Chaetognathaarrow worms


The phylum Chaetognatha, also known as arrow worms, contains nearly 200 species of mostly planktonic, bilaterally symmetrical, coelomate, worm-like organisms. The phylum contains two orders, Phragmophora and Aphragmophora. The main difference between the two is the presence of ventral transverse muscle bands in Phragmophora, which are absent in Aphragmophora. Chaetognaths may be found in marine and some estuarine environments throughout the world. About a fifth of the total species are benthic, some living just above the deep ocean floor; these are often attached to the substrate by adhesive secretions. Chaetognaths may range from 1 mm to 12 cm in length and are typically transparent, although some deep-water species may be orange in color, and phragmophorids may be opaque, due to their musculature. The common name, arrow worms is derived from their streamlined appearance, with paired lateral fins and a single caudal (tail) fin, while their scientific name comes from the hooked set of jaws that protrude lateral to the mouth. These structures are used in prey capture, with chaetognaths feeding on a number of crustacean (mainly copepods) and fish (mainly larvae) species, which they track through daily vertical migrations in the water column (these migrations may also protect them from predators). Chaetognaths are hermaphroditic, and may undergo reciprocal, nonreciprocal, or self-fertilization. ("Chaetognatha", 2012; Brusca and Brusca, 2003; Goto and Yoshida, 1985; Margulis and Chapman, 2010; Shapiro, 2012; Zhang, 2011)

Geographic Range

Chaetognaths may be found in marine and some estuarine environments throughout the world, including polar and tropical regions. (Brusca and Brusca, 2003; Margulis and Chapman, 2010; Shapiro, 2012)


Chaetognaths are mainly planktonic organisms in marine and estuarine environments. About a fifth of the total species are benthic, some living just above the deep ocean floor. They are often found in great numbers, particularly in mid-water and neritic waters, and may be found in rock pools or associated with certain oceanic currents. (Brusca and Brusca, 2003; Margulis and Chapman, 2010; Ramel, 2012)

Systematic and Taxonomic History

Since first being recorded in 1775, chaetognaths have been classified many ways, including as molluscs, arthropods, and nematodes. They were elevated to their own phylum by the German zoologist Rudolf Leuckart in 1854, a decision which is supported by numerous morphological and embryological autapomorphies, indicated below. The monophyly of this phylum is widely accepted, and molecular phylogenetic analyses of 28S ribosomal DNA have supported the monophyly of its two constituent orders, Phragmophora and Aphragmophora (without the traditionally included family Pterosagittidae). (Brusca and Brusca, 2003; Leuckart, 1854; Littlewood, et al., 2004; Papillon, et al., 2006; Telford and Holland, 1993; Telford and Holland, 1997)

Chaetognaths were originally considered deuterostomes, based on embryological observations. All molecular phylogenetic analyses performed thus far, however, strongly support the placement of Chaetognatha within protostome animals. The precise phylogenetic placement of Chaetognatha within Protostomata remains somewhat confused today, as molecular analyses have suggested Chaetognatha is a sister group to all other protostomes, Spiralia, or phylum Priapulida (within the superphylum Ecdysozoa). Their potential identity as a basal sister group to such a large metazoan group makes further examination of their phylogenetic placement essential, as it has the potential to greatly contribute to our understanding of morphological and developmental characters found in some of the earliest bilaterian organisms. (Brands, 2009; Brusca and Brusca, 2003; Dunn, et al., 2008; Edgecombe, et al., 2011; Giribet, et al., 2007; Helmkampf, et al., 2008; Littlewood, et al., 2004; Matus, et al., 2006; Papillon, et al., 2004; Paps, et al., 2009)

  • Synapomorphies
    • Their mesoderm and coelom form by enterocoely.
    • Chaetognaths have chitinous retractable jaws.
    • Chaetognaths have multilayered epithelium on their body.
    • Chaetognaths have cephalic hoods.
    • These animals have retrocerebral organs and ciliary loops whose function is unknown.

