The Chaetognatha or chaetognaths (meaning bristle-jaws) are a phylum of predatory marine worms that are a major component of plankton worldwide. Commonly known as arrow worms, they are mostly pelagic; however about 20% of the known species are benthic, and can attach to algae and rocks. They are found in all marine waters, from surface tropical waters and shallow tide pools to the deep sea and polar regions. Most chaetognaths are transparent and are torpedo shaped, but some deep-sea species are orange. They range in size from .
Chaetognaths were first recorded by the Dutch naturalist Martinus Slabber in 1775.
Arrow worms are strictly related to and possibly belonging to Gnathifera, a clade of protostomes that do not belong to either Ecdysozoa or Lophotrochozoa.
Anatomy
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. 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. 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. 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. 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
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
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.
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