Malacosteus niger, commonly known as the black dragon fish, is a species of deep-sea fish. Some additional common names for this species include: northern stoplight loosejaw, lightless loosejaw, black loosejaw, and black hinged-head. It belongs to the family Stomiidae, or dragonfishes. It is among the top predators of the open mesopelagic zone. M. niger is a circumglobal species, which means that it inhabits waters ranging from the tropics to the subarctics. Not many studies have been conducted on its feeding habits, but recent research suggests that M. niger primarily feed on calanoid copepods which is a form of zooplankton. Indeed, it appears that M. niger primarily prey on zooplankton despite its apparent morphological adaptations for the consumption of relatively large prey. Another unique adaptation for this species is its ability to produce both red and blue bioluminescence. Most mesopelagic species aren't capable of producing red bioluminescence. This is advantageous because most other species cannot perceive red light, therefore allowing M. niger to camouflage part of itself to its prey and predators. Anatomy and physiology Visual system Malacosteus niger has yellow lenses that are believed to improve the functionality of the perception of their red bioluminescence. M. niger has adapted a retinal structure of "ten layer elements," similar to those found in surface-level species and other shallow-water living species — which also perceive red light. Its retina is made up entirely of rods and no cones, with rhodopsin/porphyropsin pairs and a single opsin bound to some of its photoreceptors, which provide visual sensitivity up to 517-541 nm (this falls within the wavelength of red light). Most deep-sea fish have a single visual pigment maximally sensitive at short wavelengths, approximately matching the spectrum of both downwelling sunlight and bioluminescence. For comparison, other red light producing stomiids, such as Aristostomias and Pachystostomias, have a third pigment which allows them to perceive light up to 588 nm and 595 nm respectively. The yellow lens reduce the amount of blue light that reaches the retina and increases sensitivity to longer wavelengths, which benefits M. niger and its red bioluminescence. Yellow lens have also been identified in Echiostoma, which also produces red bioluminescence. Morphology Malacosteus niger has one of the largest relative gapes of any fish with the lower jaw being approximately one-quarter of the fishes length (Figure A). It has enlarged fangs the curve back into its mouth to prevent its prey from escaping its