Editing Chaetognatha

{{Short description|Phylum of marine worms}}
{{Automatic taxobox
| name = Arrow worms
| fossil_range = {{nowrap|{{fossil range|Lower Cambrian|Recent|ref=<ref name="Vannier2007">{{cite journal | vauthors = Vannier J, Steiner M, Renvoisé E, Hu SX, Casanova JP | title = Early Cambrian origin of modern food webs: evidence from predator arrow worms | journal =  Proceedings of the Royal Society B: Biological Sciences| volume = 274 | issue = 1610 | pages = 627–33 | date = March 2007 | pmid = 17254986 | pmc = 2197202 | doi = 10.1098/rspb.2006.3761 }}</ref>}}}}
| image = Chaetognatha.PNG
| image_caption = Chaetognatha and some examples of their diversity.
| display_parents = 8
| parent_authority = [[Rudolph Leuckart|Leuckart]], 1854
| taxon = Sagittoidea
| authority = [[Carl Friedrich Wilhelm Claus|Claus]] & [[Karl Grobben|Grobben]], 1905&nbsp;<ref>{{ITIS |taxon=Sagittoidea Claus and Grobben, 1905 |id=158655 |access-date=February 8, 2012}}</ref>
| subdivision_ranks = Orders
| subdivision = * [[Aphragmophora]]
* [[Phragmophora]]
* †[[Protoconodonta]]
* †''[[Eognathacantha]]''
* †''[[Capinatator]]''
* †''[[Eoserratosagitta]]''<ref name=dawn>{{cite journal |last1=Doguzhaeva |first1=Larisa A. |last2=Mutvei |first2=Harry |last3=Mapes |first3=Royal H. |title=Chaetognath grasping spines from the Upper Mississippian of Arkansas (USA) - Acta Palaeontologica Polonica |journal=Acta Palaeontologica Polonica |date=2002 |volume=47 |issue=3 |pages=421-430 |url=https://www.app.pan.pl/article/item/app47-421.html}}</ref>
* †''[[Paucijaculum]]''
* †''[[Protosagitta]]''
* †''[[Titerina]]''?<ref name=titerina>{{cite journal |last1=Kraft |first1=Petr |last2=Lehnert |first2=Oliver |last3=Frýda |first3=Jiři |title=Titerina, a living fossil in the Ordovician: A young protoconodont (?) and the oldest chaetognath. |journal=Acta Universitatis Carolinae - Geologica |date=1999 |volume=43 |issue=1/2 |pages=451-454 |url=https://www.researchgate.net/publication/230584744_Titerina_a_living_fossil_in_the_Ordovician_A_young_protoconodont_and_the_oldest_chaetognath}}</ref>
* †''[[Ankalodous]]''?
}}

The '''Chaetognatha''' {{IPAc-en|k|iː|'|t|ɒ|g|n|@|θ|@}} or '''chaetognaths''' {{IPAc-en|ˈ|k|iː|t|ɒ|g|n|æ|θ|s}} (meaning ''bristle-jaws'') are a [[phylum]] of predatory [[marine worm]]s that are a major component of [[plankton]] worldwide. Commonly known as '''arrow worms''', they are mostly [[nektonic]]; however about 20% of the known [[species]] are [[benthic zone|benthic]], and can attach to [[algae]] and [[rocks]]. They are found in all marine waters, from surface [[Tropics|tropical]] waters and shallow tide pools to the [[deep sea]] and [[Polar regions of Earth|polar regions]]. Most chaetognaths are transparent and are torpedo shaped, but some deep-sea species are [[Orange (colour)|orange]]. They range in size from {{convert|2|to|120|mm|in|1}}.

Chaetognaths were first recorded by the Dutch naturalist [[Martinus Slabber]] in 1775.<ref name="Perez2021"/> As of 2021, biologists recognize 133 modern species assigned to over 26 [[Genus|genera]] and eight [[Family (biology)|families]].<ref name="Perez2021"/> Despite the limited diversity of species, the number of individuals is large.<ref name="Bone">{{cite book | veditors = Bone Q, Kapp H, Pierrot-Bults AC |year=1991 |title=The Biology of Chaetognaths |publisher=[[Oxford University Press]] |place=London |isbn=978-0-19-857715-7 |ref=Bone, Kapp & Pierrot-Bults}}</ref>

Arrow worms are strictly related to and possibly belonging to [[Gnathifera (clade)|Gnathifera]], a [[clade]] of [[protostome]]s that do not belong to either [[Ecdysozoa]] or [[Lophotrochozoa]].

==Anatomy==
[[File:Ten species of chaetognaths.webp|thumb|right|400px|Drawing of ten different chaetognath species, showing morphological similarity and diversity.]]
Chaetognaths are transparent or translucent dart-shaped animals covered by a [[cuticle]]. They range in length between 1.5 mm to 105 mm in the Antarctic species ''[[Pseudosagitta gazellae]]''.<ref name="Pauly2021"/> Body size, either between individuals in the same species or between different species, seems to increase with decreasing temperature.<ref name="Pauly2021"/> The body is divided into a distinct head, trunk, and tail. About 80% of the body is occupied by primary longitudinal [[muscle]]s.<ref name="Perez2021"/>  

===Head and digestive system===
There are between four and fourteen hooked, grasping spines on each side of their head, flanking a hollow vestibule containing the mouth. The spines are used in hunting, and covered with a flexible hood arising from the neck region when the animal is swimming. Spines and teeth are made of α-[[chitin]], and the head is protected by a chitinous armature.<ref name="Perez2021"/>  
[[File:MEB back.png|thumb|left|180px|The jaw organ of a chaetognath of the genus ''[[Sagitta (arrowworm)|Sagitta]]'']]
The mouth opens into a muscular [[pharynx]], which contains glands to lubricate the passage of food. From here, a straight intestine runs the length of the trunk to an [[anus]] just forward of the tail. The intestine is the primary site of digestion and includes a pair of [[diverticulum|diverticula]] near the anterior end.<ref name=IZ/> Materials are moved about the body cavity by [[cilia]]. Waste materials are simply excreted through the skin and anus. Eukrohniid species possess an oil vacuole closely associated with the gut. This organ contains wax esters which may assist reproduction and growth outside of the production season for ''Eukrohnia hamata'' in Arctic seas.<ref>{{Cite journal|last1=Grigor|first1=Jordan J.|last2=Schmid|first2=Moritz S.|last3=Fortier|first3=Louis|date=2017-11-01|title=Growth and reproduction of the chaetognaths Eukrohnia hamata and Parasagitta elegans in the Canadian Arctic Ocean: capital breeding versus income breeding|url=https://academic.oup.com/plankt/article/39/6/910/4344873|journal=Journal of Plankton Research|language=en|volume=39|issue=6|pages=910–929|doi=10.1093/plankt/fbx045|issn=0142-7873|doi-access=free}}</ref> Owing to the position of the oil vacuole in the center of the tractus, the organ may also have implications for buoyancy, trim and locomotion.<ref>{{Cite journal|last=Pond|first=David W.|date=2012-06-01|title=The physical properties of lipids and their role in controlling the distribution of zooplankton in the oceans|url=https://academic.oup.com/plankt/article/34/6/443/1576677|journal=Journal of Plankton Research|language=en|volume=34|issue=6|pages=443–453|doi=10.1093/plankt/fbs027|issn=0142-7873|doi-access=free}}</ref>

