The light-response rhythm and seasonal entrainment of the endogenous circadian locomotor rhythm ofTalitrus saltator(Crustacea: Amphipoda)

Abstract

Talitrus saltator, a supralittoral, sand-beach amphipod, exhibits a nocturnal, circadian, locomotor rhythm entrained by the natural photoperiod (nLD). A phase-response curve (PRC) (light pulse: 2h/400 lx) of the free-running rhythm exhibits a conventional shape; steady-state delay shifts of approximately 3·5 h occur in response to light pulses around subjective dusk, with similar advance shifts when exposed to light pulses around dawn. The seasonal variation of the overt expression of the rhythm indicates that entrainment is predominantly non-parametric, while the constant phase-angle difference between the peak of locomotor activity and dawn confirms the latter as the effective field signal for entrainment. The qualitative relationship between the time scale of the PRC and nLD, together with the absence of any seasonal variation in the periodicity of the free-running rhythm (τ), suggests that entrainment occurs through the interaction of phase shifts defined by the PRC, essentially in response to the systematically changing photoperiod.