THE IMPACT OF NOCTURNAL LIGHT-PULSES ON THE ACTIVITY PATTERN OF TERRESTRIAL SNAILS (HELIX-LUCORUM) ENTRAINED TO A PHOTOPERIOD OF 12 H LIGHT/12 H DARK

The effects of nocturnal light pulses, ''asymmetrical skeleton photoperiod regimes,'' on the circadian locomotor activity rhythm in adults of the edible snail Helix lucorum previously entrained to a photoperiod regime of 12 h light : 12 h dark were examined. In all experiments the snails showed a clear bimodal rhythm of behaviour, with peaks at both dusk and dawn. Shifts occurred according to the timing of 2-h light pulses applied in the scotophase and were related to the phase of snails' entrained rhythms. It is suggested that a minimum dark period (up to 10 h) is necessary for H. lucorum to retain its normal daily rhythmicity.