Usually chaetognaths are not pigmented, however the intestines of some deep-sea species contain orange-red carotenoid pigments.<ref name="Perez2021"/>  

[[File:Chaetomorpho.png|thumb|right|160px|Anatomy of a chaetognath (''Spadella cephaloptera'')]]

===Nervous and sensory systems===
The nervous system is reasonably simple and shows a typical [[protostome]] anatomy,<ref name="Perez2021"/> consisting of a [[ganglion]]ated nerve ring surrounding the pharynx. The brain is composed of two distinct functional domains: the anterior neuropil domain and the posterior neuropil domain. The former probably controls head muscles moving the spines and the digestive system. The latter is linked to eyes and the corona ciliata. A putative sensory structure of unknown function, the retrocerebral organ, is also hosted by the posterior neuropil domain.<ref name="Perez2021"/> The ventral ganglion is the largest, but nerves extend from all the ganglia along the length of the body. 

Chaetognaths have two compound eyes, each consisting of a number of pigment-cup [[ocelli]] fused together; some deep-sea and [[troglobitic]] species have unpigmented or absent eyes.<ref name="Perez2021"/> In addition, there are a number of sensory bristles arranged in rows along the side of the body, where they probably perform a function similar to that of the [[lateral line]] in fish. An additional, curved, band of sensory bristles lies over the head and neck.<ref name=IZ/> Almost all chaetognaths have "indirect" or "inverted" eyes, according to the orientation of [[Photoreceptor cell|photoreceptor]] cells; only some [[Eukhroniidae]] species have "direct" or "everted" eyes.<ref name="Perez2021"/> A unique feature of the chaetognath eye is the lamellar structure of photoreceptor membranes, containing a grid of 35–55 nm wide circular pores.<ref name="Perez2021"/>

A significant mechanosensory system, composed of ciliary receptor organs, detects vibrations, allowing chaetognaths to detect the swimming motion of potential prey. Another organ on the dorsal part of the neck, the corona ciliata, is probably involved in chemoreception.<ref name="Perez2021"/>

===Internal organs===
The body cavity is lined by [[peritoneum]], and therefore represents a true [[coelom]], and is divided into one compartment on each side of the trunk, and additional compartments inside the head and tail, all separated completely by septa.  Although they have a mouth with one or two rows of tiny teeth, compound eyes, and a nervous system, they have no excretory or respiratory systems.<ref>[https://books.google.com/books?id=9IWaqAOGyt4C&dq=Chaetognatha+circulatory+excretory+respiratory+systems&pg=PA343 Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth]</ref><ref name="Perez2021"/> While often said to lack a circulatory system, chaetognaths do have a rudimentary hemal system resembling those of [[Annelida|annelids]].<ref name="Perez2021"/>

The arrow worm [[rhabdomere]]s are derived from [[microtubule]]s 20&nbsp;nm long and 50&nbsp;nm wide, which in turn form conical bodies that contain granules and thread structures. The cone body is derived from a [[cilium]].<ref>"Photoreception". Encyclopædia Britannica from [[Encyclopædia Britannica 2006 Ultimate Reference Suite DVD]] . 2009.</ref>

===Locomotion===
The trunk bears one or two pairs of lateral fins incorporating structures superficially similar to the [[fin ray]]s of fish, with which they are not [[homology (biology)|homologous]]. Unlike those of vertebrates, these lateral fins are composed of a thickened [[basement membrane]] extending from the [[epidermis (zoology)|epidermis]]. An additional caudal fin covers the post-anal tail.<ref name=IZ/> Two chaetognath species, ''[[Caecosagitta macrocephala]]'' and ''[[Eukrohnia fowleri]]'', have [[bioluminescent]] organs on their fins.<ref>{{cite journal | vauthors = Haddock SH, Case JF |date=20 January 1994 |title=A bioluminescent chaetognath |journal=[[Nature (journal)|Nature]] |volume=367 |issue=6460 |pages=225–226 |doi=10.1038/367225a0|bibcode=1994Natur.367..225H |s2cid=4362422 }}</ref><ref>{{cite journal | vauthors = Thuesen EV, Goetz FE, Haddock SH | title = Bioluminescent organs of two deep-sea arrow worms, Eukrohnia fowleri and Caecosagitta macrocephala, with further observations on Bioluminescence in chaetognaths | journal = The Biological Bulletin | volume = 219 | issue = 2 | pages = 100–11 | date = October 2010 | pmid = 20972255 | doi = 10.1086/BBLv219n2p100  | s2cid = 14502203 | author3-link = Steven Haddock }}</ref> 

Chaetognaths swim in short bursts using a dorso-ventral undulating body motion, where their tail fin assists with propulsion and the body fins with stabilization and steering.<ref>{{cite journal | vauthors = Jordan CE |year=1992 |title=A model of rapid-start swimming at intermediate Reynolds number: undulatory locomotion in the chaetognath ''Sagitta elegans'' |journal=[[Journal of Experimental Biology]] |volume=163 |issue=1 |pages=119–137 |doi=10.1242/jeb.163.1.119 |url=http://jeb.biologists.org/content/163/1/119|doi-access=free }}</ref> Muscle movements have been described as among the fastest in [[metazoan]]s.<ref name="Perez2021"/> Muscles are directly excitable by electrical currents or strong [[potassium|K+]] solutions; the main neuromuscular transmitter is acetylcholine.<ref name="Perez2021"/>

==Reproduction and life cycle==
All species are [[hermaphroditic]], carrying both [[ovum|eggs]] and [[spermatozoon|sperm]].<ref name="Bone" /> Each animal possesses a pair of [[testis|testes]] within the tail, and a pair of [[ovary|ovaries]] in the posterior region of the main body cavity. Immature sperm are released from the testes to mature inside the cavity of the tail, and then swim through a short duct to a [[seminal vesicle]] where they are packaged into a [[spermatophore]].<ref name=IZ/>

