Gonads are not well defined in either sex. Gametogenesis is of an extragonadian type. M. sanguinea has asynchronous spermatogenesis and oogenesis The ovaries of M. sanguinea are discrete and consist of coelomic germ-cell clusters surrounded by a thin envelope of follicle cells derived from the peritoneum. Late previtellogenic oocytes detach from the clusters and float free in the coelomic cavity where they undergo vitellogenesis as solitary cells. Oocytes that had completed vitellogenesis measure 280�C300��m. The same diameter of mature oocytes was measured in females collected in the Venice Lagoon [15]. The cytoplasmic material of the mature oocytes is asymmetrically distributed; large lipid droplets and large yolk spheres occupy the vegetal pole of the oocyte while smaller yolk spheres are situated in the animal hemisphere. The same type of eggs was observed in many other species of polychaetes such as Nereis, Platynereis, and Diopatra [17, 18]. It can be hypothesized that the differential distribution of cytoplasmic components leads to qualitative differences in blastomere cytoplasm. Elsewhere it has commonly been thought that these differences are responsible for the process of cell diversification and embryonic axis formation in early stages of embryonic development [19]. As for the majority of polychaetes [20], the male germ cells in M. sanguinea are released from the peritoneum into the coelomic cavity in mulberry-like clusters at an early stage of spermatogenesis.Contrary to what occurs in many Eunicidae [21, 22] we do not observe modifications characteristics of epitoky or shizogamy in the adults of Marphysa sanguinea from the Lagoon of Tunis. Epitokous metamorphosis was not observed too in another population of M. sanguinea from the Venice Lagoon [15]. According to Prevedelli et al. [15], the disappearance of the epitokal phase in species that colonize brackish habitats seems to be generalized. The suppression of epitoky is a common trend in estuarial species [9�C11] and is probably related to a reduction in the dispersal phase [7]; species living in unpredictable habitats such as brackish environments tend to limit spatially the gametes emission.M. sanguinea displays a seasonal and synchronous reproduction in the Lagoon of Tunis. We recorded a steady increase of the mean oocyte diameter from January to March indicating that this period corresponded to the most intense reproductive period of reproduction. However, we also found a relatively high proportion of females containing mature oocytes from November to January; so, we can consider that the reproductive period extends from November to March. The percentage of mature males was maximal from December to April. So, ovogenesis and spermatogenesis phases were quite synchronous. The reproductive cycle of M. sanguinea in the Lagoon of Tunis is very similar to that described for this species in the Venice Lagoon. Indeed Prevedelli et al.