Interaction between photoperiod and an endogenous seasonal factor in influencing the diel locomotor activity of the benthic polychaete Nereis virens Sars

The locomotor activity of Nereis virens Sars associated with food prospecting was investigated in response to photoperiod and season using an actograph. Experimental animals which had been reared under natural photoperiods were exposed to two constant photoperiodic treatments, LD 16:8 and LD 8:16, in both the autumn and winter and in the absence of tidal entrainment. Autocorrelation analysis of rhythmicity showed that during the autumn, animals under the LD 16:8 photoperiod displayed a strong nocturnal rhythm of activity, whereas animals under the LD 8:16 photoperiod showed only a weak nocturnal activity rhythm. This is believed to represent an autumn feeding cessation that is triggered when the animals pass through a critical photoperiod LDcrit 12. Later in the winter, however, animals exposed to both photoperiodic treatments showed strong rhythms of foraging activity irrespective of the imposed photoperiod. It is suggested that the autumn cessation may maximize the fitness of N. virens, a spring-breeding semelparous organism, by reducing risk during gamete maturation, while spontaneous resurgence of activity after the winter solstice permits animals that are not physiologically competent to spawn to accrue further metabolic reserves. This response is believed to be initiated by a seasonal (possibly circannual) endogenous oscillator or interval timer.