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The Chaetognatha or chaetognaths are a phylum of predatory marine worms that are a major component of plankton worldwide. Commonly known as arrow worms, about 20% of the known Chaetognatha 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 2 to 120 millimetres.
Eukrohniidae is a family of sagittoideans in the order Phragmophora. It consists of a single genus, Eukrohnia von Ritter-Záhony, 1909.
Gnathifera is a clade of generally small spiralians characterized by complex jaws made of chitin. It comprises the phyla Gnathostomulida, Rotifera, Micrognathozoa, and Chaetognatha. It may also include the Cycliophora.
Sagittidae is a family of sagittoideans in the order Aphragmophora.
Aphragmophora is an order of sagittodieans in the phylum Chaetognatha.
Phragmophora is an order of sagittoideans in the phylum Chaetognatha.
Heterokrohniidae is a family of sagittoideans in the order Phragmophora.
Krohnittellidae is a family of sagittoideans in the order Phragmophora. It consists of one genus, Krohnittella Germain & Joubin, 1912.
Bathybelidae is a family of sagittoideans in the order Aphragmophora. It consists of a single genus, Bathybelos Owre, 1973, which consists of a single species, Bathybelos typhlops Owre, 1973.
Krohnittidae is a family of chaetognaths in the order Aphragmophora. It consists of a single genus, Krohnitta von Ritter-Záhony, 1910.
Pterokrohniidae is a family of sagittoideans in the order Aphragmophora. I consists of a single genus, Pterokrohnia Srinivasan, 1986, which consists of a single species, Pterokrohnia arabica Srinavasan, 1986.
Spadellidae is a family of sagittoideans in the order Phragmophora. Spadellidae prey on plankton and commonly reside in the epipelagic zone of the ocean.
Archeterokrohnia is a genus of chaetognaths in the family Heterokrohniidae.The total body length excluding tail fin 28.5; the tail section is 55.2% of the tail fin; head blunt when hooded, triangular after preservation, head with 3.5 mm. Furthermore, the eyes are absent, the trunk section is orange throughout in life, and the organism exists around 3200 m below sea level.
Heterokrohnia is a genus of chaetognaths in the family Heterokrohniidae.
Eukrohnia fowleri is a deep-sea marine arrow worm. It is the only known bioluminescent member of the genus Eukrohnia, and one of the two known species of bioluminescent arrow worms, the other being the distantly related Caecosagitta macrocephala. The bioluminescent organ of Eukrohnia Fowlery is found along the center of its tail fin on both its dorsal and ventral side. It has a secreted bioluminescence that is thought to be coelenterazine based. While both species use luciferases in conjunction with coelenterazine for light emission, the luciferase of Eukrohnia fowleri is highly stable after 30 minutes while the luciferase of Caecosagitta macrocephala becomes inactive. So far, there is no other bioluminescent organism that uses hexagonal packing in order to hold bioluminescent materials/ E. fowleri evolved through the adaptation to hypoxic water and due to the recent oxygenation of water they have been experiencing bottleneck events. These events have been seen as one of the reasons that E. fowleri have such low biodiversity.
Caecosagitta macrocephala is a deep sea marine chaetognath that is distributed in meso- and bathypelagic layers. It has a very wide distribution that ranges from the Subantarctic to Subarctic Ocean. Cecosagitta macrocephalas have large heads, hence their name “macro-cephala”. Within their eyes are photoreceptive regions that allow them to catch weak light at bathypelagic depths. Along with their eyes, their gut or intestine has orange pigmentation and a luminous organ that gleams due to bioluminescence unlike some other species of Sagittidae. To be more precise, the luminescent organ is located on the ventral edge of each anterior lateral fin. It is the only member of the genus Caecosagitta, and only one of the two known species of bioluminescent chaetognath, the other being the distantly related Eukrohnia fowleri. C. macrocephala has a secreted bioluminescence that is thought to be coelenterazine based. The luciferase is highly unstable, being unable to survive a single freeze-thaw, and is rapidly inactivated at ice-cold temperatures.
Xenokrohnia is a genus of chaetognaths in the family Heterokrohniidae. It consists of one species, Xenokrohnia sorbei Casanova, 1993, which lives in a marine environment. The initial discovery was made from six specimens found in a deep-sea search in the Bay of Biscay for Spadella equidentata Casanova, 1987. A uniquely large (ventral) secretory gland, separate from other chaetognaths, defined the new genus and species. The gland, which is likely used to rid the body of digestive fluids, is probably due to the unique feeding habits of the species; these may include scavenging habits. The presence of an unusual and seemingly superfluous digestive utility is similar to Archeterokrohnia palpifera Casanova, 1986, which have a larger pair of pedipalps then what is common for chaetognaths.
Calispadella is a genus of chaetognaths in the family Spadellidae. It consists of one species, Calispadella alata Casanova & Moreau, 2005. It is differentiated from other species of Spadellidae by the presence of an unusually long tail segment and a rare aspect of the lateral fins, similar to that of Paraspadella gotoi Casanova, 1990. The species is noted by the complete development of seminal vesicles in juveniles and the use of lateral fins for buoyancy in the deep-sea waters where the specimens were found. It was the first to be described exclusively in the range of deep-sea hydrothermic vents.
Hemispadella is a genus of chaetognaths in the family Spadellidae. It consists of one species, Hemispadella dauvini Casanova, 1996. The species, as the generic name implies, shares numerous, but not all, of the characteristics of Spadellidae; it shares a number of other characteristics with Heterokrohniidae. The ventral ganglion, poorly described in chaetognaths, is of similar size to the Heterokrohniidae, and the larger number of teeth, and the difference in appearance and function between the anterior and posterior teeth, are similarly characteristic. The relative tail size is similar to that of the Spadellidae, although the overall size is unprecedented among the Spadellidae. The more developed nature of transverse musculature and the basis for the lateral fins are as those of Spadellidae, as well.
Paraspadella is a genus of chaetognaths in the family Spadellidae. Paraspadella was originally considered as Spadella before a revision separated that genus into three genera: Spadella, Paraspadella, and Gephyrospadella, the last of which is now synonymised to Paraspadella. The initial division was based on previous knowledge of three groups of Spadella, in a similar manner in which Sagitta was divided into a family of genera. Paraspadella is differentiated from Spadella by the presence of disparate (digital) adhesive organs, present in the former to various degrees, but entirely absent in the latter.
References
- 1 2 Tokioka, T. (1939). Three new chaetognaths from Japanese waters. Memoirs of the Imperial Marine Observatory, 7, 129–139.
- 1 2 Miyamoto, H. & Nishida, S. (2011). A new deep-sea benthopelagic chaetognath of the genus Bathyspadella (Chaetognatha) with ecological and molecular phylogenetic remarks. Journal of Natural History, 45(45), 2785–2794.
- ↑ Gasmi, S., Nève, G., Pech, N., Tekaya, S., Gilles, A. & Perez, Y. (2014). Evolutionary history of Chaetognatha inferred from molecular and morphological data: a case study for body plan simplification. Frontiers in Zoology, 11, 84.
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