Chaetognatha

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. Also a transverse musculature is present in most representants of order Phragmophora (vestigial in Eukrohniidae), but is absent in Aphragmophora. 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. 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. Usually chaetognaths are not pigmented, however the intestines of some deep-sea species contain orange-red carotenoid pigments. Locomotion The trunk bears one or two pairs of lateral fins incorporating structures superficially similar to the fin rays of fish, with which they are not homologous. Unlike those of vertebrates, these lateral fins are composed of a thickened basement membrane extending from the epidermis. An additional caudal fin covers the post-anal tail. 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. To avoid sinking they need to swim regularly, but many species have ammonium-filled vacuolated cells in the trunk, which gives them close to neutral buoyancy. Muscle movements have been described as among the fastest of any animals. Muscles are directly excitable by electrical currents or strong K+ solutions; the main neuromuscular transmitter is acetylcholine. ==Reproduction and life cycle==

All species are hermaphroditic, carrying both eggs and sperm. Eggs usually hatch after 1–3 days. Chaetognaths do not undergo metamorphosis nor they possess a well-defined larval stage, ==Behaviour==

Little is known of arrow worms' behaviour and physiology, due to the complexity in culturing them and reconstructing their natural habitat. 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. They often show a "gliding" behaviour, slowly sinking for a while, and then catching up with a quick movement of their fins. 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. The prey is detected thanks to the ciliary fence and tuft organs, sensing vibrations – individuals of Spadella cephaloptera for example will attack a glass or metal probe vibrating at an adequate frequency. To catch prey, arrow worms jump forward with a strong stroke of the tail fin. 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. ==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. 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. 58% of known species are pelagic, The highest density of chaetognaths is observed in the photic zone of shallow waters. Some species are also reported to be omnivores, feeding on algae and detritus. Chaetognaths are known to use the neurotoxin tetrodotoxin to subdue prey, possibly synthesized by Vibrio bacterial species. ==Genetics==

In 2025, the genome of the arrow worm Paraspadella 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. The genome lacks genes for the centromeric histone H3 and CENPT, both proteins involved in the centromere; a condition associated with faster rearrangements of the genome in other species. Accordingly, genomic analysis indicates a higher rate of chromosomal rearrangement in gnathiferans. The P.gotoi genome also shows evidence of a significant gene duplication event, probably due to a burst of tandem gene duplication (a rare condition in animal genomes) instead than to a whole genome duplication event, involving 3.379 gene families. Many of these genes are involved in development and ion transmembrane transport. All mitochondrial tRNA genes are absent. The MT-ATP8 and MT-ATP6 genes are also missing. Chaetognaths show a unique mitochondrial genomic diversity within individual of the same species. == Phylogeny and evolution ==

Evolution and relationship to other animals The evolutionary relationships of chaetognaths have long been enigmatic. Charles Darwin remarked that arrow worms were "remarkable for the obscurity of their affinities". Chaetognaths in the past have been traditionally, but erroneously, classed as deuterostomes by embryologists due to deuterostome-like features in the embryo. Lynn Margulis and K. V. Schwartz placed chaetognaths in the deuterostomes in their Five Kingdom classification. However, several developmental features are at odds with deuterostomes and are either akin to Spiralia or unique to Chaetognatha. The similarities between chaetognaths and nematodes mentioned above may support the protostome thesis—in fact, chaetognaths are sometimes regarded as a basal ecdysozoan or lophotrochozoan. Chaetognatha appears close to the base of the protostome tree in most studies of their molecular phylogeny. 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 bilaterian ancestors. Thus chaetognaths may be a useful model for the ancestral bilaterian. Studies of arrow worms' nervous systems suggests they should be placed within the protostomes. According to 2017 and 2019 papers, chaetognaths either belong to or are the sister group of Gnathifera. and Protosagitta spinosa Hu) 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. A more recent chaetognath, Paucijaculum samamithion Schram, has been described from the Mazon Creek biota from the Pennsylvanian of Illinois. The enigmatic Nectocaris, documented from the Burgess Shale since 1910 and described in 1976, has been recognized as a stem-chateognath in 2025. Chaetognaths were thought possibly to be related to some of the animals grouped with the conodonts. 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. There is evidence that chaetognaths were important components of the oceanic food web already in the Early Cambrian. Internal phylogeny Below is a consensus evolutionary tree of extant Chaetognatha, based on both morphological and molecular data, as of 2021. }} ==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". In the following year, August David Krohn published an early anatomical description of Sagitta bipunctata. 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. Their affinities with protostomes were clarified in 2004 by sequencing and analysis of mtDNA. == Infection by giant viruses ==

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. 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, and in 2003, for bacteria infecting Paraspadella gotoi, were in fact enveloped and spindle-shaped giant viruses with a cytoplasmic site of multiplication. The viral species infecting P. gotoi, whose maximum length is 3.1 μm, has been named Klothovirus casanovai (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 ribosomes are present in virions but their origin remains unknown (cellular, viral or only partly viral). To date, giant viruses known to infect animals are exceptionally rare. == References ==