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.[4] 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).[5] Most deep-sea fish have a single visual pigment maximally sensitive at short wavelengths, approximately matching the spectrum of both downwelling sunlight and bioluminescence.[6] 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.

Images showing morphological characteristics of Malacosteus niger.

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).[2] It has enlarged fangs the curve back into its mouth to prevent its prey from escaping its grip (Figure B). M. nigers is unique in that it does not contain gill rakers or gill teeth which is typically found in carnivorious fish species (Figure C). The anterior vertebrae appear to be unossified which enables the fish to “throw back its head” to take on relatively large prey.[7] Lastly, M. nigers lacks an ethinoid membrane (no “floor” in its mouth) which allows for it to consume bigger prey species (Figure D).[2] (Refer to image on right hand side of webpage). The lack of a floor of 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.[8]

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".[9]

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.[11] Some of its documented prey include calanoid copepods, micronekton, decapod shrimps, and other decapods.[2] 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. It has been recorded that copepods make up around 69%-83% of M. niger's diet.[11] This suggests that availability of large prey at these depths is often limited. It is suggested that its dominant feeding mode is searching for zooplanktonic prey (copepods in particular) using bioluminescence to illuminate a small search area, since M. niger experiences infrequent encounters with larger prey items. Current research suggests that M. niger has adopted this unique feeding habit in association with the abundance of prey. Recent studies in the eartern Gulf of Mexico indicate that large calanoid copepods are three orders of magnitude more abundant than either fish or shrimp.[11] More research is needed within different regions to confirm this hypothesis. 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.[11][12]