PHOTOPERIODIC CONTROL OF GROWTH AND REGENERATION IN IMMATURE JUVENILE NEREIS (NEANTHES) VIRENS
Last, K. S.; Olive, P. J. W. & Edwards, A. J.
Dept. of Marine Sciences and Coastal Management, University of Newcastle upon Tyne, Newcastle upon Tyne, UK. NE1 7RU.
Nereis (Neanthes) virens, like all nereids, has a strictly semelparous life cycle with a once per lifetime switch from somatic to reproductive growth. In this paper we explore the environmental influence of photoperiod on somatic growth rate in immature juvenile Nereis virens. Two fixed photoperiods were used in the initial experiments, long and short days denoted as LD 16:8 and LD 8:16 respectively where LD x:y represents the duration of the photophase x and the scotophase y. These LD cycles are close to the extremes experienced in the geographical location of the experimental material. All animals that experienced LD 16:8 during the ambient winter months after their birth in the spring showed elevated growth and enhanced segment proliferation rates. The rate of replacement of lost segments after ablation was similarly increased under LD 16:8. In addition, juveniles raised out of season through manipulation of temperature and photoperiod are shown to have a higher rate of segment addition when reared in long days (LD 16:8). The switch between somatic tissue growth and reproductive growth is an overt expression of a more covert response to the photoperiodic signal and is only expressed in animals that have reached a sufficiently advanced status of development. The most sensitive and easily assayed response to photoperiod is the spontaneous activity rate which has strong circadian components. This has enabled us to establish, through the use of automated activity recorders and time-lapse video film, a critical response curve. The critical photoperiod is around 12 hours of daylight with a progressive increase in activity from 12 to 16 hours of light per day. Nereis virens is highly sensitive to changes in photoperiod and such behavioural assays provide many further opportunities to elucidate the architecture of this response.