Etude du cycle biologique annuel d'une population de Sphaeroma serratum (Isopode, Flabellifere, Sphaeromatidae) du Bassin d'Arcachon

The life-cycle of Sphaeroma serratum is studied in a population occurring in Arcachon, by means of samples collected monthly from January 1973 to September 1976. The following categories were distinguished: the sexually undifferentiated larvae, the juvenile and adult males, the females at sexual repose (stage 1), the ovigerous females (stage 2), the females with full brood pouches (stage 3), the females with empty brood pouches (stage 4). The sexual activity of females lasts from December (first advent of stage 2 females) until September (disappearance of stage 3 females; predominance of stage 1 females). Numerous juvenile males are observed from August to March. Puberty moults are very frequent from November to March and a great number of adult males is found during the breeding season. The larvae, present in the samples during the whole year, are more numerous from June to October. At the end of this period, the sex of the young born at the beginning of the summer can be determined. The females produce only one brood per year. It seems that the life span of males and females is about two years. The Arcachon population shows a typical annual variation in the sex-ratio: the males generally remain predominant. Comparison of the results obtained during different years shows that a temperature increase has an accelerating effect on the ovary in vitellogenesis but does not act on the ovary in previtellogenesis, while it influences the duration of embryonic development. In the area of geographical distribution of S. serratum, the region of Arcachon appears like a transitional zone between northern regions with a short breeding period and southern regions in which the rise in temperature leads to a reduction of the winter sexual repose.