Although quite monstrous in appearance, Malacosteus niger does not get very big, growing to a maximum length of around 25cm. Not much is known about its biology.
The scaleless, very thin, black skin is easily damaged during capture in a net, which is why a lot of museum specimens have bald patches.
M. niger has two large light organs, or photophores, on each side of its head:
Like many deep-sea fishes, M. niger uses a chemical called luciferin to produce blue light (460-490 nanometres in wavelength). This is capable of the greatest penetration through the water and can therefore be seen from further away than other wavelengths.
To create red light, a protein in the suborbital photophore absorbs blue light and then emits light of a greater wavelength. This light then passes through a special filter which increases the wavelength even further to over 700nm.
The ability of Malacosteus species to produce red light appears to be nearly unique, shared by only two other groups of closely related fishes and a beetle.
It is almost impossible to study the sex lives of deep sea organisms due to the obvious difficulty in observing such behaviour, therefore a lot of speculation has arisen.
One interesting feature of M. niger is that it shows sexual dimorphism - the males have a larger postorbital photophore than the females. It is possible that a male could use this light as part of a courtship display or at least to alert potential mates in the vicinity to his presence and availability, but we may never know for sure.