ABSTRACT: Environmental factors influencing the rate of oogenesis in the seasonal cycle of the semelparous polychaete Nereis (Neanthes) virens Sars have been investigated in specimens reared under commercial conditions (concrete tanks under natural light supplied with a mixture of power station heated and ambient sea water to maintain ca 18°C all year round). The rate of oocyte growth is strongly influenced by the external photoperiod and the responses to static 24 h LD (light-dark) cycles have been investigated. Oocyte growth is more rapid when specimens are exposed to a 24 h LD cycle in which the photophase is below a critical value between 12 and 13 h. The critical photoperiod transition in natural environments is therefore at the autumn equinox. N. virens are able to respond to transfer between LD 16:8 h and LD 8:16 h at all times of the year, which suggests the operation of a continuously consulted system of photoperiodic control. The rate of oocyte growth in competent specimens is a linear function of the number of LD 8:16 h cycles experienced. The critical photoperiod divides the solar year into 2 nearly equal periods, with the short day period (winter) favouring oocyte growth. There is a synergism between the effects of photoperiod and seasonally reducing temperature. During the calendar winter transition to LD 8:16 h or other LD cycles below the critical value initiates rapid oocyte growth whatever the temperature but in the calendar summer this only occurs if the external temperature is also reduced to 12°C or below. The response to photoperiod and temperature is moderated by internal factors and under commercial production systems elevated rates of oocyte growth can only be achieved in females that are about 1 yr old. The oocyte growth induced by exposure to LD 8:16 h light cycles and low temperature is similar to, but slightly less than, that induced by hormone deprivation, i.e. supra-oesophageal ganglion ablation. The results are discussed in relation to the control of seasonal cycles in Nereidae, the role of the endocrine system and in the context of the life history theory for long lived semelparous organisms with variable age at maturity.

KEY WORDS: Nereis (Neanthes) virens · Photoperiodism · Critical day length · Polychaeta · Oogenesis · Temperature · Seasonal reproduction · Semelparity · Nereidae

Published in MEPS Vol. 172 (1998) on October 22
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1998