''Caecosagitta macrocephala'' is a deep sea marine

chaetognath 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, about 20% of the known Chaetognatha species are benthic zone, be ...

that is distributed in meso- and bathypelagic layers. It has a very wide distribution that ranges from the Subantarctic to Subarctic Ocean. 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. 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.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. 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 ''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 mac ...

''. ''C. macrocephala'' has a secreted bioluminescence that is thought to be

coelenterazine Coelenterazine is a luciferin, a molecule that emits light after reaction with oxygen, found in many aquatic organisms across eight phyla. It is the substrate of many luciferases such as ''Renilla reniformis'' luciferase (Rluc), ''Gaussia'' lucif ...

based. The luciferase is highly unstable, being unable to survive a single freeze-thaw, and is rapidly inactivated at ice-cold temperatures. ''Caecosagitta macrocephalas 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. ''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."

References

Chaetognatha Bioluminescent arrow worms Monotypic protostome genera {{Chaetognatha-stub