Chaetognatha
Arrow worms Temporal range:
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Spadella cephaloptera | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Subkingdom: | Eumetazoa |
Clade: | ParaHoxozoa |
Clade: | Bilateria |
Clade: | Nephrozoa |
(unranked): | Protostomia |
(unranked): | Spiralia |
Clade: | Gnathifera |
Phylum: | Chaetognatha Leuckart , 1854
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Class: | Sagittoidea Claus & Grobben, 1905 [2] |
Orders | |
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The Chaetognatha
Chaetognaths were first recorded by the Dutch naturalist Martinus Slabber in 1775.[3] As of 2021, biologists recognize 133 modern species assigned to over 26 genera and eight families.[3] Despite the limited diversity of species, the number of individuals is large.[4]
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
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.[5] Body size, either between individuals in the same species or between different species, seems to increase with decreasing temperature.[5] The body is divided into a distinct head, trunk, and tail. About 80% of the body is occupied by primary longitudinal muscles.[3]
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.[3]
The mouth opens into a muscular
Usually chaetognaths are not pigmented, however the intestines of some deep-sea species contain orange-red carotenoid pigments.[3]
Nervous and sensory systems
The nervous system is reasonably simple and shows a typical protostome anatomy,[3] consisting of a ganglionated 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.[3] The dorsal 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
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.[3]
Internal organs
The body cavity is lined by
The arrow worm
Locomotion
The trunk bears one or two pairs of lateral fins incorporating structures superficially similar to the
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.
Reproduction and life cycle
All species are
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
The eggs are mostly planktonic, except in a few species such as
The life spans of chaetognaths are variable but short; the longest recorded was 15 months in Sagitta friderici.[16]
Behaviour
Little is known of arrow worms' behaviour and physiology, due to the complexity in culturing them and reconstructing their natural habitat.[3] 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.[3] They often show a "gliding" behaviour, slowly sinking for a while, and then catching up with a quick movement of their fins.[14] 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.[3] The prey is detected thanks to the ciliary fence and tuft organs, sensing vibrations[3] - individuals of Spadella cephaloptera for example will attack a glass or metal probe vibrating at an adequate frequency.[14] To catch prey, arrow worms jump forward with a strong stroke of the tail fin.[3] 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.[14]
Ecology
Chaetognaths are found in all world's oceans, from the poles to tropics, and also in
The highest density of chaetognaths is observed in the
All chaetognaths are
Genetics
Mitochondrial genome
The
Chaetognaths show a unique mitochondrial genomic diversity within individual of the same species.[25]
Phylogeny
External
The evolutionary relationships of chaetognaths have long been enigmatic.
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Summary of relationships of gnathiferans in recent studies including Chaetognatha within the clade, with disputed relationships represented as polytomies[28][29][30][31][32] |
Chaetognaths in the metazoan tree of life, when considered the sister group of Gnathifera.[3]
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Molecular
Internal
Below is a consensus evolutionary tree of Chaetognatha, based on both morphological and molecular data, as of 2021.[3]
Chaetognatha |
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Fossil record
Due to their soft bodies, chaetognaths fossilize poorly. Even so, several fossil chaetognath species have been described.
Chaetognaths were thought possibly to be related to some of the animals grouped with the
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".[47][5] 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.[48][5] Among the early zoologists describing arrow worms, there is Charles Darwin, who took notes about them during the voyage of the Beagle and in 1844 dedicated a paper to them.[49] In the following year, August David Krohn published an early anatomical description of Sagitta bipunctata.[50][18]
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.»[51][5]
The modern systematics of Chaetognatha begins in 1911 with Ritter-Záhony[52][18] and is later consolidated by Takasi Tokioka in 1965[53][5][18] and Robert Bieri in 1991.[54] 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.[18]
Chaetognaths were for a while considered as belonging or affine to the
Infection by giant viruses
In 2018, reanalysis of electron microscopy photographs from the 1980s allowed scientists to identify a
References
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- ^ "Sagittoidea Claus and Grobben, 1905". Integrated Taxonomic Information System. Retrieved February 8, 2012.
- ^ ISBN 978-1-4822-3582-1. Retrieved 2023-08-14.
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- ^ Systema Naturae 2000 Taxon: Phylum Chaetognatha per Margulis and Schwartz Archived November 27, 2005, at the Wayback Machine (select Margulis & Schwartz in 'Classification by')—last retrieved November 25, 2006
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- ^ Systema Naturae 2000 Taxon: Phylum Chaetognatha per Cavalier-Smith Archived November 27, 2005, at the Wayback Machine (select Cavalier-Smith in 'Classification by')—last retrieved November 25, 2006
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- ^ Hu SX (2005). "Taphonomy and palaeoecology of the Early Cambrian Chengjiang Biota from Eastern Yunnan, China". Berliner Paläobiologische Abhandlungen. 7: 1–197.
- ^ Szaniawski H (2005). "Cambrian chaetognaths recognized in Burgess Shale fossils" (PDF). Acta Palaeontologica Polonica. 50 (1): 1–8. Archived (PDF) from the original on 2022-10-09.
- PMC 10796117.
- ^ Szaniawski H (2002). "New evidence for the protoconodont origin of chaetognaths" (PDF). Acta Palaeontologica Polonica. 47 (3): 405–419. Archived (PDF) from the original on 2022-10-09.
- PMID 17254986.
- ^ Slabber, Martinus (1778). Natuurkundige Verlustigingen, Behelzende Microscopise Waarneemingen Van de in—En Uitlandse Water—En Land-Dieren. Haarlem: J. Bosch. pp. 46–48.
- ^ 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
- ^ Darwin, C. "Observations on the Structure and Propagation of the Genus Sagitta." Ann. Mag. Nat. Hist. 1844, 13, 1–6.
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- ^ 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)
- ^ Ritter-Záhony R. (1911) "Revision der Chaetognathan." Deutsche Sudpolar Expedition 1901–1903. Band 13, Zoologie 5. Hft. 1: 1–72.
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- ^ Bieri, Robert. "Systematics of the Chaetognatha." in The biology of chaetognaths (1991): 122-136.
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- ^ Roxane-Marie Barthélémy, Eric Faure, Taichiro Goto: Serendipitous Discovery in a Marine Invertebrate (Phylum Chaetognatha) of the Longest Giant Viruses Reported till Date. In: Biology, 2019, Abstract
External links
- Image of Pseudosagitta gazellae with a krill in its gut from the Tasmanian Aquaculture and Fisheries Institute
- Chaetognatha of the World – last retrieved December 13, 2006
- Eric Fauré, Roxane-Marie Barthélémy: Specific mitochondrial ss-tRNAs in phylum Chaetognatha. In: Journal of Entomology and Zoology Studies 7(3), April 2019, pp. 304-315. hal-02130653