Cover-seeking behavior and shelter use by juvenile and adult crayfish, Procambarus clarkii: Potential importance in species invasion

Cover-seeking behavior of juvenile and adult crayfish, Procambarus clarkii, was examined in relation to the importance of darkness (shadow) and thigmo tactic cues. Crayfish were observed individually in small aquaria 5 times p er day, with at least 30 min between observations, for a 3-day period. Thei r positions were recorded as either within or outside shelter(s) provided. Experiment Ia presented crayfish with a choice between a clear thigmotactic shelter or open area. Adults were observed in the open area significantly more than in the shelter. Juveniles were also observed in the open area mor e often, but the difference was not statistically significant. Experiment I b presented crayfish with a choice between a dark thigmotactic shelter and open area. Adults were observed more often in the open area, but the differ ence was not statistically significant. Juveniles were observed in the dark thigmotactic shelter significantly more often than in the open area. In Experiment II, crayfish were given a simultaneous choice among a clear t higmotactic shelter, a dark thigmotactic shelter, and open area. Adults wer e observed in both open area and dark thigmotactic shelter significantly mo re often than in the clear thigmotactic shelter. Juveniles were observed in the dark thigmotactic shelter significantly more often than in the open ar ea and were never observed in the clear shelter. For Experiment III, crayfish were given a simultaneous choice among a dark reduced-thigmotactic shelter, a clear thigmotactic shelter, and open area. Adults chose the dark reduced-thigmotactic shelter significantly more often than the clear thigmotactic shelter, but not significantly more often than the open area. Juveniles chose the dark reduced-thigmotactic shelter signi ficantly more often than either the open area or the clear thigmotactic she lter. Experiment IV gave crayfish a choice among a dark reduced-thigmotactic shel ter, a dark thigmotactic shelter, and open area. Adults chose the dark redu ced-thigmotactic shelter significantly more often. Juveniles chose the dark thigmotactic shelter significantly more often than the dark reduced-thigmo tactic shelter, but not significantly more often than the open area. Darkne ss appears to be the controlling factor in the cover-seeking behavior of bo th juvenile;md adult P. clarkii. The preferential use of larger shelters by adult P. clarkii could reduce th e limiting effects of shelter availability and might reduce competition for shelter between adults and juveniles, increasing juvenile recruitment. Dif ferences observed in shelter use between adult crayfishes, P. clarkii and O . rusticus, which prefer the same shelter as juveniles, may give P. clarkii an advantage over O. rusticus in a species invasion.