Given its ecological importance, the phylum Chaetognatha is surprisingly poorly known. Chaetognaths (commonly known as "arrow worms") are marine predators that typically locate their prey by detecting vibrations produced by copepods and other zooplankton, then use sharp hooks and teeth at the front of the body to grab their victims and immobilize them with neurotoxins. Chaetognaths, most of which are distinctly transparent, are important predators in many marine food webs. Around 120 to 125 species of chaetognaths are known. Most are planktonic, but a small number of species are benthic or live just above the ocean floor. Although species diversity is low, chaetognaths can be very abundant, sometimes dominating the biomass in mid-water plankton sampling tows. Many chaetognaths undergo daily vertical migrations, rising to surface waters at night and sinking downward during the day, possibly to avoid predators. These vertical movements are facilitated by ammonia-filled vacuolated cells in the trunk, which regulate buoyancy. (Brusca and Brusca 2003; Margulis and Chapman 2010; Jennings et al. 2010) At least one chaetognath species (the cosmopolitan Caecosagitta macrocephala, which is generally found below 700 meters) is bioluminescent (Haddock and Case 1994).
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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).