DISTRIBUTION AND REPRODUCTIVE-BIOLOGY OF THE BLACKCHIN GUITARFISH, RHINOBATOS-CEMICULUS (PISCES, RHINOBATIDAE), IN TUNISIAN WATERS (CENTRALMEDITERRANEAN)

Aspects of the reproductive biology of Rhinobatos cemiculus are descri bed from specimens collected in Tunisian waters. Sizes at first sexual maturity of males and females are 1000 mm and 1100 mm total length (T L), respectively. Adult females are generally larger than adult males, the maximum TL for males and females being 1920 mm and 2300 mm, respe ctively. The smallest gravid female observed was 1220 mm TL. R. cemicu lus is an aplacental viviparous species, with each female having two o varies and two uteri, both functional. Ripe oocytes in the ovaries, ov a, embryos and fully developed fetuses in the uteri are symmetrically distributed. The gestation period could last for a maximum of eight mo nths. Vitellogenesis proceeds in parallel with gestation, and at the t ime of parturition a crop of ripe oocytes is ready to be ovulated. Ovu lation and parturition occur during winter and summer, respectively. T here is probably one litter per year. The mean TL and mean weight of f ully developed fetuses are 39.6 mm and 115.1 g, respectively. A comput ed chemical balance of development, based on the mean dry weights of f ully developed fetuses and ripe oocytes, is 1.01. This low value is du e to the fact that R. cemiculus is purely a lecithotrophic species. Fe cundity ranges from 5 to 12 young per litter. Ovarian fecundity and ut erine fecundity are slightly correlated with the size of females. Fema les are more numerous than males in the total sample, as well as in ut ero and at the juvenile stage; however, this is not the case for subad ults and adults. This phenomenon is probably due to segregation of the sexes at different depths during certain stages of the reproductive c ycle rather than to a high rate of mortality among subadult and adult females.