Molt-related and size-dependent differences in the escape response and post-threat behavior of the American lobster, Homarus americanus.

Abstract

Videotaped recordings of adult lobsters of different molt stages were analyzed. The escape response of adults was compared with that of juveniles recorded in an earlier study. Juvenile lobsters always respond to a threat with escape behavior irrespective of their molt stage, but in adults the probability of eliciting a response was a function of molt stage: more hard-shelled (intermolt stage C) and (premolt stage D) animals tailflipped than did soft-shelled (postmolt stages A and B) animals. The number, frequency, and duration of tailflips, and the average distance swum by animals in each molt stage were measured for the entire escape response, for the initial power swim, and for the subsequent swims. These measurements were used to compute several parameters: velocity, acceleration, force, and work; average distance traveled in a tailflip for each kilogram of body weight (distance/kg/tailflip); and average distance traveled for each bodylength (distance/bodylength). Among adults, intermolt (stage C) lobsters traveled significantly farther and faster than postmolt animals (stages A and B). Among juveniles, late postmolt (stage B) animals traveled farther. Among adults, although the total number of tailflips and the duration of the response were not significantly different among molt stages, the number of tailflips/second (frequency) and distance traveled/kg/tailflip were greater for intermolt animals. In juvenile intermolts, however, frequency and distance/kg/tailflip were markedly lower than in the premolt stages. Although values were lower than intermolts and premolts, postmolt adults sustained their swimming frequency, distance/kg/tailflip, and distance/bodylength for the entire escape distance (as did postmolt juveniles). These parameters then dropped off sharply for both adult and juvenile intermolt and premolt animals in the second half of the escape distance. Post-threat behaviors reveal that stage D animals have the highest aggression index and often attack the presented stimulus, whereas stage A animals are the least likely to approach the stimulus and typically back away in a non-aggressive posture. Thus, although effects of the molt cycle on adult and juvenile escape behavior are similar in some ways, other physical characteristics of adults, such as weight, allometry, and physiology, seem to become important in determining the likelihood of escape behavior and the characteristics of the escape swim in each molt stage.