Fitness components of seasonal reproduction: an analysis using Nereis virens as a life history model

The reproductive cycles of the majority of marine invertebrates are strongl y seasonal with highly synchronised patterns of spawning. It has become acc epted that a distinction can be made between proximate factors and ultimate factors in the causation of seasonal reproduction. We briefly review these concepts and the hypotheses that have been developed about the nature of t he ultimate factors that cause the fitness benefit and that 'explain' the a daptive significance of seasonal reproduction. We use a model based on the life history of the polychaete Nereis virens and demonstrate that seasonal reproduction may have impacts on adult components of fitness (i.e. reproduc tive value at age of first maturity), as well as on fertilisation rate and survival to age at first maturity. The results are discussed in the context of the programme PNDR and the related UK Natural Environment Research Coun cil thematic programme 'Developmental Physiology of Marine Organisms (NERC- DEMA). (C) 2000 Ifremer/CNRS/IRD/Editions scientifiques et medicales Elsevi er SAS.