The Southern stoplight loosejaw, Malacosteus australis, is a species of barbeled dragonfish.

Malacosteus australis (Southern stoplight loosejaw)

Malacosteus niger (Stoplight loosejaw)

In 2007, Kenaley examined over 450 stoplight loosejaw specimens and revised the genus to contain two species, M. niger and the new M. australis.

The red photophore of Malacosteus thus allows it to illuminate prey without being detected.

It is yet unclear how Malacosteus captures such small planktonic prey given the open structure of its mouth.

This characteristic is only found in three genera of dragonfish: Malacosteus, Aristostomias, and Pachystomias.

The name Malacosteus is derived from the Greek malakos meaning "soft" and osteon meaning "bone".

Malacosteus niger (Stoplight loosejaw)

Relative to its size, Malacosteus has one of the widest gapes of any fish, with a lower jaw measuring one-quarter of the fish's length.

The unexpected diet of Malacosteus is theorized to be a result of the small volumes that it searches for food, in which large prey items are rare.

No vertebrates are known to synthesize chlorophyll derivatives, and Malacosteus is believed to obtain these derivatives from the copepods it consumes.

The red photophore of Malacosteus consists of a pigmented sac with a reflective inner lining and an internal mass of gland cells.

As most of their prey organisms are not capable of perceiving light at those wavelengths, this allows Malacosteus to hunt with an essentially invisible beam of light.

However, contrary to its apparent morphological specialization, the diet of Malacosteus consists primarily of zooplankton, chiefly large calanoid copepods, with smaller numbers of krill, shrimps, and fishes.

The stoplight loosejaws are small, deep-sea dragonfishes of the genus Malacosteus, classified either within the subfamily Malacosteinae of the family Stomiidae, or in the separate family Malacosteidae.

The rapid attenuation of red light in sea water gives Malacosteus a shorter visual range than species that use blue light, and it does not migrate vertically into more productive waters like other stomiids.

The other factor believed to be partly responsible for Malacosteus diet is its unique visual system, which uses a derivative of chlorophyll as a photosensitizer that absorbs long-wave light (around 700 nm) and then indirectly stimulates the fish's two visual pigments, which have maximum absorbances at only 520 and 540 nm.