enhances attacks on an unfamiliar conspecific odor plume. The second shows that increasing differences in crusher claw size between opponents cause gradually decreasing fight duration; this negative correlation is maintained when either vision or olfaction is blocked. Blocking either sense caused similar increases in fight duration. Two additional experiments tested visual memory of opponents. Blindfolding the loser of a first fight abolished the decrease in second-fight duration seen in control fights, similar to previous results based on blocking olfaction. Finally, painting the "face" (carapace around the eyes and rostrum) of the winner in a second fight did not prevent recognition by the loser. Together, these results demonstrate that vision makes contributions to lobster communication, specifically opponent evaluation.