During mating, each individual places a spermatophore onto the neck of its partner after rupture of the seminal vesicle. The sperm rapidly escape from the spermatophore and swim along the midline of the animal until they reach a pair of small pores just in front of the tail. These pores connect to the [[oviduct]]s, into which the developed eggs have already passed from the ovaries, and it is here that fertilisation takes place.<ref name=IZ/> The seminal receptacles and oviducts accumulate and store spermatozoa, to perform multiple fertilisation cycles.<ref name="Perez2021"/> Some [[benthic]] members of [[Spadellidae]] are known to have elaborate [[courtship ritual]]s before copulation,<ref name="Perez2021"/> for example ''[[Paraspadella gotoi]]''.<ref name="Ball2006"/>

The eggs are mostly planktonic, except in a few species such as ''[[Ferosagitta hispida]]'' that attaches eggs to the substrate.<ref name="Perez2021"/> In ''[[Eukrohnia]]'', eggs develop in marsupial sacs or attached to [[alga]]e.<ref>{{Cite journal|last1=Terazaki|first1=M.|last2=Miller|first2=C. B.|date=1982-11-01|title=Reproduction of meso- and bathypelagic chaetognaths in the genus Eukrohnia|journal=Marine Biology|language=en|volume=71|issue=2|pages=193–196|doi=10.1007/BF00394629|bibcode=1982MarBi..71..193T |s2cid=84519862|issn=1432-1793}}</ref> Eggs usually hatch after 1–3 days. Chaetognaths do not undergo [[metamorphosis]] nor they possess a well-defined [[larva]]l stage,<ref name=IZ/><ref name="Perez2021"/> an unusual trait among marine invertebrates;<ref name="Ball2006"/> however there are significant morphological differences between the newborn and the adult, with respect to proportions, chitinous structures and fin development.<ref name="Perez2021"/><ref name="Daponte2004"/> 

The life spans of chaetognaths are variable but short; the longest recorded was 15 months in ''[[Sagitta friderici]]''.<ref name="Daponte2004">{{cite journal |doi=10.1016/j.icesjms.2004.03.006 |title=Sagitta friderici Ritter-Záhony (Chaetognatha) from South Atlantic waters: Abundance, population structure, and life cycle |date=2004 |last1=Daponte |first1=M.C |last2=Capitanio |first2=F.L |last3=Nahabedian |first3=D.E |last4=Viñas |first4=M.D |last5=Negri |first5=R.M |journal=ICES Journal of Marine Science |volume=61 |issue=4 |pages=680–686 |doi-access=free |bibcode=2004ICJMS..61..680D }}</ref>

==Behaviour==
Little is known of arrow worms' behaviour and physiology, due to the complexity in culturing them and reconstructing their natural habitat.<ref name="Perez2021"/> It is known that they feed more frequently with higher temperatures. Planktonic chaetognaths often must swim continuously,  with a "hop and sink" behaviour, to keep themselves in the desired location in the water layer, and swim actively to catch prey. They all tend to keep the body slightly slanted with the head pointing downwards.<ref name="Perez2021"/> They often show a "gliding" behaviour, slowly sinking for a while, and then catching up with a quick movement of their fins.<ref name="Ball2006"/> Benthic species usually stay attached to substrates such as rocks, algae or sea grasses, more rarely on top or between sand grains, and act more strictly as ambush predators, staying still until prey passes by.<ref name="Perez2021"/>  The prey is detected thanks to the ciliary fence and tuft organs, sensing vibrations<ref name="Perez2021"/>  – individuals of ''Spadella cephaloptera'' for example will attack a glass or metal probe vibrating at an adequate frequency.<ref name="Ball2006"/> To catch prey, arrow worms jump forward with a strong stroke of the tail fin.<ref name="Perez2021"/> Once in contact with prey, they withdraw the hood over the grasping spines, so that it forms a cage around the prey and bring it in contact with the mouth. They swallow their prey whole.<ref name="Ball2006"/>

==Ecology==
Chaetognaths are found in all world's oceans, from the poles to tropics, and also in [[brackish]] and [[estuarine]] waters. They inhabit very diverse environments, from hydrothermal vents to deep ocean seafloor, to seagrass beds and marine caves.<ref name="Perez2021"/>  The majority are planktonic, and they are often the second most common component of [[zooplankton]], with a biomass ranging between 10 and 30% that of [[copepod]]s.<ref name="Perez2021"/>  In the [[Canada Basin]], chaetognaths alone represent ~13% of the zooplankton biomass.<ref name="Kosobokova2010">{{cite journal |doi=10.1016/j.dsr2.2009.08.009 |title=Diversity and vertical distribution of mesozooplankton in the Arctic's Canada Basin |date=2010 |last1=Kosobokova |first1=Ksenia N. |last2=Hopcroft |first2=Russell R. |journal=Deep Sea Research Part II: Topical Studies in Oceanography |volume=57 |issue=1–2 |pages=96–110 |bibcode=2010DSRII..57...96K }}</ref> As such, they are ecologically relevant and a key food source for fishes and other predators, including commercially relevant fishes such as [[mackerel]] or [[sardines]].<ref name="Choo2022">{{cite journal |doi=10.3897/zookeys.1106.80184 |issue=1106 |doi-access=free |title=Taxonomic reassessment of chaetognaths (Chaetognatha, Sagittoidea, Aphragmophora) from Korean waters |date=2022 |last1=Choo |first1=Seohwi |last2=Jeong |first2=Man-Ki |last3=Soh |first3=Ho Young |journal=ZooKeys |pages=165–211 |pmid=36760822 |pmc=9848744 |bibcode=2022ZooK.1106..165C }}</ref> 58% of known species are pelagic,<ref name="Pauly2021"/> while about a third of species are [[epibenthic]] or [[meiobenthic]], or inhabit the immediate vicinity of the substrate.<ref name="Perez2021"/> Chaetognaths have been recorded up to 5000 and possibly even 6000 meters of depth.<ref name="Pauly2021">{{cite journal |doi=10.3390/jmse9121397 |doi-access=free |title=The Sizes, Growth and Reproduction of Arrow Worms (Chaetognatha) in Light of the Gill-Oxygen Limitation Theory (GOLT) |date=2021 |last1=Pauly |first1=Daniel |last2=Liang |first2=Cui |last3=Xian |first3=Weiwei |last4=Chu |first4=Elaine |last5=Bailly |first5=Nicolas |journal=Journal of Marine Science and Engineering |volume=9 |issue=12 |page=1397 }}</ref> 

The highest density of chaetognaths is observed in the [[photic zone]] of shallow waters.<ref name="Perez2021"/> Larger chaetognath species tend to live deeper in water, but spend their juvenile stages higher in the water column.<ref name="Ball2006"/> Arrow worms however engage in [[diel vertical migration]], spending the day at lower depths to avoid predators, and coming close to the surface at night. Their position in the water column can depend on light, temperature, salinity, age and food supply. They cannot swim against [[oceanic current]]s, and they are used as a hydrological indicator of currents and water masses.<ref name="Perez2021"/> 

