Endogenous locomotor activity rhythm of two sympatric species of Talitrids (Crustacea, Amphipoda) from the supralittoral zone of Bizerte lagoon (Northern Tunisia)

Samples of fresh adults of two sympatric species, Orchestia montagui and Orchestia gammarellus, from the supralittoral zone of Bizerte lagoon (Northern of Tunisia) at Menzel Jmil (37 degrees 138N 09 degrees 551E) were housed in a controlled environment cabinet. Locomotor activity rhythms were recorded in individual animals in spring, at a constant temperature of 18 +/- 1 degrees C; firstly, under lightdark (LD) cycle and constant darkness (DD), and secondly, under continuous light with high (140 lux) and low (5 lux) luminous intensities during the first and second week, respectively. Results showed that whatever the regimen imposed and species studied, periodogram analysis indicated a circadian rhythm with the presence of ultradian component close to 12h. Double plotted actograms and wave forms highlighted two different locomotor behaviors; in fact, individuals of O. montagui are active during photophase when placed under LD cycle whereas, individuals of O. gammarellus concentrated their activity during the scotophase. The comparison of rhythm characteristics showed that the highest rhythmicity was observed under entraining condition both for O. montagui and O. gammarellus with 77% and 60%, respectively. Moreover, the most important stability and activity time were better defined under continuous light with high luminous intensity for O. montagui and entraining conditions for O. gammarellus. On the other hand, results showed a highly standard deviation explaining the important intraspecific variation. We concluded thus the presence of diurnal and nocturnal trends adopted by these two talitrids. This strategy allowed these concurrent species to minimize their interactions. These two sympatric species, which have the same food preferences, do not share the same temporal niche. This is a strategy to allow the diminution of interspecific competition intensity. This result is discussed in this paper.