Seasonal variation of the locomotor behavior of two sympatric species, Orchestia montagui and Orchestia gammarellus (Amphipoda, Talitridae), under aperiodic regimens

摘要

Locomotor activity rhythms of Orchestia montagui and Orchestia gammarellus were investigated in populations collected from Bizerte lagoon banks. The locomotor rhythm of these two species was recorded in individual animals under continuous light and constant darkness over four seasons. Results revealed that the mortality was higher under continuous light than constant darkness. Whatever the season and species, a great variability of locomotor rhythm ranging from unimodal patterns to multimodal patterns via bimodal patterns was observed. In addition, double-plotted actograms obtained for the two species showed the presence of a clear drift to the right. This drift was more important under continuous light than constant darkness, lengthening thus the circadian period. Furthermore, the ultradian component was observed. Whatever the season, circadian rhythmicity was shorter under continuous light than constant darkness. Individuals of O. montagui were less active under continuous light than constant darkness whatever the season; whereas, those of O. gammarellus were more active under continuous light than constant darkness, especially in summer and autumn. The seasonal locomotor activity pattern variability of these sympatric species will be discussed as an adaptive strategy to cope with environmental conditions.