Deep-sea dragonfish are the only fish that can generate and see red light, which they use to detect prey and sneak up on them, whilst remaining invisible themselves. This unique ability has made them the masters of the deep-ocean by living in a world of their own.
Dragonfish are a group of deep-sea fish that reside between 200-2000m below the surface in the region known as the Bathyal zone. At this depth almost no sunlight reaches the creatures that reside there, leaving them to spend their lives in almost total darkness. As a result bioluminescence, the ability to create your own light, has evolved to become a vital tool for communication, navigation and hunting in the creatures that live there. For most this has meant being able to produce and detect blue-light which works best in their dark world. However some dragonfish have taken their own route and instead both create and see light in the red-end of the spectrum, giving them the unique ability to be able to see and hunt other creatures whilst remaining undetected themselves.
Light in the deep
Water is over 800 times denser than air, which makes it much harder for light waves to pass through it. That is why only the first few hundred metres of the ocean are illuminated by sunlight and the rest is left in total darkness. However light is made up of many different wavelengths, responsible for the different colours we see, which can travel through water at different speeds. The short wavelengths towards the red-end of the spectrum are quickly absorbed by water meaning they do not penetrate very deep into the ocean, whereas longer wavelengths like blue and green can shine much deeper. Which is what gives our oceans their rich blue colour.
So in the deep oceans if you wanted to make your own light, via bioluminescence, it makes sense that you would make it blue rather than red. This is why almost all light created by sea creatures is blue, because it allows them to see further and be seen by others from further away. As a result evolution has shifted life in the deep to adapt accordingly, predators use blue lures and light to hunt for food and prey create red-based camouflages that absorb blue light to hide from those predators. However dragonfish are the exception to this rule by both producing and being able to see shortwave red light, in addition to regular blue-light, giving them a significant advantage over their deep-sea relatives.
Of the 57 species of dragonfish, belonging to the Stomiidae family of fish, only nine have developed the ability to produce and see red light. They were first discovered by researchers at the turn of the century and since then lots of studies have looked into what allows these outliers to be able to do this. It turns out that they use specialized light-emitting cells, known as photophores, which are located below their eyes. These cells create bioluminescence in very much the same way as other deep-sea creatures, by using an enzyme called coelenterazine which emits photons when introduced to free radicals. However unlike the photophores of blue light-emitters that filter out red light, theirs instead filters out blue leaving only red.
The dragonfish use these photophores as searchlights sending out red-shifted light that no other creature can see. This allows them to see their prey who have naturally evolved red camouflage to better avoid the blue bioluminescence of other predators. However this only works if the dragonfish are also able to see the red-light which reflects back at them. To do this they have evolved large eyes which contain an unusual pigment that that boosts their vision in the red-end of the spectrum, meaning they are the only creatures that can actually see the light they are producing. Experts also believe that being the only deep-sea creatures to produce and detect red light means they may also be able to communicate with each other on their own personal wavelength.
Whilst being able to produce and see red light gives dragonfish a significant advantage in terms of being able to see their prey without being detected, there is still the major limitation that red light does not travel very far underwater. To get around this they must therefore get up close and personal with their prey before they strike. To do this they have to remain completely invisible in the dark ocean, which they do in a number of ways.
Firstly they are incredibly dark in colour, with some species such as the pacific blackdragon being classified as ultra-black, meaning they can absorb over 99.5% of the light that hits their skin. In addition to this camouflage most dragonfish also have specialized transparent teeth, which means they can open their large mouths and prepare to strike at their prey without giving away their position. Some red-seeing dragonfish can also skirt around having to sneak up on their prey by getting the prey to come to them. These species, such as the stoplight loosejaw, use blue-light lures in addition to their red vision to attract prey before striking at close distance.
However regardless of which technique they use to land their prey, it is clear that these unique deep-sea hunters are true masters of light in the deep, using their own private wavelength to light up their surroundings without alerting other to their presence.