Endogenous locomotor activity rhythm of two sympatric species of talitrids (Crustacea, Amphipoda) from a sandy beach of Tuscany, Italy

Abstract. The biological clock of living organisms, which is an evolutionary effect of their temporal adaptation to the environment, is known to be entrained by natural periodic variables; the behavioural and physiological response to these variables can be traced by laboratory recordings under constant conditions. Adult individuals of two sympatric talitrid species (Talitrus saltator and Orchestia gammarellus), inhabiting different niches of the same coastal ecosystem in the Maremma Regional Natural Park (Grosseto, Italy), were studied for their locomotor activity rhythm. Recordings were carried out by an automatic electronic apparatus and lasted for 21 days in constant darkness and controlled temperature and humidity. At the end of the recording sessions, the cephalic length and antennal tagmas of the tested specimens were measured and sex determined. Statistical analysis showed a relevant difference in rhythm expression between the two species, which was related to their different physiology and ecology and highlighted a differentiated adaptation to terrestrial life of the two species. O. gammarellus showed a longer mean period and T. saltator a higher mean definition, confirming a finer use of the biological clock in the latter species. This study leads to the general conclusion of a higher dependence of T. saltator from the circadian cycle as compared to O. gammarellus. This behaviour can be directly linked with the colonisation of the aphytoic sandy beach by T. saltator, while O. gammarellus in this area occupies the vegetation of the dune slack, accordingly showing a more variable rhythm, less dependant on daily cycle.