Physical Description

Chaetognaths range from 1 mm to 12 cm in length and are typically transparent, although some deep-water species may be orange in color (carotenoid pigmentation), and phragmophorids may be opaque, due to their musculature. They are bilaterally symmetrical and have long, streamlined bodies, which may be divided into head, trunk, and tail regions. They have paired lateral fins and a single tail fin. The mouth is located ventrally on the head, and is set into a vestibule; this structure is typically associated with grasping spines or hooks, located laterally to the mouth, as well as teeth, which are in located at the front of the mouth. Some species have serrated hooks and/or cuspidate teeth. A hood (anterolateral body wall fold) may be pulled over the head to enclose the vestibule. (Brusca and Brusca, 2003; Ghirardelli, 1968; Margulis and Chapman, 2010; Shapiro, 2012)

Chaetognaths are covered in a thin, flexible cuticle on top of the epidermis. Epidermal cells are mainly squamous and have interlocking margins; they may be stratified. Epidermal cells covering the fins are elongated and the cells lining the vestibule are columnar rather than squamous. The cuticle is not continuous and, where it is not present, there are many secretory cells in the epidermis. There is a basement membrane present between the epidermis and body wall; the body wall is made up of four quadrants of dorsolateral and ventrolateral longitudinal muscles. The body cavities are most likely derived from enterocoelic cavities, which form during development. The body cavity has a tripartite arrangement, with a head cavity (protocoel, reduced in space by the cephalic musculature), and paired trunk and tail coeloms with dorsal and ventral longitudinal mesenteries, which correspond to the mesocoel and matcoel, respectively. Transverse septa separate the body regions. The body fluid has a variety of cells and cell-like structures, although their functions are largely unknown. The fluid-filled coeloms, body wall, basement membrane, and cuticle all provide support to the body. They do not have circulatory, respiratory, or excretory organs; gases are diffused across the body wall and fluid transport is via cilial action within the body cavities. (Brusca and Brusca, 2003; Margulis and Chapman, 2010; Shapiro, 2012)

A few species of deep sea chaetognaths, including Eukrohnia fowleri and Caecosagitta macrocephala, are bioluminescent. (Thuesen, et al., 2010)


Chaetognaths are hermaphroditic. Cross-fertilization is most common, although some species will self-fertilize. Fertilization is typically internal and eggs may be released into the water, deposited on the sea floor or other substrate, or brooded in pouches near the tail. Cleavage is radial, holoblastic, and equal, leading to a coeloblastula. Development is direct and accomplished quickly, typically from zygote to juvenile within 48 hours. (Brusca and Brusca, 2003; Margulis and Chapman, 2010)


Chaetognaths may undergo reciprocal, nonreciprocal, or self-fertilization. Some benthic species have been documented performing a mating "dance," with an individual depositing balls of sperm onto a mate. (Brusca and Brusca, 2003; Goto and Yoshida, 1985)

Chaetognaths have paired ovaries located in their trunks and paired testes located in their tails. Sperm mature before eggs (which makes self-fertilization less likely), and are stored in coelomic cavities within the tail until they are released in clusters outside the body via a pair of seminal vesicles. Ovaries have oviducts, which lead to genital pores located near the trunk-tail junction. In populations of at least a few species, breeding occurs twice a year, and hatching occurs from April to June and late September to December (typically fewer hatchlings). (Brusca and Brusca, 2003; Ghirardelli, 1968; Ramel, 2012; Zo, 1973)

Outside of a few species (such as members of genus Eukronhnia) that brood their young until they are ready to swim, chaetognaths exhibit no parental investment beyond the production of gametes. (Brusca and Brusca, 2003)

  • Parental Investment
  • female parental care
  • pre-fertilization
    • provisioning
  • pre-hatching/birth
    • protecting
      • female


Most cold water chaetognaths have a longer life expectancy than those in tropical waters, two years versus six weeks, respectively. ("Arrow Worms — Phylum Chaetognatha", 2002)


Many species within this phylum are known to undergo daily vertical migrations, rising to the surface at night to follow prey and sinking during the day, which provides protection from predators. These worms have ammonia-filled vacuolated cells in their trunks, which help them to regulate their depth in the water column. Pelagic chaetognaths move by contracting the longitudinal muscles of their right and left sides alternately, creating forward, darting motions. Fins do not appear to aid in locomotion, instead acting as stabilizers. (Brusca and Brusca, 2003; Margulis and Chapman, 2010)

