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Abstract. Biological zonation in brackish habitats is usually related to the gradients of several factors, of which salinity is often taken as the most important. In contrast, recent papers, mainly on Mediterranean lagoons, have emphasized the role of other factors such as water movement and trophic status. In this paper, the distribution of polychaetes in the Orbetello lagoon (a poly‐hyperhaline pond on the northern Tyrrhenian coast) is compared with the gradients of a number of environmental parameters. Polychaete distribution turned out to be under the control of a variety of highly interrelated factors. The study of biotic zonation as a function of a single driving factor is thus overly simplistic even when dealing with ‘simple’ environments, as lagoons are often defined. Several factors and their synergistic effects must always be taken into account. Summary A total of 79 species was collected from 13 sites in the Orbetello lagoon, comprising both brackish‐water and marine species as well as a large number of species typical of organically enriched habitats. Through correspondence analysis, three major polychaete assemblages were recognized, linked to three different zones of the lagoon: 1) the channels connecting the lagoon to the sea; 2) the western lagoon; 3) the eastern lagoon (including a small part of the western lagoon situated near the town of Orbetello). Such a zonation was shown ‐ by Trend Surface and linear correlation analyses ‐ to be consistent with the spatial trends of a number of environmental parameters such as vivification/confinement, water movement, dissolved oxygen, poikilohalinity, and poikilothermy (in the order of importance); in contrast, salinity (minimal values), sediment grain size, and primary productivity showed little or no relation to polychaete distribution. However, almost all environmental parameters were significantly correlated with each other, thus making it impossible to identify the principal factor. This demonstrates that biotic zonation cannot be explained as the resultant of the action of a single driving factor (e. g., salinity or confinement), even in environments which are usually considered to be relatively simple from a synecological point of view, such as brackish habitats. Both the synergism among the various abiotic factors and the influence of biotic interactions must always be taken into account.

Abortive eggs were found in parthenogenetic ovarioles of Megoura viciae Buckton when environmental conditions favour the appearance of males. By means of different rearing environments, eight experimental categories of parthenogenetic females were obtained. In each of these categories male sex determination took place in a different position within the ovariole. The position in which abortive eggs appeared always coincided with that in which the eggs were determined as males. RÉSUMÉ VIABILITÉ DE L'ŒUF ET DÉTERMINATION DU SEXE CHEZ MEGOURA VICIAE Dans les ovarioles parthénogénétiques de Megoura viciae, les œufs abortifs ne s'observent qu'avec des conditions favorables à la détermination du sexe mâle. Par modification des conditions d'élevage on a reálisé huit séries expérimentales de femelles parthénogénétiques. Les séries différaient par le moment de l'ontogénèse où une photopériode à photophase courte succédait à une photopériode à photophase longue. Ainsi le sexe mâle était déterminé pour chaque série, à un niveau différent de l'ovariole. L'examen des ovarioles a montré que la position de l'œeuf abortif coïncide toujours avec celle où a lieu la détermination du sexe mâle. Il n'y a jamais formation d'œufs abortifs, ni apparition de mâle dans le premier ovocyte de chaque ovariole. Des hypothèses ont été formulées pour expliquer ce phénomène.

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