All chaetognaths are [[ambush predators]], preying on other planktonic animals, mostly copepods and [[cladocera]]ns<ref name=IZ>{{cite book |author= Barnes, Robert D. |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |location= Philadelphia, PA|pages= 1046–1050|isbn= 978-0-03-056747-6}}</ref><ref name="Perez2021"/> but also amphipods, krill and fish larvae.<ref name="Choo2022"/> Adults can feed on younger individuals of the same species.<ref name="Sullivan1980">{{cite journal |doi=10.4319/lo.1980.25.2.0317 |title=In situ feeding behavior of Sagitta elegans and Eukrohnia hamata (Chaetognatha) in relation to the vertical distribution and abundance of prey at Ocean Station "P"1 |date=1980 |last1=Sullivan |first1=Barbara K. |journal=Limnology and Oceanography |volume=25 |issue=2 |pages=317–326 |bibcode=1980LimOc..25..317S }}</ref> Some species are also reported to be omnivores, feeding on algae and detritus.<ref>{{Cite journal|last1=Grigor|first1=Jordan J.|last2=Schmid|first2=Moritz S.|last3=Caouette|first3=Marianne|last4=St.-Onge|first4=Vicky|last5=Brown|first5=Thomas A.|last6=Barthélémy|first6=Roxane-M.|date=2020-07-01|title=Non-carnivorous feeding in Arctic chaetognaths|url=http://www.sciencedirect.com/science/article/pii/S0079661120301270|journal=Progress in Oceanography|language=en|volume=186|pages=102388|doi=10.1016/j.pocean.2020.102388|bibcode=2020PrOce.18602388G |s2cid=225636394 |issn=0079-6611|url-access=subscription}}</ref> 
Chaetognaths are known to use the [[neurotoxin]] [[tetrodotoxin]] to subdue prey,<ref>{{cite book |last=Thuesen |first=Erik V. |chapter = The Tetrodotoxin Venom of Chaetognaths | pages = 55–60 | veditors = Bone Q, Kapp H, Pierrot-Bults AC |title=The Biology of chaetognaths |date=1991 |publisher=Oxford University Press |isbn=978-0-19-857715-7}}</ref> possibly synthesized by ''[[Vibrio]]'' bacterial species.<ref name="Perez2021"/>

==Genetics==
===Mitochondrial genome===
The [[mtDNA]] of the arrow worm ''[[Spadella cephaloptera]]'' has been sequenced in 2004, and at the time it was the smallest metazoan mitochondrial genome known, being 11,905 base pairs long<ref name="Papillon2004"/> (it has now been surpassed by the mitchondrial genome of the [[ctenophore]] ''Mnemiopsis leidyi'', which is 10,326 bp long).<ref>{{cite journal |doi=10.3390/life11070663 |doi-access=free |title=Mitochondrial Genomic Landscape: A Portrait of the Mitochondrial Genome 40 Years after the First Complete Sequence |date=2021 |last1=Formaggioni |first1=Alessandro |last2=Luchetti |first2=Andrea |last3=Plazzi |first3=Federico |journal=Life |volume=11 |issue=7 |page=663 |pmid=34357035 |pmc=8303319 |bibcode=2021Life...11..663F }}</ref> All mitochondrial [[tRNA]] genes are absent. The [[MT-ATP8]] and [[MT-ATP6]] genes are also missing.<ref name="Papillon2004"/> The [[mtDNA]] of ''[[Paraspadella gotoi]]'', also sequenced in 2004, is even smaller (11,403 bp) and it shows a similar pattern, lacking 21 of the 22 usually present tRNA genes and featuring only 14 of the 37 genes normally present. <ref name="Helfenbein2004">{{cite journal |doi=10.1073/pnas.0400941101 |title=The mitochondrial genome of ''Paraspadella gotoi'' is highly reduced and reveals that chaetognaths are a sister group to protostomes |date=2004 |last1=Helfenbein |first1=Kevin G. |last2=Fourcade |first2=H. Matthew |last3=Vanjani |first3=Rohit G. |last4=Boore |first4=Jeffrey L. |journal=Proceedings of the National Academy of Sciences |volume=101 |issue=29 |pages=10639–10643 |pmid=15249679 |pmc=489987 |doi-access=free |bibcode=2004PNAS..10110639H }}</ref> 

Chaetognaths show a unique mitochondrial genomic diversity within individual of the same species.<ref name="Marletaz2017">{{cite journal |doi=10.1093/gbe/evx090 |title=Extreme Mitogenomic Variation in Natural Populations of Chaetognaths |date=2017 |last1=Marlétaz |first1=Ferdinand |last2=Le Parco |first2=Yannick |last3=Liu |first3=Shenglin |last4=Peijnenburg |first4=Katja TCA |journal=Genome Biology and Evolution |volume=9 |issue=6 |pages=1374–1384 |pmid=28854623 |pmc=5470650 }}</ref>

== Phylogeny ==
[[File:NS and jaws Chaetognathifera.png|thumb|Hypothesis of the possible homology between the jaw parts and nervous system parts of extant and extinct Chaetognathifera taxa. Putative homologies between jaw parts and nervous system respectively have the same color. Questionable, and/or, disputable homologies are in grey. Non oberved part of the nervous system are in dotted lines. Only the anterior part of each organism is represented for the jaws. Modified from Bekkouche and Gąsiorowski 2022<ref name="Gynognathifera">{{cite journal |last1=Bekkouche |first1=Nicolas |last2=Gąsiorowski |first2=Ludwik |date=2022 |title=Careful amendment of morphological data sets improves phylogenetic frameworks: re-evaluating placement of the fossil ''Amiskwia sagittiformis'' |journal=Journal of Systematic Palaeontology |volume=20 |pages=1–14 |doi=10.1080/14772019.2022.2109217|url=https://hal.sorbonne-universite.fr/hal-03828531/file/Manuscript_Amiskwia_Bekkouche_and_Gasiorowski_Hall_version_compressed.pdf }}</ref>]]
===External===
The evolutionary relationships of chaetognaths have long been enigmatic. [[Charles Darwin]] remarked that arrow worms were "remarkable for the obscurity of their affinities".<ref name="Ball2006">{{cite journal |doi=10.1016/j.cub.2006.07.006 |title=Phylogeny: The Continuing Classificatory Conundrum of Chaetognaths |date=2006 |last1=Ball |first1=Eldon E. |last2=Miller |first2=David J. |journal=Current Biology |volume=16 |issue=15 |pages=R593–R596 |pmid=16890517 |s2cid=18793650 |doi-access=free |bibcode=2006CBio...16.R593B |hdl=1885/26305 |hdl-access=free }}</ref> Chaetognaths in the past have been traditionally, but erroneously, classed as [[deuterostomes]] by [[embryologist]]s due to deuterostome-like features in the embryo. [[Lynn Margulis]] and K. V. Schwartz placed chaetognaths in the deuterostomes in their ''Five Kingdom'' classification.<ref>[http://sn2000.taxonomy.nl/Taxonomicon/TaxonTree.aspx?id=40615 Systema Naturae 2000 Taxon: Phylum Chaetognatha per Margulis and Schwartz] {{webarchive |url=https://web.archive.org/web/20051127153818/http://sn2000.taxonomy.nl/Taxonomicon/TaxonTree.aspx?id=40615 |date=November 27, 2005 }} (select Margulis & Schwartz in 'Classification by')—last retrieved November 25, 2006</ref> However, several developmental features are at odds with deuterostomes and are either akin to [[Spiralia]] or unique to Chaetognatha.<ref name="Perez2021"/>
 