Communication and Perception

Chaetognaths have a central nervous system with a large cerebral ganglion, dorsal to the pharynx. Additional ganglia, which serve muscles and sensory organs of the head, arise from this structure. They also have a pair of circumenteric connective nerves, emerging from the rear of the cerebral ganglion and extending (posterioventrally) to meet in a ventral ganglion in the epidermis of the trunk. This ganglion controls swimming motion and also gives rise to many pairs of nerves, which create a subepidermal nerve plexus. (Brusca and Brusca, 2003)

Chaetognaths have a pair of compound eyes below the epidermis, on the head. They are made up of five inverted pigment-cup ocelli, one large ocellus directed laterally and four smaller ones directed medially; this gives these worms a nearly uninterrupted field of vision. Their eyes do not typically have lenses and likely do not form images, but are used for light reception and body orientation. The ocelli also contain ciliated receptor cells. (Brusca and Brusca, 2003; Ghirardelli, 1968; Ramel, 2012)

Chaetognaths have a ciliary loop (corona ciliata) on the dorsal surface of the head-trunk juncture, made up of two rings of cilial cells that may be chemoreceptive and/or aid in sperm transfer. They are covered in patches of ciliary fans, which enhance the detection of vibrations in the water. (Brusca and Brusca, 2003; Ghirardelli, 1968)

Food Habits

Chaetognaths are carnivorous predators, particularly of copepods. They are also known to feed on other crustaceans and small fishes. Benthic species are typically ambush predators. They use adhesive secretions to affix themselves to substrate and extend their mouths and vestibules, as well as their associated hooks. When prey is detected by a worm (by the cilia on the body), the head darts forward and prey is captured, using the hooks. Prey is swallowed whole. Panktonic species dart forward in the water column to catch prey within reach, using their grasping spines to pull prey in. The majority of these worms inject their prey with a neurotoxin (tetrodotoxin); it has been hypothesized that chaetognaths have a commensal relationship with bacteria (from genus Vibro) in their heads or guts, which actually produce the toxin. (Brusca and Brusca, 2003; Margulis and Chapman, 2010; Ramel, 2012; Shapiro, 2012)


Chaetognaths are prey to many larger organisms including fishes, whales, other marine invertebrates, and molluscs. (Shapiro, 2012)

Ecosystem Roles

A number of tetrodotoxin producing bacteria have been isolated from the guts of chaetognaths; these are likely responsible for the production of the toxin used by the worms in prey capture. They may be hosts to parasitic digeneans, nematodes, and metacestodes; infections may be the result of the worm ingesting infested copepods, which serve as intermediate hosts. In turn, they may pass these parasites on to their predators (particularly fishes). They may also serve as hosts to ectoparasites such as copepods and dinoflagellates. (DaPonte, et al., 2008; McLean and Nielsen, 1989; Thuesen and Kogure, 1989; Øresland, 1986)

Mutualist Species
  • Vibrio species
  • Vibrio alginolyticus
Commensal/Parasitic Species

Economic Importance for Humans: Positive

Chaetognaths are important to humans not only in terms of scientific research possibilities, but also as prey items for a variety of fish eaten by humans. (Shapiro, 2012)

  • Positive Impacts
  • research and education

Economic Importance for Humans: Negative

Chaetognaths may negatively impact humans if they pass along parasites to fishes eaten by humans. (Shapiro, 2012)

Conservation Status

Neither this phylum, nor its constituent species is considered at risk of becoming threatened or endangered. (Shapiro, 2012)

  • IUCN Red List [Link]
    Not Evaluated

Other Comments

In spite of being soft-bodied, chaetognaths are known in the fossil record, dating back to the early Cambrian Period. (Chen and Huang, 2002; Szaniawski, 2005; Vannier, et al., 2007)


Jeremy Wright (author), University of Michigan-Ann Arbor, Leila Siciliano Martina (editor), Animal Diversity Web Staff.



lives on Antarctica, the southernmost continent which sits astride the southern pole.

Arctic Ocean

the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

World Map


Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

World Map


living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

World Map


living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map


living in the southern part of the New World. In other words, Central and South America.

