''Pachystomias microdon'', the smalltooth dragonfish, is a species of

barbeled dragonfish Stomiidae is a family of deep-sea ray-finned fish, including the barbeled dragonfishes. They are quite small, usually around 15 cm, up to 26 cm. These fish are apex predators and have enormous jaws filled with fang-like teeth. They ar ...

found in the oceans at depths of from . This species grows to a length of SL. This species is the only known species in its genus.

Red Light Bioluminescence

Production of far-red bioluminescence

''Pachystomias'' is one of three deep-sea fish that can produce red light

bioluminescence Bioluminescence is the production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some b ...

, along with ''

Aristostomias ''Aristostomias'' is a genus of barbeled dragonfishes native to the ocean depths in the Pacific, Atlantic and Indian oceans. Red Light Bioluminescence Production of red light bioluminescence Similar to other deep-sea organisms that are capa ...

'' and '' Malacosteus''. In addition to producing blue light via postorbital photophores, ''Pachystomias'' also possess suborbital and preorbital cephalic photophores that are capable of producing far-red bioluminescence, with wavelength emissions of over 650 nm. Compared to other bioluminescent fish, ''Pachystomias'' has a uniquely large suborbital photophore, which extends from the orbit to the roof of the mouth, while the preorbital photophore is much smaller. The suborbital and preorbital organs have been observed to produce both bright flashes as well as steady glows.

Visual systems

Deep-sea fishes that are able to detect light typically have visual pigments sensitive to blue and green light, ranging from 470-490 nm. However, deep-sea loose-jawed dragonfish, including ''Pachystomias,'' are sensitive to long-wave light and are able to detect their own bioluminescence. ''Pachystomias'' has at least three long-wave shifted pigments that can detect wavelengths of up to 595 nm.

Adaptive significance

''Pachystomias'' are able to both produce and see far-red wavelengths. Because the ability to detect red light is rare, it is thought that this adaptation could serve deep-sea loose-jawed dragonfish by acting as a prey-detection system, as well as for intraspecific communication.

References

* Stomiidae Taxa named by Albert Günther Monotypic marine fish genera {{Stomiiformes-stub