Chaetognaths are carnivores and swim very actively in order to catch prey. They detect their prey by sensing movement with the ciliar tufts on their body. The chaetognath uses its hooks to grab prey, which is usually a small copepod whose nicely rounded shape makes it easy for the chaetognath to swallow. However, arrow worms can feed on anything that is of a certain size, including fish larvae, other chaetognath species, and even phytoplankton. The function of the teeth is not known. Arrow worms appear to immobilize their prey after capture. Tetrodotoxin, which is probably synthesized by the bacterium Vibrio algynoliticus, has been isolated from the head of several chaetognath species. How chaetognaths acquire this bacterium is still unknown, but tetrodoxin can cause immobilization. However, direct observations of feeding chaetognaths are very scarce, and it is not known how many species of chaetognaths may have or use the venom.
Digestion is rapid, and it is very rare that arrow worms are found with full guts. It is estimated that chaetognaths consume between two and 50 prey in a day, the latter number coming from laboratory experiments to establish the worms' food saturation point. Chaetognaths play an important part in the food chain. Estimates have shown that they comprise 30% of the biomass of copepods. Chaetognaths are mainly eaten by fishes, but they are also prey for larger carnivorous animals. Chaetognaths do acquire parasites from prey but not very frequently; the most common parasites ingested by chaetognaths are trematodes, nematodes, and cestodes (helminths), although the latter occurs very infrequently. There is no host-specifc parasite known in chaetognaths, which is remarkable for such an old group.
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