World Map

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map


living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

World Map


on or near the ocean floor in the deep ocean. Abyssal regions are characterized by complete lack of light, extremely high water pressure, low nutrient availability, and continuous cold (3 degrees C).


Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

brackish water

areas with salty water, usually in coastal marshes and estuaries.


an animal that mainly eats meat


uses smells or other chemicals to communicate


the nearshore aquatic habitats near a coast, or shoreline.


having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.


active at dawn and dusk

  1. active during the day, 2. lasting for one day.

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature


an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

female parental care

parental care is carried out by females


union of egg and spermatozoan


having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.


a distribution that more or less circles the Arctic, so occurring in both the Nearctic and Palearctic biogeographic regions.

World Map

Found in northern North America and northern Europe or Asia.

internal fertilization

fertilization takes place within the female's body


offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).


having the capacity to move from one place to another.


specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.


active during the night


found in the oriental region of the world. In other words, India and southeast Asia.

World Map


reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.


An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).


generates and uses light to communicate


an animal that mainly eats fish


an animal that mainly eats plankton


the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.


the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.


structure produced by the calcium carbonate skeletons of coral polyps (Class Anthozoa). Coral reefs are found in warm, shallow oceans with low nutrient availability. They form the basis for rich communities of other invertebrates, plants, fish, and protists. The polyps live only on the reef surface. Because they depend on symbiotic photosynthetic algae, zooxanthellae, they cannot live where light does not penetrate.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season


remains in the same area


reproduction that includes combining the genetic contribution of two individuals, a male and a female


lives alone


uses touch to communicate


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.


an animal which has an organ capable of injecting a poisonous substance into a wound (for example, scorpions, jellyfish, and rattlesnakes).


movements of a hard surface that are produced by animals as signals to others


uses sight to communicate


2002. "Arrow Worms — Phylum Chaetognatha" (On-line). Canada's Polar Life. Accessed April 01, 2013 at

2012. "Chaetognatha" (On-line). World Register of Marine Species. Accessed April 01, 2013 at

Brands, S. 2009. "Classification: Phylum Chaetognatha" (On-line). Systema Naturae 2000. Accessed April 01, 2013 at

Brusca, R., G. Brusca. 2003. Invertebrates (2nd Edition). Sunderland, MA: Sinauer Associates.

Chen, J., D. Huang. 2002. A possible Lower Cambrian chaetognath (arrow worm). Science, 298: 187.

DaPonte, M., A. Gil de Pertierra, M. Palmieri, M. Ostrowski de Nunez. 2008. Monthly occurrence of parasites of the chaetognath Parasagitta friderici off Mar del Plata, Argentina. Journal of Plankton Research, 30/5: 567-576. Accessed April 02, 2013 at

Dunn, C., A. Hejnol, D. Matus, K. Pang, W. Browne, S. Smith, E. Seaver, G. Rouse, M. Obst, G. Edgecombe, M. Sørensen, S. Haddock, A. Schmidt-Rhaesa, A. Okusu, R. Kristensen, W. Wheeler, M. Martindale, G. Giribet. 2008. Broad phylogenomic sampling improves resolution of the animal tree of life. Nature, 452: 745-749.

Edgecombe, G., G. Giribet, C. Dunn, A. Hejnol, R. Kristensen, R. Neves, G. Rouse, K. Worsaae, M. Sørensen. 2011. Higher-level metazoan relationships: recent progress and remaining questions. Organisms Diversity and Evolution, 11: 151-172.

Ghirardelli, E. 1968. Some aspects of the biology of chaetognaths. Pp. 271-375 in F Russell, M Yonge, eds. Advances in Marine Biology, Vol. 6. London, England: Academic Press. Accessed April 01, 2013 at

Giribet, G., C. Dunn, G. Edgecombe, G. Rouse. 2007. A modern look at the Animal Tree of Life. Zootaxa, 1668: 61-79.

Goto, T., M. Yoshida. 1985. The mating sequence of the benthic arrowworm Paraspadella schizoptera. The Biological Bulletin, 169: 328-333. Accessed April 01, 2013 at

Helmkampf, M., I. Bruchhaus, B. Hausdorf. 2008. Multigene analysis of lophophorate and chaetognath phylogenetic relationships. Molecular Phylogenetics and Evolution, 46/1: 206-214.

