Consequences of oocyte form modifications inEupolymnia nebulosa(Annelida; Polychaeta)

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Abstract
The changes in form of intracoelomic oocytes during their developmental stages and the change from mature to the external fertilized (i.e., egg) phases during the spawning process were analyzed in Eupolymnia nebulosa with reference to the life-history strategy of the species. Accurate description of the real form of both oocytes and eggs was the objective of the study. All developmental stages of oocytes floating freely in the coelom (solitary oocytes) showed a flattened form. Increase in oocyte thickness was not reflected in a proportional increase in diameter. Therefore, by simply measuring diameters, a significant component of oocyte growth would not have been recorded. Different relationships between diameter and thickness of oocytes for the Mediterranean (slope=0.436, intercept=-4.507) and English Channel, (slope=0.321, intercepe-2.199) populations of E. nebulosa have been observed. The implications of this difference for the speciation problem of the “cosmopolitan” E. nebulosa are discussed. The development of flattened oocytes into spherical newly spawned eggs has also been noted. Although no direct demonstration has been made, our results provide strong supporting evidence for the operation of a size-dependent selection mechanism during the spawning process. This mechanism can be directly linked with the life-cycle strategy of the Mediterranean E. nebulosa populations, while the implications of its existence in the English Channel populations remain unclear. The results demonstrate the importance of considering the real form of gametes when dealing with the study of life-history strategies (viz. oocyte growth linked to different environmental or endogenous control mechanisms or to different spawning mechanisms).