Fertilization Process in the Arrow Worm Spadella cephaloptera (Chaetognatha)

To reveal the fertilization process of arrow worms, oocytes from fixed specimens of Spadella cephaloptera were isolated and observed by staining fluorescent nucleic acid dyes. Fully grown oocytes with a germinal vesicle (GV oocytes) are associated with an accessory fertilization cell (AFC), which is easily removed from the oocyte during isolation. Oocytes after germinal vesicle breakdown (GVBD oocytes) do not have an AFC and are at the metaphase of the first meiosis. In most cases GVBD oocytes contain a small bright spot that is regarded as a condensed sperm chromatin. In rare cases the sperm chromatin in GVBD oocytes has the form of a thin-thread (20 μm long) which corresponds to the sperm nucleus immediately after fertilization. Therefore, fertilization occurs at the first meiotic metaphase after disappearance of the AFC. Numerous zygotes from a single ovary have an hourglass shape, indicative of passage of ovulating eggs into the oviducal complex. Thus, ovulation occurs after fertilization in S. cephaloptera as reported in the genus Sagitta.