{{cladogram|caption=Summary of relationships of gnathiferans in recent studies including Chaetognatha within the clade, with disputed relationships represented as polytomies<ref name=marletaz19>{{Cite journal | last5 = Rokhsar | first5 = Daniel S.| last4 = Satoh | first4 = Noriyuki | last3 = Goto | first3 = Taichiro | last2 = Peijnenburg | first2 = Katja T. C. A. | last1 = Marlétaz | first1 = Ferdinand | year = 2019 | title = A new spiralian phylogeny places the enigmatic arrow worms among gnathiferans | journal = Current Biology | volume = 29 | issue = 2 | pages = 312–318.e3 | doi=10.1016/j.cub.2018.11.042 | pmid = 30639106| doi-access = free | bibcode = 2019CBio...29E.312M| url = https://discovery.ucl.ac.uk/10097250/1/Phylogeny_chaeto_rv9s_pre.pdf }}</ref><ref name=vinther19>{{cite journal | first1 = Jakob | last1 = Vinther | first2 = Luke A. | last2 = Parry | year = 2019 | title = Bilateral jaw elements in Amiskwia sagittiformis bridge the morphological gap between gnathiferans and chaetognaths | journal = Current Biology | volume = 29 | issue = 5 | pages = 881–888.e1 | doi = 10.1016/j.cub.2019.01.052 | pmid = 30799238 | doi-access = free | bibcode = 2019CBio...29E.881V | url = https://research-information.bris.ac.uk/files/183275665/Collated_for_pure.pdf }}</ref><ref name=frobius17>{{cite journal | last1 = Fröbius | first1 = Andreas C. | last2 = Funch | first2 = Peter | year = 2017 | title = Rotiferan Hox genes give new insights into the evolution of metazoan bodyplans | journal = Nature Communications | volume = 8 | issue = 1 | pages = 9 | doi = 10.1038/s41467-017-00020-w | pmid = 28377584 | pmc = 5431905 | bibcode = 2017NatCo...8....9F }}</ref><ref name=laumer15>{{cite journal | first1 = Christopher E. | last1 = Laumer | first2 = Nicolas | last2 = Bekkouche | first3 = Alexandra | last3 = Kerbl | first4 = Freya | last4 = Goetz | first5 = Ricardo C. | last5 = Neves | first6 = Martin V. | last6 = Sørensen | first7 = Reinhardt M. | last7 = Kristensen | first8 = Andreas | last8 = Hejnol | first9 = Casey W. | last9 = Dunn | first10 = Gonzalo | last10 = Giribet | first11 = Katrine | last11 = Worsaae | year = 2015 | title = Spiralian phylogeny informs the evolution of microscopic lineages | journal = Current Biology | volume = 25 | issue = 15 | pages = 2000–2006 | doi = 10.1016/j.cub.2015.06.068 | pmid = 26212884 | doi-access = free | bibcode = 2015CBio...25.2000L }}</ref><ref name=sielaff16>{{cite journal | title = Phylogeny of Syndermata (syn. Rotifera): Mitochondrial gene order verifies epizoic Seisonidea as sister to endoparasitic Acanthocephala within monophyletic Hemirotifera | journal = Molecular Phylogenetics and Evolution | volume = 96 | year = 2016 | pages = 79–92 | doi = 10.1016/j.ympev.2015.11.017 | last1 = Sielaff | first1 = Malte | last2 = Schmidt | first2 = Hanno | last3 = Struck | first3 = Torsten H. | last4 = Rosenkranz | first4 = David | last5 = Mark Welch | first5 = David B. | last6 = Hankeln | first6 = Thomas | last7 = Herlyn | first7 = Holger | pmid = 26702959 | bibcode = 2016MolPE..96...79S }}</ref>
|clades={{Clade|style=font-size:100%; line-height:100%
   |label1=[[Spiralia]]
   |1={{clade
      |label1=[[Gnathifera (clade)|Gnathifera]]
      |1={{clade
         |1=[[Gnathostomulida]]
         |2={{Clade
            |1=[[Micrognathozoa]]
            |2='''Chaetognatha'''
            |label3=[[Rotifera]]
            |3={{clade
               |1=[[Seisonida]]
               |2=[[Acanthocephala]]
               |3=[[Bdelloidea]]
               |4=[[Monogononta]]
               }}
            }}
         }}
      |2=[[Platytrochozoa]]
      }}
   }}
|align=right}}

{{cladogram|caption=Chaetognaths in the [[metazoa]]n tree of life, when considered the sister group of Gnathifera.<ref name="Perez2021"/>
|clades={{Clade|style=font-size:100%; line-height:100%
   |label1=Metazoa
   |1={{clade
       |1=[[Ctenophora]]
       |2=[[Porifera]]
       |3=[[Cnidaria]]
       |4=[[Placozoa]]
       |5={{clade
          |1=[[Xenacoelomorpha]]
          |2={{clade
             |1=[[Deuterostomia]]
             |2={{clade
                |1=[[Ecdysozoa]]
                |2={{clade
                   |1=[[Lophotrochozoa]]
                   |2={{clade
                      |1=[[Gnathifera (clade)|Gnathifera]]
                      |2='''Chaetognatha'''
                      }}
                   }} |label2=[[Spiralia]]
                }} | label2=[[Protostomia]]
             }} | label2=[[Nephrozoa]]
          }} |label5=[[Bilateria]]
       }}
   }}
}}

