Perception of the steroid hormone 20-hydroxyecdysone modulates agonistic interactions in Homarus americanus

Abstract

The American lobster, Homarus americanus , becomes more aggressive just before moulting. This increased aggression is accompanied by an increase in the blood and urine titres of the moulting hormone, 20-hydroxyecdysone (20-HE). During combats with conspecifics, lobsters urinate upon each other to signal their aggressive and/or sexual status. We videotaped staged combats between large intermoult nongravid female lobsters exposed to 20-HE, α-ecdysone or artificial sea water (ASW) and small nongravid female lobsters that could not smell. The nephropores of both combatants were plugged to prevent urine-release. Aggressive, defensive and avoidance behaviours were ranked according to aggressiveness in a rank of aggression hierarchy, which included opponent-directed and (nonopponent) redirected behaviours. Exposure to 20-HE increased the state of arousal of large lobsters: they performed more highly aggressive behaviours, more defensive and more avoidance behaviours than either α-ecdysone-exposed or ASW-exposed lobsters. The small opponents of large 20-HE-exposed lobsters performed more aggressive behaviours. The summed aggressive intensity of all behaviours of both lobsters in a fight was greater in 20-HE fights than in ASW or α-ecdysone fights. The difference in aggressive intensity of all behaviours between the 20-HE-exposed lobsters and their opponents was significantly smaller in 20-HE fights than in α-ecdysone or ASW fights. Our results indicate that 20-HE exposure alters the agonistic behaviour of a lobster, which then evokes an increase in the aggressive behaviour of an opponent lobster. The findings suggest that 20-HE acts as a pheromone modulating aggressive interactions in lobsters.