Leuckart, R. 1854. Zoologische Untersuchungen. Giessen, Germany: J. Ricker'sche Buchhandlung.

Littlewood, D., M. Telford, R. Bray. 2004. Protostomes and Platyhelminthes: The Worm's Turn. Pp. 209-236 in J Cracraft, M Donoghue, eds. Assembling the Tree of Life. Oxford, UK: Oxford University Press.

Margulis, L., M. Chapman. 2010. Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth, 4th Edition, reprinted with corrections. Philadelphia, PA: Academic Press.

Matus, D., R. Copley, C. Dunn, A. Hejnol, H. Eccleston, K. Halanych, M. Martindale, M. Telford. 2006. Broad taxon and gene sampling indicate that chaetognaths are protostomes. Current Biology, 16/15: R575-R576.

McLean, N., C. Nielsen. 1989. Oodinium jordani n. sp., a dinoflagellate (Dinoflagellata: Oodinidae) ectoparasitic on Parasagitta elegans (Chaetognatha). Diseases of Aquatic Organisms, 7: 61-66. Accessed April 02, 2013 at

Papillon, D., Y. Perez, X. Caubit, Y. Le Parco. 2004. Identification of Chaetognaths as Protostomes Is Supported by the Analysis of Their Mitochondrial Genome. Molecular Biology and Evolution, 21/11: 2122-2129.

Papillon, D., Y. Perez, X. Caubit, Y. Le Parco. 2006. Systematics of Chaetognatha under the light of molecular data, using duplicated ribosomal 18S DNA sequences. Molecular Phylogenetics and Evolution, 38/3: 621-634.

Paps, J., J. Baguñà, M. Riutort. 2009. Bilaterian Phylogeny: A Broad Sampling of 13 Nuclear Genes Provides a New Lophotrochozoa Phylogeny and Supports a Paraphyletic Basal Acoelomorpha. Molecular Biology and Evolution, 26/10: 2397-2406.

Ramel, G. 2012. "The Phylum Chaetognatha" (On-line). Earthlife. Accessed April 01, 2013 at

Shapiro, L. 2012. "Chaetognatha" (On-line). Encyclopedia of Life. Accessed April 01, 2013 at

Szaniawski, H. 2005. Cambrian chaetognaths recognized in Burgess Shale fossils. Acta Palaeontologica Polonica, 50/1: 1-8.

Telford, M., P. Holland. 1997. Evolution of 28S ribosomal DNA in chaetognaths: duplicate genes and molecular phylogeny. Journal of Molecular Evolution, 44/2: 135-144.

Telford, M., P. Holland. 1993. The phylogenetic affinities of the chaetognaths: a molecular analysis. Molecular Biology and Evolution, 10/3: 660-676.

Thuesen, E., F. Goetz, S. Haddock. 2010. Bioluminescent organs of two deep-sea arrow worms, Eukrohnia fowleri and Caecosagitta macrocephala, with further observations on bioluminescence in chaetognaths. The Biological Bulletin, 219/2: 100-111. Accessed April 01, 2013 at

Thuesen, E., K. Kogure. 1989. Bacterial production of tetrodotoxin in four species of Chaetognatha. The Biological Bulletin, 176: 191-194. Accessed April 02, 2013 at

Thuesen, E. 2009. "Chaetognatha" (On-line). Accessed April 01, 2013 at

Vannier, J., M. Steiner, E. Renvoise, S. Hu, J. Casanova. 2007. Early Cambrian origin of modern food webs: evidence from predator arrow worms. Proceedings of the Royal society B: Biological Science, 274: 627-633.

Zhang, Z. 2011. Animal biodiversity: An introduction to higher-level classification and taxonomic richness. Zootaxa, 3148: 7-12.

Zo, Z. 1973. Breeding and growth of the chaetognath Parasagitta elegans in Bedford Basin. Limnology and Oceanography, 18/5: 750-756. Accessed April 01, 2013 at

Øresland, V. 1986. Parasites of the chaetognath Sagitta setosa in the western English Channel. Marine Biology, 92/1: 87-91. Accessed April 02, 2013 at