Spermatogenesis in appendicularians

Spermatogenesis was studied in species of oikopleurid and fritillarid appendicularians, particularly in O. dioica reared in the laboratory of Villefranche-sur-Mer. In all species the spermatogenesis proceeds highly synchronised in the whole testis, thanks to the presence of syncytial structures. Two models were recognised: 1) In O. dioica, a large cytoplasmic area contains most of the organelles and the germ cell nuclei hanging at the periphery. The germ cells undergo series of mitoses, meiosis and spermiogenesis, while they remain attached to the common cytoplasmic area by bridges through which mtochondria migrate. Finally, the syncytial area reduces markedly and spermatozoa segegate and mature all together. 2) In F. pellucida and F. borealis the syncytium contains organelles and two populations of large and small nuclei. The large one produces "nuage" and occupies the centre of wide areas at the periphery of which the small nuclei of germ cells lie. As spermatogenesis progresses, the large nuclei and the cytoplasmic areas reduce in volume, while germ cells, still attached to syncytium, increase highly in number and undergo meiosis and spermiogenesis. Finally, the large nuclei degenerate while the late spermatids, all possessing a single mitochondrion, segregate. In all species, spermatozoa are small, oligopyrene and flagellate. They mature synchronised and, although in great number, they are appropriately arranged, thus to utilise better the space for the incoming spawing.