''Malacosteus niger'', commonly known as the black dragon fish, is a species of
deep-sea fish
Deep-sea fish are fish that live in the darkness below the sunlit surface waters, that is below the epipelagic or photic zone of the sea. The lanternfish is, by far, the most common deep-sea fish. Other deep sea fishes include the flashlight ...
. Some additional common names for this species include: northern stoplight loosejaw, lightless loosejaw, black loosejaw, and black hinged-head.
[Harold, A. 2015]
''Malacosteus niger''.
The IUCN Red List of Threatened Species. Downloaded on 20 February 2016. It belongs to the family
Stomiidae
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 are ...
, or dragonfishes. It is among the top predators of the open
mesopelagic zone
The mesopelagic zone (Greek μέσον, middle), also known as the middle pelagic or twilight zone, is the part of the pelagic zone that lies between the photic epipelagic and the aphotic bathypelagic zones. It is defined by light, and begins ...
.
''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
copepod
Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (inhabiting sea waters), some are benthos, benthic (living on the ocean floor) ...
s which is a form of
zooplankton
Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...
. 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
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 ...
. 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
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 ...
. ''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
Rhodopsin, also known as visual purple, is a protein encoded by the RHO gene and a G-protein-coupled receptor (GPCR). It is the opsin of the rod cells in the retina and a light-sensitive receptor protein that triggers visual phototransduction ...
/porphyropsin pairs and a single
opsin
Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become Retinylidene proteins, but are usually still called opsins regardless. Most pro ...
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
''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 ''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
![M](https://upload.wikimedia.org/wikipedia/commons/e/e2/M._niger_Morphology_Characteristics.jpg)
''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 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. Lastly, ''M. nigers'' lacks an ethinoid membrane (no “floor” in its mouth) which allows for it to consume bigger prey species (Figure D).
(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.
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".
Ecological and geographical distribution
''Malacosteus niger'' is a circumglobal species and has a large geographic range. It can typically be found from the Arctic latitudes of 66° North and from 30° South in the Southern Hemisphere.
While ''M. nigers'' is found throughout the world, it appears to be widely distributed and found often in the Eastern Central Atlantic Ocean. ''M. niger'' does not leave the mesopalgic and is known to have a vertical range from 500 meters to 1000 meters.
Interestingly, it is believed to be the only member of the family that does not go through diel/vertical migration, which means that it does not migrate up to the surface like other fish species.
Diet
While the morphology of ''M. niger'' with huge fangs and an enormous gape is typical for its family and suggests adaptations to
piscivory
A piscivore () is a carnivorous animal that eats primarily fish. The name ''piscivore'' is derived . Piscivore is equivalent to the Greek-derived word ichthyophage, both of which mean "fish eater". Fish were the diet of early tetrapod evolut ...
, its diet in fact contains a substantial proportion of
zooplankton
Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...
.
Some of its documented prey include calanoid
copepod
Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (inhabiting sea waters), some are benthos, benthic (living on the ocean floor) ...
s, micronekton, decapod shrimps, and other
decapods
The Decapoda or decapods (literally "ten-footed") are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crabs, lobsters, crayfish, shrimp and prawns. Most decapods are scavengers. The order is estim ...
.
''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 copopods make up around 69%-83% of ''M. niger''
's diet.
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.
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
Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to a ...
derivative, is the copepods themselves.
Red bioluminescence
''Malacosteus niger''
's unique adaptation of producing red bioluminescence is only found in two other deep-sea dwelling creatures, ''Aristostomias'' and ''Pachystomias''. This rare form of bioluminescence can reach up to 700 nm in the deep-sea and cannot be perceived by green and blue bioluminescent organisms, thus granting ''M. niger'' a considerable advantage while hunting for food.
Other deep-sea fish capable of detecting far-red bioluminescence, including ''
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 ''
Pachystomias
''Pachystomias microdon'', the smalltooth dragonfish, is a species of barbeled dragonfish 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 Biolumine ...
'' are able to do so using visual pigments. ''M. niger'' lacks these same long-wave pigments, and instead increases its sensitivity to red light using a chlorophyll-derived photosensitizer.
''Malacosteus Niger'' has a tear-drop shaped, dark brown, suborbital photophore which is used to emit red light at an emission maxima of 710 nanometers. Removal of the top brown photophore layer causes a shift of the emission spectra to shorter wavelengths of around 650 nanometers. The photophores contain red-fluorescent material which is made to fluoresce via energy transfer from chemical reactions. Control of the photophore is maintained via innervation through branches of the fifth cranial nerve and this photophore. It is reported to be controlled independently of the postorbital blue photophore and has been noted to fluoresce for longer durations. The photophore is composed of a large pigmented sac containing a mass of scarlet gland cells. A thick, reflective layer lines the pigment sac, with occasional strands of reflective tissue running through the glandular core of the photophore. The outer layer is composed of large epithelial cells which merge into an inner, darker stained layer. The presumed function of this layer is to provide the brown layer through which fluorescence is filtered. The cells of the glandular core are characterized by a dense rough endoplasmic reticulum.
References
{{Taxonbar, from=Q3361606
Stomiidae
Fish described in 1848
Taxa named by William Orville Ayres