Chaetognaths commonly called " arrowworms " or " glass worms " are translucid, carnivorous animals shaped like torpedos. They are very abundant in seas and oceans, only outnumbered by copepods, and represent an important food source for fish. Most of the 150 species of chaetognaths are planctonic (Sagitta). The Chaetognath life cycle is about 1-3 months and a few benthic species (Spadella) can be raised easily in the laboratory. Chaetognaths are hermaphrodites but mate between individuals; their reproduction involves a spectacular exchange of packets of sperm. They are particularly well suited for studying gametogenesis and the germ line, as had been noted by Wilson in 1905 in " Cell Development and Inheritance ". They possess a particulate germ granule that is inherited in one of the first two blastomeres and by the first four germ cell precursors. The germ granule cycle and formation can be directly observed in Spadella and Sagitta, species which contain a Vasa-like protein (D.Carré, C.Djediat, C.Sardet (2002) Formation of a large vasa-positive Germ Granule and its inheritance by Germ Cells in the enigmatic Chaetognaths. Development, 129, 661-670).
To learn more about Chaetognath development see http://biodev.obs-vlfr.fr/recherche/biomarcell/choetognats/chaetognaths.htm Christian Sardet
FORMATION OF GERM CELLS IN CHAETOGNATHS THE LIFE CYCLE OF CHAETOGNATS To see videos and diagrams go to http://biodev.obs-vlfr.fr/recherche/biomarcell/choetognats/chaetognaths.htm Formation of a large Vasa-positive Germ Granule and its inheritance by Germ Cells in the enigmatic Chaetognaths For the full story download the PDF version of Danièle Carré, Chakib Djediat, Christian Sardet, Development, Development, 129, February 2002 . For videos and further info go to: http://biodev.obs-vlfr.fr/recherche/biomarcell/choetognats/germplasm.htm Ancient histological observations dating from a century ago described the presence in eggs of a large granule presumed to be a germ plasm and its probable inheritance in 4 primary germ cells (PGCs). Using videomicroscopy, electron microscopy and immunocytochemistry (labelling with anti-Vasa antibodies) we have followed the cycle of aggregation and dispersion of germ plasm and nuage material in eggs, embryos, PGCs and oocytes in several species of benthic (Spadella) and planctonic (Sagitta) chaetognaths. The germ plasm (green) is within and around the germinal vesicle (GV).During maturation it probably fragments into minute granules (1). At fertilization (2) and spawning (3) many small granules line the vegetal cortex (v) and then aggregate during amphimixy (4). They form a large granule at mitosis (see films)(5). This large granule is segregated into one of the first 2 blastomeres and continues to be inherited by only one blastomere until the 32 cell stage (7); it then splits into 2 at the 64 cell stage (8). The germ plasm is then found in four presumptive Primordial Germ Cells (pPGC's) at the tip of the archenteron in the gastrula (9). The four PGC's become the right and left male and female stem cells in the juvenile (10), which give rise to two ovaries and two testis in the adult.