Seasonal variation of locomotor activity rhythm of Orchestia gammarellus in the supralittoral zone of Ghar Melh lagoon (North-East of Tunisia)

Abstract

Locomotor activity rhythm is recorded in individual animals of O. gammarellus from the supralittoral zone of the old harbor of Ghar El Melh lagoon. The rhythm was investigated over the four seasons under two simultaneous experimental regimens; during the first week, animals are kept in light–dark cycle (nLD) in phase with natural diel cycle. During the second week, animals are maintained in constant darkness (DD). Analysis of actograms and waveform curves showed globally that individuals of O. gammarellus concentrated their activity during the experimental and subjective night, respectively during the first and second week of experiment. In addition, animals exhibited circadian and ultradian components. Under light–dark cycle, period is significantly shorter in autumn (τ LD = 23h43′ ± 0h34′) than in the other seasons. Whereas, under constant darkness, the situation is reversed and we noted that the longest circadian period is observed in autumn (τ DD = 26h ± 1h28′). The study of the mean activity times calculated for four seasons showed that whatever the regimen imposed, the most important activity of individuals, observed during autumn, is equal to 15 h 30′ ± 3h24′ and 14h40′ ± 1h21′ under light–dark cycle and constant darkness respectively. In addition, the locomotor activity rhythm is more stable in summer whatever the regimen is.