Seasonal variation of the endogenous rhythm in two sympatric amphipods: Talitrus saltator and Talorchestia deshayesii from Bizerte beach (northern Tunisia)

Abstract

The locomotor activity rhythm of the supralittoral sandhoppers, Talitrus saltator and Talorchestia deshayesii, was investigated over four seasons to reveal the impact of environmental variation on these sympatric species collected from Bizerte beach (37°19′N–9°51′E). For each season, thirty adult individuals were collected by hand and transferred to individual actographs equipped with an infrared recording system, where they were subjected to two consecutive experimental light/dark regimens (natural light/dark cycle (LD) and continuous darkness (DD)) in a controlled environment cabinet at 18°C ± 0.5°C. Periodogram analysis of the resulting actograms indicates a predominantly circadian rhythm with a period close to 24 h, with a less predictable semi-diurnal component of about 12 h in both T. saltator and T. deshayesii. A seasonal comparison of the various rhythm characteristics showed that the greater mortality was observed in winter (36.6%) and in spring (70%), respectively, among the population of T. saltator and T. deshayesii; whereas, in summer, this rate was equal to zero whatever the species. Moreover, irrespective of the season and the photoperiodic regimen, circadian rhythmicity was more important than the semi-diurnal one for all seasons and both under natural Light/Dark (nLD) cycle and free-running conditions. In addition, the stability of the circadian rhythm was better defined in spring, whatever the photoperiodic regimen imposed and the species. Similarities and differences observed between T. saltator and T. deshayesii over four seasons reflect an ecological plasticity of their biological clock adapted to different changing environmental conditions.