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ABSTRACT The cover-seeking behavior of juvenile and adult crayfish, Orconectes rusticus. was examined in relation to the relative importance of darkness and thigmotactic cues. Crayfish (N = 12 per experiment) were observed in individual aquaria 5 times per day, with at least 30 min between observations, for a 3-day period. Their positions were recorded as either within or outside the provided cover(s). When presented with a choice between a clear thigmotactic cover or open area (Experiment Ia), adult crayfish were observed in the cover significantly more often. Juvenile crayfish were observed in the open area significantly more often, but burrowed into the gravel to provide a measure of darkness (shadow) and tactile sensation. Experiment Ib presented crayfish with a choice between a dark thigmotactic cover and an open area; both adults and juveniles were observed in the cover significantly more than in the open area. In Experiment II, crayfish were given a simultaneous choice among a clear thigmotactic cover, a dark thigmotactic cover, and open area. Both age groups were observed in the dark cover more often than the other 2 conditions and were never observed in the clear cover. Although a clear cover was acceptable to adult crayfish (Experiment Ia), it was not acceptable to juveniles. Adults appeared to prefer dark cover to clear cover; juveniles altered their environment to provide shadowed cover, rather than utilize a clear cover. Thus, juveniles also appear to prefer "darkness" and will alter their environment accordingly. In Experiment III, crayfish were given a simultaneous choice among: a reduced-thigmotactic dark cover; a clear thigmotactic cover; and open area. Adults chose the large dark cover significantly more than the open area, but not significantly more than the clear thigmotactic cover. Adults that used the clear thigmotactic cover pulled gravel into their shelters, thus providing a measure of darkness. Juveniles chose the large dark cover significantly more than either the clear thigmotactic cover or the open area. Darkness appears to be the controlling factor in cover-seeking of juveniles; adults will modify cover providing only thigmotactic cues when dark thigmotactic cover is absent. Darkness plus thigmotactic cues, however, is the most preferred cover for both age groups.


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