20-hydroxyecdysone causes increased aggressiveness in female American lobsters, Homarus americanus

Lobsters become transiently more aggressive before ecdysis. This aggressive ness accompanies an increase in hemolymph titers of 20-hydroxyecdysone (20- HE). Combats between intermolt female lobsters, injected with premolt level s of 20-HE, and larger, saline-injected opponents were videotaped. Aggressi ve, defensive, and avoidance behaviors were ranked according to aggressiven ess in a Rank of Aggression hierarchy, which included opponent-directed and (nonopponent) redirected behaviors. Treated animals performed more and mor e highly aggressive behaviors than saline-injected controls. Opponents of t reated animals performed fewer aggressive behaviors than saline-injected co ntrol opponents. Controls performed more defensive behaviors than treated a nimals, when redirected behaviors were considered. Differences in avoidance behaviors among the four types of combatants were not significant. The tot al aggressive content was the same in treated and control fights, but the i nteractions between combatants in the two fights were significantly differe nt. Treated animals were equally as aggressive and defensive as their oppon ents; controls were relatively less aggressive and more defensive than thei r opponents. These results correlate with molt-cycle variations in behavior , 20-HE titers, and the effects of 20-HE and molt-differentiated hemolymph on the claw opener muscle. They suggest that 20-HE orchestrates intrinsic, cellular, and nuclear events that produce the molt-cycle transformations in agonistic behavior and aggressive state of lobsters. (C) 2001 Academic Pre ss.