| Arrow worms | |
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| Chaetognatha and some examples of their diversity. | |
Scientific classification | |
| Kingdom: | Animalia |
| Subkingdom: | Eumetazoa |
| Clade: | ParaHoxozoa |
| Clade: | Bilateria |
| Clade: | Nephrozoa |
| Clade: | Protostomia |
| Clade: | Spiralia |
| Clade: | Gnathifera |
| Clade: | Cucullophora |
| Phylum: | Chaetognatha Leuckart, 1854 |
| Class: | Sagittoidea Claus &Grobben, 1905 [2] |
| Orders | |
TheChaetognatha/kiːˈtɒɡnəθə/ orchaetognaths/ˈkiːtɒɡnæθs/ (meaningbristle-jaws) are aphylum of predatorymarine worms that are a major component ofplankton worldwide. Commonly known asarrow worms, they are mostlypelagic; however about 20% of the knownspecies arebenthic, and can attach toalgae androcks. They are found in all marine waters, from surfacetropical waters and shallow tide pools to thedeep sea andpolar regions. Most chaetognaths are transparent and are torpedo shaped, but some deep-sea species are orange. They range in size from 2 to 120 millimetres (0.1 to 4.7 in).
Chaetognaths were first recorded by the Dutch naturalistMartinus Slabber in 1775.[5] As of 2021, biologists recognize 133 modern species assigned to over 26genera and eightfamilies.[5] Despite the limited diversity of species, the number of individuals is large.[6]
Arrow worms are strictly related to and possibly belonging toGnathifera, aclade ofprotostomes that do not belong to eitherEcdysozoa orLophotrochozoa.
Chaetognaths are transparent or translucent dart-shaped animals covered by acuticle. They range in length between 1.5 mm to 105 mm in the Antarctic speciesPseudosagitta gazellae.[7] Body size, either between individuals in the same species or between different species, seems to increase with decreasing temperature.[7] The body is divided into a distinct head, trunk, and tail. The longitudinalmusculature, which makes up approximately 80% of the body's volume,[5] contracts against ahydrostatic skeleton consisting of a fluid-filled body cavity enclosed by the cuticle and a dense sheath of helically arranged collagen fibrils.[8][9] Also a transverse musculature is present in most representants of order Phragmophora (vestigial in Eukrohniidae), but is absent in Aphragmophora.[10]
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.[5]
The mouth opens into a muscularpharynx, which contains glands to lubricate the passage of food. From here, a straight intestine runs the length of the trunk to ananus just forward of the tail. The intestine is the primary site of digestion and includes a pair ofdiverticula near the anterior end.[11] Materials are moved about the body cavity bycilia. 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 forEukrohnia hamata in Arctic seas.[12] 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.[13]
Usually chaetognaths are not pigmented, however the intestines of some deep-sea species contain orange-red carotenoid pigments.[5]
The nervous system is reasonably simple and shows a typicalprotostome anatomy,[5] consisting of aganglionated 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.[5] 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-cupocelli fused together; some deep-sea andtroglobitic species have unpigmented or absent eyes.[5] 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 thelateral line in fish. An additional, curved, band of sensory bristles lies over the head and neck.[11] Almost all chaetognaths have "indirect" or "inverted" eyes, according to the orientation ofphotoreceptor cells; only someEukhroniidae species have "direct" or "everted" eyes.[5] A unique feature of the chaetognath eye is the lamellar structure of photoreceptor membranes, containing a grid of 35–55 nm wide circular pores.[5]
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.[5]
The body cavity is lined byperitoneum, and therefore represents a truecoelom, 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.[14][5] While often said to lack a circulatory system, chaetognaths do have a rudimentary hemal system resembling those ofannelids.[5]
The arrow wormrhabdomeres are derived frommicrotubules 20 nm long and 50 nm wide, which in turn form conical bodies that contain granules and thread structures. The cone body is derived from acilium.[15]
The trunk bears one or two pairs of lateral fins incorporating structures superficially similar to thefin rays of fish, with which they are nothomologous. Unlike those of vertebrates, these lateral fins are composed of a thickenedbasement membrane extending from theepidermis. An additional caudal fin covers the post-anal tail.[11] Two chaetognath species,Caecosagitta macrocephala andEukrohnia fowleri, havebioluminescent organs on their fins.[16][17]
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.[18] To avoid sinking they need to swim regularly, but many species haveammonium-filled vacuolated cells in the trunk, which gives them close to neutral buoyancy.[19][20] Muscle movements have been described as among the fastest of any animals.[5] Muscles are directly excitable by electrical currents or strongK+ solutions; the main neuromuscular transmitter is acetylcholine.[5]
All species arehermaphroditic, carrying botheggs andsperm.[6] Each animal possesses a pair oftestes within the tail, and a pair ofovaries 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 aseminal vesicle where they are packaged into aspermatophore.[11]
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 theoviducts, into which the developed eggs have already passed from the ovaries, and it is here that fertilisation takes place.[11] The seminal receptacles and oviducts accumulate and store spermatozoa, to perform multiple fertilisation cycles.[5] Somebenthic members ofSpadellidae are known to have elaboratecourtship rituals before copulation,[5] for exampleParaspadella gotoi.[21]
