Daily activity of the white-clawed crayfish, Austropotamobius pallipes (Lereboullet): a comparison between field and laboratory studies

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Abstract
The daily activity of the threatened crayfish, Austropotamobius pallipes (Lereboullet), was studied both in the field, through direct counts of active animals along a stream section, and in the laboratory, by means of actographs. In the field, crayfish were mostly found after dawn, with a slight but significant difference among seasons. In winter, only two specimens were seen to be active. Distinguishing among size classes, medium-sized crayfish were more abundant in spring and summer, while large ones mostly occurred in autumn. The former always concentrated their activity at night-time; to the contrary, larger specimens were more active late at night in spring and at dusk in autumn. Sex-ratio, analysed only in summer, was balanced throughout the 24h. Nocturnal activity, although differing among seasons, was similar in size classes, temperatures and sexes. In the laboratory, activity (evaluated as photo-cell interruptions) increased with temperature and occurrence throughout a 24-h cycle (the highest peak shifting from midnight to dusk). Only in the laboratory, a low peak occurred at dawn, but disappeared when a shelter was available. The interaction between the response of A. pallipes to temperature and its nocturnal habit is examined. Questions still remain on the adaptive value of the environmental factors influencing the cycle of freshwater decapods' activity.