Molecular [[phylogeny]] shows that Chaetognatha are, in fact, [[protostomes]]. [[Thomas Cavalier-Smith]] places them in the protostomes in his ''Six Kingdom'' classification.<ref>[http://sn2000.taxonomy.nl/Taxonomicon/TaxonTree.aspx?id=40615 Systema Naturae 2000 Taxon: Phylum Chaetognatha per Cavalier-Smith] {{webarchive |url=https://web.archive.org/web/20051127153818/http://sn2000.taxonomy.nl/Taxonomicon/TaxonTree.aspx?id=40615 |date=November 27, 2005 }} (select Cavalier-Smith in 'Classification by')—last retrieved November 25, 2006</ref> The similarities between chaetognaths and nematodes mentioned above may support the protostome thesis—in fact, chaetognaths are sometimes regarded as a basal [[ecdysozoa]]n or [[lophotrochozoa]]n.<ref>{{cite journal | vauthors = Matus DQ, Copley RR, Dunn CW, Hejnol A, Eccleston H, Halanych KM, Martindale MQ, Telford MJ | title = Broad taxon and gene sampling indicate that chaetognaths are protostomes | journal = Current Biology | volume = 16 | issue = 15 | pages = R575–R576 | date = August 2006 | pmid = 16890509 | doi = 10.1016/j.cub.2006.07.017 | s2cid = 11284138 | doi-access = free | bibcode = 2006CBio...16.R575M }}</ref> Chaetognatha appears close to the base of the protostome tree in most studies of their molecular phylogeny.<ref>{{cite journal | vauthors = Marlétaz F, Martin E, Perez Y, Papillon D, Caubit X, Lowe CJ, Freeman B, Fasano L, Dossat C, Wincker P, Weissenbach J, Le Parco Y | title = Chaetognath phylogenomics: a protostome with deuterostome-like development | journal = Current Biology | volume = 16 | issue = 15 | pages = R577-8 | date = August 2006 | pmid = 16890510 | doi = 10.1016/j.cub.2006.07.016 | s2cid = 18339954 | doi-access = free | bibcode = 2006CBio...16.R577M }}</ref> This may explain their deuterostome embryonic characters. If chaetognaths branched off from the protostomes before they evolved their distinctive protostome embryonic characters, they might have retained deuterostome characters inherited from early [[bilateria]]n ancestors. Thus chaetognaths may be a useful model for the ancestral bilaterian.<ref>{{cite journal | vauthors = Papillon D, Perez Y, Caubit X, Le Parco Y | title = Identification of chaetognaths as protostomes is supported by the analysis of their mitochondrial genome | journal = Molecular Biology and Evolution | volume = 21 | issue = 11 | pages = 2122–9 | date = November 2004 | pmid = 15306659 | doi = 10.1093/molbev/msh229 | doi-access = free }}</ref>
Studies of arrow worms' nervous systems suggests they should be placed within the protostomes.<ref>{{cite journal | vauthors =Rieger V, Perez Y, Müller CH, Lipke E, Sombke A, Hansson BS, Harzsch S | date = February 2010 |title=Immunohistochemical analysis and 3D reconstruction of the cephalic nervous system in Chaetognatha: Insights into the evolution of an early bilaterian brain? |journal=Invertebrate Biology |volume=129 |issue=1 |pages=77–104 |doi=10.1111/j.1744-7410.2010.00189.x}}</ref><ref>{{cite journal | vauthors = Harzsch S, Müller CH | title = A new look at the ventral nerve centre of Sagitta: implications for the phylogenetic position of Chaetognatha (arrow worms) and the evolution of the bilaterian nervous system | journal = Frontiers in Zoology | volume = 4 | pages = 14 | date = May 2007 | pmid = 17511857 | pmc = 1885248 | doi = 10.1186/1742-9994-4-14 | doi-access = free }}</ref> According to 2017 and 2019 papers, chaetognaths either belong to<ref>{{cite journal | vauthors = Fröbius AC, Funch P | title = Rotiferan Hox genes give new insights into the evolution of metazoan bodyplans | journal = Nature Communications | volume = 8 | issue = 1 | pages = 9 | date = April 2017 | pmid = 28377584 | pmc = 5431905 | doi = 10.1038/s41467-017-00020-w | bibcode = 2017NatCo...8....9F }}</ref><ref>{{cite journal | vauthors = Marlétaz F, Peijnenburg KT, Goto T, Satoh N, Rokhsar DS | title = A New Spiralian Phylogeny Places the Enigmatic Arrow Worms among Gnathiferans | language = en | journal = Current Biology | volume = 29 | issue = 2 | pages = 312–318.e3 | date = January 2019 | pmid = 30639106 | doi = 10.1016/j.cub.2018.11.042 | doi-access = free | bibcode = 2019CBio...29E.312M | url = https://discovery.ucl.ac.uk/10097250/1/Phylogeny_chaeto_rv9s_pre.pdf }}</ref> or are the sister group of  [[Gnathifera (clade)|Gnathifera]].<ref name="Perez2021"/>

===Internal===
Below is a consensus evolutionary tree of extant Chaetognatha, based on both morphological and molecular data, as of 2021.<ref name="Perez2021">{{cite book | last1=Perez | first1=Yvan | last2=Müller | first2=Carsten H.G. | last3=Harzsch | first3=Steffen |editor-last=Schierwater | editor-first=Bernd | editor-last2=DeSalle | editor-first2=Rob | title=Invertebrate Zoology: A Tree of Life Approach | publisher=CRC Press | year=2021 | chapter=Chapter 15: Chaetognatha | isbn=978-1-4822-3582-1 | chapter-url=https://books.google.com/books?id=Bk4vEAAAQBAJ&pg=PA231 | access-date=2023-08-14 | page=231}}</ref>

{{clade |style=line-height:100%;
|label1=Chaetognatha
|1={{clade
   |1={{clade
      |1=[[Heterokrohniidae]]
      |2=[[Eukrohniidae]]
      }}
   |2={{clade
      |1=[[Spadellidae]]
      |2={{clade
         |1=[[Krohnittidae]]
         |2=[[Sagittidae]]
         }}
      }}
   }}
}}