The eggs are mostly planktonic, except in a few species such asFerosagitta hispida that attaches eggs to the substrate.[5] InEukrohnia, eggs develop in marsupial sacs or attached toalgae.[22] Eggs usually hatch after 1–3 days. Chaetognaths do not undergometamorphosis nor they possess a well-definedlarval stage,[11][5] an unusual trait among marine invertebrates;[21] however there are significant morphological differences between the newborn and the adult, with respect to proportions, chitinous structures and fin development.[5][23]
The life spans of chaetognaths are variable but short; the longest recorded was 15 months inSagitta friderici.[23]
Little is known of arrow worms' behaviour and physiology, due to the complexity in culturing them and reconstructing their natural habitat.[5] 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.[5] They often show a "gliding" behaviour, slowly sinking for a while, and then catching up with a quick movement of their fins.[21] 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.[5] The prey is detected thanks to the ciliary fence and tuft organs, sensing vibrations[5] – individuals ofSpadella cephaloptera for example will attack a glass or metal probe vibrating at an adequate frequency.[21] To catch prey, arrow worms jump forward with a strong stroke of the tail fin.[5] 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.[21]
Chaetognaths are found in all world's oceans, from the poles to tropics, and also inbrackish andestuarine waters. They inhabit very diverse environments, from hydrothermal vents to deep ocean seafloor, to seagrass beds and marine caves.[5] The majority are planktonic, and they are often the second most common component ofzooplankton, with a biomass ranging between 10 and 30% that ofcopepods.[5] In theCanada Basin, chaetognaths alone represent ~13% of the zooplankton biomass.[24] As such, they are ecologically relevant and a key food source for fishes and other predators, including commercially relevant fishes such asmackerel orsardines.[25] 58% of known species are pelagic,[7] while about a third of species areepibenthic ormeiobenthic, or inhabit the immediate vicinity of the substrate.[5] Chaetognaths have been recorded up to 5000 and possibly even 6000 meters of depth.[7]
The highest density of chaetognaths is observed in thephotic zone of shallow waters.[5] Larger chaetognath species tend to live deeper in water, but spend their juvenile stages higher in the water column.[21] Arrow worms however engage indiel 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 againstoceanic currents, and they are used as a hydrological indicator of currents and water masses.[5]
All chaetognaths areambush predators, preying on other planktonic animals, mostly copepods andcladocerans[11][5] but also amphipods, krill and fish larvae.[25] Adults can feed on younger individuals of the same species.[26] Some species are also reported to be omnivores, feeding on algae and detritus.[27] Chaetognaths are known to use theneurotoxintetrodotoxin to subdue prey,[28] possibly synthesized byVibrio bacterial species.[5]
In 2025, the genome of the arrow wormParaspadella gotoi has been published. The genome is 257 Mb long, consists of 9 chromosomes and includes 22.072 protein-coding genes, with 20% of the sequence being occupied by repeats.[29] The genome lacks genes for the centromerichistone H3 andCENPT, both proteins involved in thecentromere; a condition associated with faster rearrangements of the genome in other species. Accordingly, genomic analysis indicates a higher rate of chromosomal rearrangement ingnathiferans. TheP.gotoi genome also shows evidence of a significantgene duplication event, probably due to a burst of tandem gene duplication (a rare condition in animal genomes) instead than to awhole genome duplication event, involving 3.379 gene families. Many of these genes are involved in development andion transmembrane transport.[29] Chaetognaths seem to include as many as 2.250 unique gene families, a large number compared with the 157 unique gene families ofMollusca or 124 unique gene families ofEchinodermata.[29] Nearly half the chaetognath genes undergotrans-splicing.[29]
ThemtDNA of the arrow wormSpadella cephaloptera has been sequenced in 2004, and at the time it was the smallest metazoan mitochondrial genome known, being 11,905 base pairs long[30] (it has now been surpassed by the mitchondrial genome of thectenophoreMnemiopsis leidyi, which is 10,326 bp long).[31] All mitochondrialtRNA genes are absent. TheMT-ATP8 andMT-ATP6 genes are also missing.[30] ThemtDNA ofParaspadella 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.[32]
Chaetognaths show a unique mitochondrial genomic diversity within individual of the same species.[33]
The evolutionary relationships of chaetognaths have long been enigmatic.Charles Darwin remarked that arrow worms were "remarkable for the obscurity of their affinities".[21] Chaetognaths in the past have been traditionally, but erroneously, classed asdeuterostomes byembryologists due to deuterostome-like features in the embryo.Lynn Margulis and K. V. Schwartz placed chaetognaths in the deuterostomes in theirFive Kingdom classification.[35] However, several developmental features are at odds with deuterostomes and are either akin toSpiralia or unique to Chaetognatha.[5]
| Summary of relationships of gnathiferans in recent studies including Chaetognatha within the clade, with disputed relationships represented as polytomies[36][37][38][39][40] |
