Malacosteus niger

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.[2] 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).[3] 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.

M. niger is unique from other stomiid genera because its jaw is over 30% its whole body length (average adult length of 24cm according to the Australian Museum), which aids in catching larger prey.[4] Its jaw is also made up of large, fang-like teeth, its lower-jaw lacks a floor to the oral cavity, and it can open its jaw greater than 120˚.[4] The lack of a floor of to the oral cavity allows for decreased resistive forces which allows M. niger to close its mouth rapidly and easily trap its prey. This adaptation also minimizes the amount of energy required for M. niger to close its mouth, thus permitting it to quickly latch onto fast-swimming prey.[4]

The postorbital photophore in this species is larger than in M. australis. It also differs in lateral photophore count, as well as in morphological characters. The maximum known length is 25.6 cm (10.1 in). Its specific epithet niger is Latin for "black".[5]

While the morphology of M. niger with huge fangs and an enormous gape is typical for its family and suggests adaptations to piscivory, its diet in fact contains a substantial proportion of zooplankton.[8] Some of its documented prey include calanoid copepods, micronekton, decapod shrimps, and other decapods.[6] M. niger digests its prey within a diel cycle, meaning the copepods it consumes at nighttime are digested by the afternoon the following day, which requires it to be constantly feeding on these small prey to sustain its energy; copepods make up 69-83% of its diet.[6] It is suggested that its dominant feeding mode is searching for zooplanktonic prey (copepods in particular) using bioluminescence to illuminate a small search area, with infrequent encounters with larger prey items. The likely origin of the pigment necessary for detecting its long wavelength bioluminescence, a chlorophyll derivative, is the copepods themselves.[8][9]