== Fossil record ==
Due to their soft bodies, chaetognaths fossilize poorly. Even so, several fossil chaetognath species have been described.<ref name="Vannier2007" /> Chaetognaths first appear during the [[Cambrian]] Period. Complete body fossils have been formally described from the Lower [[Cambrian]] [[Maotianshan shales]] of [[Yunnan, China]] (''Eognathacantha ercainella'' Chen & Huang<ref>{{cite journal | vauthors = Chen JY, Huang DY | title = A possible Lower Cambrian chaetognath (arrow worm) | journal = Science | volume = 298 | issue = 5591 | pages = 187 | date = October 2002 | pmid = 12364798 | doi = 10.1126/science.1075059 | s2cid = 544495 }}</ref> and ''Protosagitta spinosa'' Hu<ref>{{cite journal | vauthors = Hu SX |year=2005 |title=Taphonomy and palaeoecology of the Early Cambrian Chengjiang Biota from Eastern Yunnan, China |journal=Berliner Paläobiologische Abhandlungen |volume=7 |pages=1–197}}</ref>) and the Middle Cambrian [[Burgess Shale]] of British Columbia (''[[Capinatator praetermissus]]''.) A Cambrian [[stem-group]] chaetognath, ''[[Timorebestia]]'', first described in 2024, was much larger than modern species, showing that chaetognaths occupied different roles in marine ecosystems compared to today.<ref name="Park2024">{{cite journal |doi=10.1126/sciadv.adi6678 |title=A giant stem-group chaetognath |date=2024 |last1=Park |first1=Tae-Yoon S. |last2=Nielsen |first2=Morten Lunde |last3=Parry |first3=Luke A. |last4=Sørensen |first4=Martin Vinther |last5=Lee |first5=Mirinae |last6=Kihm |first6=Ji-Hoon |last7=Ahn |first7=Inhye |last8=Park |first8=Changkun |last9=De Vivo |first9=Giacinto |last10=Smith |first10=M. Paul |last11=Harper |first11=David A. T. |last12=Nielsen |first12=Arne T. |last13=Vinther |first13=Jakob |journal=Science Advances |volume=10 |issue=1 |pages=eadi6678 |doi-access=free |pmid=38170772 |pmc=10796117 |bibcode=2024SciA...10I6678P }}</ref> A more recent chaetognath, ''Paucijaculum samamithion'' Schram, has been described from the [[Mazon Creek]] biota from the [[Pennsylvanian (geology)|Pennsylvanian]] of Illinois. 


Chaetognaths were thought possibly to be related to some of the animals grouped with the [[conodont]]s. The conodonts themselves, however, have been shown to be dental elements of [[vertebrates]]. It is now thought that [[protoconodont]] elements (e.g., ''[[Protohertzina]] anabarica'' Missarzhevsky, 1973), are probably grasping spines of chaetognaths rather than teeth of conodonts. Previously chaetognaths in the Early Cambrian were only suspected from these protoconodont elements, but the more recent discoveries of body fossils have confirmed their presence then.<ref>{{cite journal | vauthors = Szaniawski H |year=2002 |title=New evidence for the protoconodont origin of chaetognaths |journal=[[Acta Palaeontologica Polonica]] |volume=47 |issue=3 |url=http://www.app.pan.pl/archive/published/app47/app47-405.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.app.pan.pl/archive/published/app47/app47-405.pdf |archive-date=2022-10-09 |url-status=live | pages=405–419 }}</ref> There is evidence that chaetognaths were important components of the oceanic [[food web]] already in the Early Cambrian.<ref>{{cite journal |doi=10.1098/rspb.2006.3761 |title=Early Cambrian origin of modern food webs: Evidence from predator arrow worms |date=2007 |last1=Vannier |first1=J. |last2=Steiner |first2=M. |last3=Renvoisé |first3=E. |last4=Hu |first4=S.-X |last5=Casanova |first5=J.-P |journal=Proceedings of the Royal Society B: Biological Sciences |volume=274 |issue=1610 |pages=627–633 |pmid=17254986 |pmc=2197202 }}</ref>

==History==
The first known description of a chaetognath has been published by Dutch naturalist [[Martinus Slabber]] in the 1770s; he also coined the name "arrow worm".<ref name="Slabber1778">{{cite book |last1=Slabber |first1=Martinus |title=Natuurkundige Verlustigingen, Behelzende Microscopise Waarneemingen Van de in—En Uitlandse Water—En Land-Dieren |date=1778 |publisher=J. Bosch |location=Haarlem |pages=46–48 |url=https://books.google.com/books?id=f8rDmjs4abkC&pg=PA3}}</ref><ref name="Pauly2021"/> The zoologist [[Henri Marie Ducrotay de Blainville]] also briefly mentioned probable chaetognaths but he understood them as pelagic mollusks. The first description of a currently accepted species of chaetognath, ''[[Sagitta bipunctata]]'', is from 1827.<ref name="Quoy1827">Quoy, J.R.C.; Gaimard, J.P. "Observations Zoologiques Faites à Bord de l’Astrolabe, en Mai 1826, dans le Détroit de Gibraltar (suite et fin). Description des genres Biphore, Carinaire, Hyale, Flèche, Cléodore, Anatife et Briarée." ''Ann. Sci. Nat.'' 1827, 10, 225–239</ref><ref name="Pauly2021"/> Among the early zoologists describing arrow worms, there is [[Charles Darwin]], who took notes about them during the [[Second voyage of HMS Beagle|voyage of the ''Beagle'']] and in 1844 dedicated a paper to them.<ref name="Darwin1844">Darwin, C. "Observations on the Structure and Propagation of the Genus Sagitta." ''Ann. Mag. Nat. Hist.'' 1844, 13, 1–6.</ref> In the following year, [[August David Krohn]] published an early anatomical description of ''Sagitta bipunctata''.<ref name="Krohn1845">{{cite journal |doi=10.1080/037454809496523 |title=XXXI.— ''Anatomical and physiological observations on'' Sagitta bipunctata |date=1845 |last1=Krohn |first1=M.A. |journal=Annals and Magazine of Natural History |volume=16 |issue=106 |pages=289–304 |url=https://www.biodiversitylibrary.org/part/60372 }}</ref><ref name="Choo2022"/>

The term "chaetognath" has been coined in 1856 by [[Rudolf Leuckart]]. He was also the first to propose that the genus ''Sagitta'' belonged to a separate group: «At the moment, it seems most natural to regard the Sagittas as representatives of a small group of their own that makes the transition from the real annelids (first of all the lumbricines) to the nematodes, and may not be unsuitably named Chaetognathi.»<ref name="Leuckart1856">Leuckart, R. Nachträge und Berichtigungen zu dem ersten Bande von J. van Der Hoeven’s Handbuch der Zoologie. Eine Systematisch Geordnete Übersicht der Hauptsächlichste Neueren Leistungen:über die Zoologie der Wirbellosen Thiere; L. Voss: Leipzig, Germany, 1856. (In German)</ref><ref name="Pauly2021"/>

The modern [[systematics]] of Chaetognatha begins in 1911 with Ritter-Záhony<ref name="Ritter-Zahony1911">Ritter-Záhony R. (1911) "Revision der Chaetognathan." ''Deutsche Sudpolar Expedition 1901–1903''. Band 13, Zoologie 5. Hft. 1: 1–72.</ref><ref name="Choo2022"/> and is later consolidated by [[Takasi Tokioka]] in 1965<ref name="Tokioka1965">{{cite journal |doi=10.5134/175381 |title=The Taxonomical Outline of Chaetognatha |date=1965 |last1=Tokioka |first1=Takasi |journal=Publications of the Seto Marine Biological Laboratory |volume=12 |issue=5 |pages=335–357 |s2cid=81053929 |doi-access=free }}</ref><ref name="Pauly2021"/><ref name="Choo2022"/> and Robert Bieri in 1991.<ref name="Bieri1991">Bieri, Robert. "Systematics of the Chaetognatha." in ''The biology of chaetognaths'' (1991): 122–136.</ref> Tokioka introduced the orders [[Phragmophora]] and [[Aphragmophora]], and classified four families, six genera, for a total of 58 species – plus the extinct ''[[Amiskwia]]'', classified as a true primitive chaetognath in a separate class, Archisagittoidea.<ref name="Choo2022"/>

