Fish can't see red light and loosejaws fishes use this fact to hunt

Fish can’t see red light and some species use this fact to hunt

There’s a deep sea species that takes advantage of the fact that fish can’t see red light, and illuminates its prey with a beam of red bioluminescence so it can hunt with an effectively invisible beam of light

 

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. They are found worldwide, outside of the Arctic and Subantarctic, in the mesopelagic zone below a depth of 500 meters (1,640 ft). This genus once contained three nominal species: M. niger (the type), M. choristodactylus, and M. danae, with the validity of the latter two species being challenged by different authors at various times. In 2007, Kenaley examined over 450 stoplight loosejaw specimens and revised the genus to contain two species, M. niger and the new M. australis.[1]

Malacosteus and the related genera Aristostomias and Pachystomias are the only fishes that produce red bioluminescence. 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. Furthermore, Malacosteus is unique amongst animals in using a chlorophyll derivative to perceive red light.[1] The name Malacosteus is derived from the Greek malakos meaning “soft” and osteon meaning “bone”.[2] Another common name for these fishes is “rat-trap fish”, from the unusual open structure of their jaws

 

As long wavelengths of light (i.e. red) do not reach the deep sea, many deep-sea organisms are red-colored (effectively appearing black) and are insensitive to red wavelengths. The red photophore of Malacosteus thus allows it to illuminate prey without being detected. These fishes exhibit a number of adaptations for feeding on large prey. The “open” structure of its jaws reduces water resistance, allowing them to be snapped shut more quickly, while large recurved teeth and powerful jaw closing muscles assure a secure hold on prey items. The connection between the head and the body is reduced, with unossified vertebrae, allowing the cranium to be tilted back and the jaws thrust forward for a wider gape. Finally, the gills are exposed to the outside, allowing the fish to continue respiring while slowly swallowing large prey.[6]

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. It is yet unclear how Malacosteus captures such small planktonic prey given the open structure of its mouth.[5] 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. 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. Therefore, its strategy may be one of “snacking” on copepods, which are three orders of magnitude more abundant than fishes at its native depths, in between larger meals.[5]

 

The red photophore of Malacosteus.

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. No vertebrates are known to synthesize chlorophyll derivatives, and Malacosteus is believed to obtain these derivatives from the copepods it consumes.[7] The red photophore of Malacosteus consists of a pigmented sac with a reflective inner lining and an internal mass of gland cells. Inside the gland cells, blue-green light is produced via the same chemical reaction found in other stomiids, which is then absorbed by a protein that fluoresces in a broad red band. This light is then reflected out through the photophore apterture, where it passes through a brown filter, yielding a far-red light with a maximum absorbance at 708 nm (almost infra-red). In live fish, the suborbital and postorbital photophores both flash vigorously, the suborbital at a slower rate