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).