Chaetognaths were for a while considered as belonging or affine to the [[deuterostomes]], but suspects of their affinities among [[Spiralia]] or other protostomes were already present as early as 1986.<ref name="Papillon2004">{{cite journal |doi=10.1093/molbev/msh229 |title=Identification of Chaetognaths as Protostomes is Supported by the Analysis of Their Mitochondrial Genome |date=2004 |last1=Papillon |first1=Daniel |last2=Perez |first2=Yvan |last3=Caubit |first3=Xavier |last4=Le Parco |first4=Yannick |journal=Molecular Biology and Evolution |volume=21 |issue=11 |pages=2122–2129 |pmid=15306659 |doi-access=free }}</ref> Their affinities with protostomes were clarified in 2004 by sequencing and analysis of [[mtDNA]].<ref name="Papillon2004"/>

== Infection by giant viruses ==
[[File:Comparison of the size of giant viruses to a common virus (HIV) and bacteria (E. coli).tif|thumb|Comparison of size between various viruses and the bacteria ''E. coli'' ]]
In 2018, reanalysis of electron microscopy photographs from the 1980s allowed scientists to identify a [[giant virus]] (''Meelsvirus'') infecting ''Adhesisagitta hispida''; its site of multiplication is nuclear and the virions (length: 1.25 μm) are enveloped.<ref>{{cite journal | vauthors = Shinn GL, Bullard BL | title = Ultrastructure of Meelsvirus: A nuclear virus of arrow worms (phylum Chaetognatha) producing giant "tailed" virions | journal = PLOS ONE | volume = 13 | issue = 9 | pages = e0203282 | date = 2018-09-19 | pmid = 30231047 | pmc = 6145532 | doi = 10.1371/journal.pone.0203282 | editor-first = Carmen | editor-last = San Martin | bibcode = 2018PLoSO..1303282S | doi-access = free }}</ref> In 2019, reanalysis of other previous studies has shown that structures that were taken in 1967 for bristles present on the surface of the species ''[[Spadella]] cephaloptera'',<ref>{{cite journal| vauthors = Horridge GA, Boulton PS |date=1967-11-14|title=Prey detection by Chaetognatha via a vibration sense|journal= Proceedings of the Royal Society of London. Series B. Biological Sciences|volume=168|issue=1013|pages=413–419|doi=10.1098/rspb.1967.0072|bibcode=1967RSPSB.168..413H|s2cid=86422882}}</ref> and in 2003, for bacteria infecting ''[[Paraspadella]] gotoi'',<ref>{{cite journal | vauthors = Casanova JP, Duvert M, Goto T | title = Ultrastructural study and ontogenesis of the appendages and related musculature of Paraspadella (Chaetognatha) | journal = Tissue & Cell | volume = 35 | issue = 5 | pages = 339–51 | date = October 2003 | pmid = 14517101 | doi = 10.1016/S0040-8166(03)00055-7 }}</ref> were in fact enveloped and spindle-shaped giant viruses with a cytoplasmic site of multiplication.<ref name="Barthélémy2019">Roxane-Marie Barthélémy, Eric Faure, Taichiro Goto: [https://www.semanticscholar.org/paper/Serendipitous-Discovery-in-a-Marine-Invertebrate-of-Barth%C3%A9l%C3%A9my-Faure/84c0a5481ea8fab48329ac6d28e81a091c59164a ''Serendipitous Discovery in a Marine Invertebrate (Phylum Chaetognatha) of the Longest Giant Viruses Reported till Date''.] In: ''Biology'', 2019, [https://www.hilarispublisher.com/abstract/serendipitous-discovery-in-a-marine-invertebrate-phylum-chaetognatha-of-the-longest-giant-viruses-reported-to-date-24968.html Abstract]</ref> The viral species infecting ''P. gotoi'', whose maximum length is 3.1 μm, has been named ''[[Klothovirus casanovai]]'' (''[[Clotho|Klotho]]'' being the Greek name for one of the three Fates whose attribute was a spindle, and ''casanovai'', in tribute to Pr J.-P. Casanova who devoted a large part of his scientific life to the study of chaetognaths). The other species has been named ''[[Megaklothovirus horridgei]]'' (in tribute to [[Adrian Horridge]], the first author of the 1967 article). On a photograph, one of the viruses ''M. horridgei'', although truncated, is 3.9 μm long, corresponding to about twice the length of the bacteria ''[[Escherichia coli]]''. Many [[Ribosome|ribosomes]] are present in virions but their origin remains unknown (cellular, viral or only partly viral). To date, giant viruses known to infect [[metazoans]] are exceptionally rare.

== References ==
{{Reflist|32em}}

== External links ==
{{Commons category|Chaetognatha}}
* [https://web.archive.org/web/20080720014039/http://www.tafi.org.au/zooplankton/imagekey/chaetognatha/images/sagitta_anatomy_full.jpg Image of ''Pseudosagitta gazellae'' with a krill in its gut from the Tasmanian Aquaculture and Fisheries Institute]
* [https://web.archive.org/web/20070311002237/http://nlbif.eti.uva.nl/bis/chaetognatha.php?menuentry=zoeken&id=&selected=wetenschap Chaetognatha of the World] – last retrieved December 13, 2006
* Eric Fauré, Roxane-Marie Barthélémy: [https://www.semanticscholar.org/paper/Specific-mitochondrial-ss-tRNAs-in-phylum-Faur%C3%A9-Barth%C3%A9l%C3%A9my/72f9c512738c88fd2b97fec882468743849c958e Specific mitochondrial ss-tRNAs in phylum Chaetognatha]. In: Journal of Entomology and Zoology Studies 7(3), April 2019, pp.&nbsp;304–315. [https://hal-amu.archives-ouvertes.fr/hal-02130653/document hal-02130653]

{{Plankton}}
{{Animalia}}
{{Life on Earth}}
{{Taxonbar|from1=Q192416|from2=Q2356636}}
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[[Category:Chaetognatha| ]]
[[Category:Planktology]]
[[Category:Protostome phyla]]
[[Category:Extant Cambrian first appearances]]
[[Category:Taxa named by Karl Grobben]]
[[Category:Gnathifera (clade)]]