Decreased response or alternative defensive strategies in escape: two different types of long-term memory in the crab Chasmagnathus

An opaque screen moving overhead elicits an escape response in the crab Cha smagnathus that after a few presentations habituates for a long period (lon gterm habituation). Two types of long-term habituation were previously desc ribed: the (context-signal)-long-term habituation yielded by spaced trainin g - context dependent, cycloheximide sensitive and long lasting; and the (s ignal)-long-term habituation yielded by massed training - context independe nt, cycloheximide insensitive and shorter lasting. Present research is focu sed on the defensive strategies crabs display during acquisition of both lo ng-term habituations, using video analysis as the main method of study. Asi de from the escape response, Chasmagnathus shows a rigid motionless display , an alternative defensive response we term freezing response. The escape r esponse is predominantly exhibited at night and in summer months, while fre ezing occurs during day light hours and in wister months. During acquisitio n of (signal)-long-term habituation, the escape response vanishes without b eing replaced by freezing. During acquisition of (context-signal)-long-term habituation, the escape response vanishes and is replaced by a strong free zing that finally becomes the only defensive strategy. The former, but not the latter, meets the current concept of habituation.