Seasonal variation of locomotor activity rhythm of<i>Orchestia montagui</i>in the supralittoral zone of Bizerte lagoon (North of Tunisia)

Abstract

Locomotor activity rhythms of Orchestia montagui were investigated in a population from the supralittoral zone of Bizerte lagoon. These rhythms were recorded in individual animals over the four seasons under two simultaneous experimental regimens; during the first week, the animals were kept in light–dark cycle in phase with the natural diel cycle. During the second week, animals were maintained in constant darkness (DD). Results revealed that, whatever the season, actograms and mean activity curves showed globally that individuals of O. montagui concentrated their activity during the experimental and subjective day. In addition, animals exhibited a diurnal circadian rhythm of locomotor activity with an ultradian component. Under light–dark cycle, circadian periods determined by periodogram analysis in four seasons were appreciably similar and were close to 24 h. While under constant darkness, circadian period was longer in winter (τDD = 25 h 54′ ± 1 h 14′) and summer (τDD = 25 h 47′ ± 0 h 45′) than in spring (τDD = 24 h 26′ ± 1 h 51′) and autumn (τDD = 24 h 44′ ± 2 h 22′). In addition, the study of the mean activity times calculated for four seasons showed that, whatever the regimen imposed, the most important activity of individuals was observed in summer and it was equal to 11 h 51′ ± 5 h 10′ and 16 h 27′ ± 5 h 44′ under entraining conditions and constant darkness, respectively. With reference to environmental stability and variability, the differences of locomotor activity characteristics observed are explained as a need for plasticity to adapt to environmental changes.