| Chaetognaths in themetazoan tree of life, when considered the sister group of Gnathifera.[5] |
Molecularphylogeny shows that Chaetognatha are, in fact,protostomes.Thomas Cavalier-Smith places them in the protostomes in hisSix Kingdom classification.[41] The similarities between chaetognaths and nematodes mentioned above may support the protostome thesis—in fact, chaetognaths are sometimes regarded as a basalecdysozoan orlophotrochozoan.[42] Chaetognatha appears close to the base of the protostome tree in most studies of their molecular phylogeny.[43] 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 earlybilaterian ancestors. Thus chaetognaths may be a useful model for the ancestral bilaterian.[44]Studies of arrow worms' nervous systems suggests they should be placed within the protostomes.[45][46] According to 2017 and 2019 papers, chaetognaths either belong to[47][48] or are the sister group ofGnathifera.[5]
In 2025, the sequencing ofParaspadella gotoi revealed that chaetognaths underwent both extensivegene loss andgene duplication and feature an unusual number of phylum-specific gene families that seem to have an important role in shaping novel cell types. One of the duplication events involved theHox cluster, probably playing a role in sculpting the unique chaetognath body plan.[29] Overall, according to researchers, the chaetognath body plan seems to have evolved not by incorporating and adapting the ancestral bilaterian genetic toolkit, but by discarding most of it and then evolving its own new genes, thus explaining at least in part the enigmatic uniqueness of these animals.[29]
Due to their soft bodies, chaetognaths fossilize poorly. Even so, several fossil chaetognath species have been described.[1] Chaetognaths first appear during theCambrian Period. Complete body fossils have been formally described from the LowerCambrianMaotianshan shales ofYunnan, China (Eognathacantha ercainella Chen & Huang[49] andProtosagitta spinosa Hu[50]) and the Middle CambrianBurgess Shale of British Columbia (Capinatator praetermissus.) A Cambrianstem-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.[51] A more recent chaetognath,Paucijaculum samamithion Schram, has been described from theMazon Creek biota from thePennsylvanian of Illinois. The enigmaticNectocaris, documented from theBurgess Shale since 1910 and described in 1976, has been recognized as a stem-chateognath in 2025.[52]
Chaetognaths were thought possibly to be related to some of the animals grouped with theconodonts. The conodonts themselves, however, have been shown to be dental elements ofvertebrates. It is now thought thatprotoconodont 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.[53] There is evidence that chaetognaths were important components of the oceanicfood web already in the Early Cambrian.[54]
Below is a consensus evolutionary tree of extant Chaetognatha, based on both morphological and molecular data, as of 2021.[5]
The first known description of a chaetognath has been published by Dutch naturalistMartinus Slabber in the 1770s; he also coined the name "arrow worm".[55][7] The zoologistHenri 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.[56][7] Among the early zoologists describing arrow worms, there isCharles Darwin, who took notes about them during thevoyage of theBeagle and in 1844 dedicated a paper to them.[57] In the following year,August David Krohn published an early anatomical description ofSagitta bipunctata.[58][25]
The term "chaetognath" has been coined in 1856 byRudolf Leuckart. He was also the first to propose that the genusSagitta 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.»[59][7]
The modernsystematics of Chaetognatha begins in 1911 with Ritter-Záhony[60][25] and is later consolidated byTakasi Tokioka in 1965[61][7][25] and Robert Bieri in 1991.[62] Tokioka introduced the ordersPhragmophora andAphragmophora, and classified four families, six genera, for a total of 58 species – plus the extinctAmiskwia, classified as a true primitive chaetognath in a separate class, Archisagittoidea.[25]
Chaetognaths were for a while considered as belonging or affine to thedeuterostomes, but suspects of their affinities amongSpiralia or other protostomes were already present as early as 1986.[30] Their affinities with protostomes were clarified in 2004 by sequencing and analysis ofmtDNA.[30]
In 2018, reanalysis of electron microscopy photographs from the 1980s allowed scientists to identify agiant virus (Meelsvirus) infectingAdhesisagitta hispida; its site of multiplication is nuclear and the virions (length: 1.25 μm) are enveloped.[63] In 2019, reanalysis of other previous studies has shown that structures that were taken in 1967 for bristles present on the surface of the speciesSpadella cephaloptera,[64] and in 2003, for bacteria infectingParaspadella gotoi,[65] were in fact enveloped and spindle-shaped giant viruses with a cytoplasmic site of multiplication.[66] The viral species infectingP. gotoi, whose maximum length is 3.1 μm, has been namedKlothovirus casanovai (Klotho being the Greek name for one of the three Fates whose attribute was a spindle, andcasanovai, 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 namedMegaklothovirus horridgei (in tribute toAdrian Horridge, the first author of the 1967 article). On a photograph, one of the virusesM. horridgei, although truncated, is 3.9 μm long, corresponding to about twice the length of the bacteriaEscherichia coli. Manyribosomes are present in virions but their origin remains unknown (cellular, viral or only partly viral). To date, giant viruses known to infectmetazoans are exceptionally rare.
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