Fertilization
and Development of Chaetognaths
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).
Their phylogenetic position is considered to be between protostomes and
deuterostomes.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).
