CONSEQUENCES OF OOCYTE FORM MODIFICATIONS IN EUPOLYMNIA-NEBULOSA (ANNELIDA, POLYCHAETA)

The changes in form of intracoelomic oocytes during their developmenta l stages and the change from mature to the external fertilized (i.e., egg) phases during the spawning process were analyzed in Eupolymnia ne bulosa with reference to the life-history strategy of the species. Acc urate description of the real form of both oocytes and eggs was the ob jective of the study. All developmental stages of oocytes floating fre ely in the coelom (solitary oocytes) showed a flattened form. Increase in oocyte thickness was not reflected in a proportional increase in d iameter. Therefore, by simply measuring diameters, a significant compo nent of oocyte growth would not have been recorded. Different relation ships between diameter and thickness of oocytes for the Mediterranean (slope=0.436, intercept=-4.507) and English Channel (slope=0.321, inte rcept=-2.199) populations of E. nebulosa have been observed. The impli cations of this difference for the speciation problem of the ''cosmopo litan'' E. nebulosa are discussed. The development of flattened oocyte s into spherical newly spawned eggs has also been noted. Although no d irect demonstration has been made, our results provide strong supporti ng evidence for the operation of a size-dependent selection mechanism during the spawning process. This mechanism can be directly linked wit h the life-cycle strategy of the Mediterranean E. nebulosa populations , while the implications of its existence in the English Channel popul ations remain unclear. The results demonstrate the importance of consi dering the real form of gametes when dealing with the study of life-hi story strategies (viz. oocyte growth linked to different environmental or endogenous control mechanisms or to different spawning mechanisms) .