Arrow worms are a unique phylum of carnivorous plankton, with about 120 known species. They are abundant in the water column, with an estimated global population of around 340 billion. Their chitinous spines are used for hunting and injecting toxins. They are considered a model for early bilaterians and are closely related to nematodes.
Arrow worms are a very unusual group of “worms” not obviously related to any other worm or other animal. They make up their own phylum, Chaetognatha, one animal phylum among about 37 in total. In terms of numbers, arrowworms are among the most abundant phyla on the planet—only arthropods, nematodes, and a few others even come close. Arrowworms inhabit the water column, usually in the pelagic zone, within 200 m (656 ft), where most plankton live, although they can be found down to the lower mesopelagic zone, 1000 m (3,280 ft). Alongside copepods, the crustaceans “insects of the sea”, ketognaths are the most common form of plankton.
Their name – Chaetognatha – comes from the Greek chaite for long hair and gnathos for jaw. The long hair is a reference to the barbed and chitinous spines found in pairs on the head, their main hunting tool. The spines are held in a cap while swimming. Arrow worms are carnivores, eating all the plankton they can find. Their size is about 3-5 cm (1-2 inches), about the length of a fingernail, although some individuals can reach 10 cm (4 inches). There are only about 120 known species of ketognaths in 20 genera, but despite the small number of species, their abundance is difficult for the human mind to imagine.
We can give a rough estimate of the number of arrow worms worldwide. Measured densities range from about 1 to 30 individuals per cubic meter of illuminated water, with an average of five. Since the World Ocean has an area of about 340 million square kilometers, excluding Arctic and Antarctic waters, and the pelagic zone is 200 m deep, we can estimate a world total of about 340 billion, which is probably within the order of greatness of being correct. Interestingly, this also allows us to estimate that human biomass exceeds arrow worm biomass.
Because ketognaths are transparent, scientists can put them under a microscope and observe the entire process of their digestion. They estimate that arrow worms eat between 3 and 50 prey items per day, including numerous types of larvae, copepods and other small crustaceans, and other arrow worms. They pierce these organisms’ thin layers of protective tissue with their chitin spines, then inject them with toxins, such as the neurotoxin tetrodotoxin, to cause death.
Arrow worms are considered a good model for an early bilaterian. They are thought to be basal protostomes (one of the two major divisions of animals), although elements of their embryological development resemble deuterostomes. This is thought to be because they represent a very early offshoot of the protostomes from the deuterostomes. This event probably occurred long ago, in the Ediacaran period. Molecular and morphological studies have indicated that the ketognaths are most closely related to nematodes, and may in fact be related to the common ancestor of the ecdysozoans (the group comprising any animal that sheds its cuticle).
Protect your devices with Threat Protection by NordVPN
