Eupolymnia nebulosa (Montagu) is a widely distributed terebellid polychaete. It builds its tubes in the coastal shelf, always in areas with mixed soft and hard bottoms. From a long-term survey carried out in the Bay of Banyuls, France (NW Mediterranean), the coincidence of an advancement of the spawning period with a delay of the thermocline breakdown has been pointed out. We postulate that a persistent high temperature could act on gamete development by stimulating oocyte growth, subsequently causing the advancement of cocoon release. During 1992 and 1993, we have used a between-individuals experimental approach based on: (1) the use of oocyte "net" growth; (2) the identification of differences in oocyte growth rate among females; (3) the determination of non significantly different pairs of "net" oocyte size distributions at the beginning of the experiment to compare them at the end. The effect of temperature on oocyte growth was not detected at a population level.However, a positive individual response to the maintenance of a high temperature has been demonstrated. Thus, the lack of a significant response by the population does not imply the absence of any response by individuals. We also propose a model of oocyte growth dynamics interacting with temperature. The model is based on our experimental results, but also on the following previous observations: (1) the occupation by oogenesis and oocyte growth of periods of both rising and declining temperature; and (2) the dispersed appearance of oocyte size distributions at the onset of the spawning period.
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