Stoplight loosejaw
The stoplight loosejaws are small, deep-sea dragonfishes of the genus Malacosteus, classified either within the subfamily Malacosteinae of the family Stomiidae, or in the separate family Malacosteidae. They are found worldwide, outside of the Arctic and Subantarctic, in the mesopelagic zone below a depth of 500 meters (1,600 feet). This genus once contained three nominal species: M. niger (the type), M. choristodactylus, and M. danae, with the validity of the latter two species being challenged by different authors at various times. In 2007, Kenaley examined over 450 stoplight loosejaw specimens and revised the genus to contain two species, M. niger and the new M. australis.[1]
Malacosteus and the related genera Aristostomias, Chirostomias and Pachystomias are the only fishes that produce red bioluminescence. As most of their prey organisms are not capable of perceiving light at those wavelengths, this allows Malacosteus to hunt with an essentially invisible beam of light. Furthermore, Malacosteus is unique amongst animals in using a chlorophyll derivative to perceive red light.[1] The name Malacosteus is derived from the Greek malakos meaning "soft" and osteon meaning "bone".[2] Another common name for these fishes is "rat-trap fish", from the unusual open structure of their jaws.[3]
These fishes have a wide distribution in all oceans: M. niger is found between 66° N and 33° S, except for the Mediterranean Sea, while M. australis is found in the southern transition zone between 25° to 45° S, where it is bound by the Antarctic Circumpolar Current. M. niger appears to be replaced by M. australis south of 30° S, while M. australis does not occur north of that latitude outside of the Indian Ocean and the Indo-Australian Archipelago. Both species are usually found below a depth of 500 meters (1,600 feet) in midwater. They are the only known stomiids that do not seem to conduct significant diel vertical migrations.[1]
Malacosteus has an elongated body with short, blunt snouts and large eyes that face forward, granting binocular vision. Unlike other stomiids, it has a single round nostril on each side in front of the eye. Relative to its size, Malacosteus has one of the widest gapes of any fish, with a lower jaw measuring one-quarter of the fish's length. The lower jaw has no ethmoid membrane (floor) and is attached only by the hinge and a modified tongue bone. There are several large, fang-like teeth in the front of the jaws, followed by many small barbed teeth. There are several groups of pharyngeal teeth that serve to direct food down the esophagus.[1][5]
The pectoral and pelvic fins are moderately long, containing 3-4 and 6 fin rays respectively. The dorsal and anal fins are placed far back on the body and contain 18-20 and 19-22 rays respectively. The caudal fin is small, with the lower lobe larger than the upper. There are three bioluminescent photophores near the eyes: beneath the eye is a large, teardrop-shaped suborbital photophore that emits red light. Behind it is an ovoid postorbital photophore that emits green light; this photophore is larger in males than females. These red and green photophores are evocative of traffic lights, hence the fish's common name. The third is tiny and round, located between the eye and the large red photophore. Several rows and clusters of blue photophores are present on the sides and belly. In addition, there are small photophores and accessory areas of white luminous tissue scattered over the head and body. The skin is thin and scaleless; the coloration is black.[1]
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