Caecosagitta macrocephala

Caecosagitta
Scientific classification
Kingdom:	Animalia
Phylum:	Chaetognatha
Class:	Sagittoidea
Order:	Aphragmophora
Family:	Sagittidae
Genus:	Caecosagitta; Tokioka, 1965[1]
Species:	Caecosagitta macrocephala; (Fowler, 1904)[2]

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. [3] Cecosagitta macrocephalas have large heads due to their name “macro-cephala”. Within their eyes are photoreceptive regions that allow them to catch weak light at bathypelagic depths.[3] 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.[4] To be more precise, the luminescent organ is located on the ventral edge of each anterior lateral fin.[4] 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.[1] C. macrocephala has a secreted bioluminescence that is thought to be coelenterazine based.[1] The luciferase is highly unstable, being unable to survive a single freeze-thaw, and is rapidly inactivated at ice-cold temperatures.

C. 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. [4]

The C. 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."[3]

References

Tokioka, T. (1965). The taxonomical outline of Chaetognatha. Publications of the Seto Marine Biological Laboratory, 12(5), 335–357.
Fowler, G. H. (1905). Biscayan plankton. Part III.—The Chætognatha. Transactions of the Linnean Society of London, (2)10, 55–87.
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.
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.

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[Tokioka-1] Tokioka, T. (1965). The taxonomical outline of Chaetognatha. Publications of the Seto Marine Biological Laboratory, 12(5), 335–357.

[Fowler-2] Fowler, G. H. (1905). Biscayan plankton. Part III.—The Chætognatha. Transactions of the Linnean Society of London, (2)10, 55–87.

[Miyamoto_et_al.-3] 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.

[Thuesen_et_al.-4] 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.