The influence of photoperiod on oocyte growth and its role in the control of the reproductive cycle of the polychaete Harmothoe imbricata (L.)

The date of spawning of the first of the two batches of oocytes produced by Harmothoe imbricata each year is known to be influenced by both temperature and day length conditions during the preceeding period of vitellogenesis, acting directly on the rate of oocyte growth. The influence of day length is shown to be in the form of a critical day length effect, with days of 11 h of daylight or more acting as long days and advancing the date of spawning. It is suggested that this critical day length effect serves to synchronize the spawning of the first cohort of oocytes, and it may also entrain an endogenous time sense underlying the reproductive cycle of this species.