Caecosagitta

{{short description|Species of worm}} {{Speciesbox |image = Caecosagitta_macrocephala_463758980.jpg |genus = Caecosagitta |parent_authority = Tokioka, 1965<ref name=Tokioka>Tokioka, T. (1965). The taxonomical outline of Chaetognatha. ''Publications of the Seto Marine Biological Laboratory'', 12(5), 335–357.</ref>| |species = macrocephala |authority = (Fowler, 1904)<ref name=Fowler>Fowler, G. H. (1905). Biscayan plankton. Part III.—The Chætognatha. ''Transactions of the Linnean Society of London'', (2)10, 55–87.</ref> }}

'''''Caecosagitta macrocephala''''' is a deep sea marine [[Chaetognatha|chaetognath]] that is distributed in meso- and bathypelagic layers. It has a very wide distribution that ranges from the Subantarctic to Subarctic Ocean. <ref name="Miyamoto et al.">Miyamoto, H., Machida, R. & Nishida, S. (2010). Genetic diversity and cryptic speciation of the deep-sea chaetognath ''Caecosagitta macrocephala'' (Fowler, 1904). ''Deep-Sea Research Part II: Topical Studies in Oceanography'', 57(24–26), 2211--2219.</ref> 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.<ref name="Miyamoto et al." /> 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.<ref name="Thuesen et al.">Thuesen, E. V., Goetz, F. E. & Haddock, S. H. (2010). Bioluminescent organs of two deep-sea arrow-worms, ''Eukrohnia fowleri'' and ''Caecosagitta macrocephala'', with further observations on bioluminescence in chaetognaths. ''Biological Bulletin'', 219(2), 100–111.</ref> To be more precise, the luminescent organ is located on the ventral edge of each anterior lateral fin.<ref name="Thuesen et al." /> 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]]''.<ref name=Tokioka/> ''C. macrocephala'' has a secreted bioluminescence that is thought to be [[coelenterazine]] based.<ref name=Tokioka/> The luciferase is highly unstable, being unable to survive a single freeze-thaw, and is rapidly inactivated at ice-cold temperatures.

''Caecosagitta macrocephala'''s bioluminescent organs consist of hexagonal chambers containing elongate ovoid particles—the organelles holding bioluminescent materials. No other luminous organism is known to use hexagonal packing to hold bioluminescent materials. <ref name="Thuesen et al."/>

''Caecosagitta macrocephala'' found in the north-western Pacific and centre-east to south Atlantic Oceans, were found to consists of at least two cryptic species due to "speciation of the bathypelagic species from a mesopelagic precursor."<ref name="Miyamoto et al."/>

==References== {{Reflist}}

[[Category:Chaetognatha]] [[Category:Bioluminescent arrow worms]] [[Category:Monotypic protostome genera]]