It is super weird to think of a world without sunlight – yet many deep-sea dwelling species live beyond the reach of solar illumination. In these sun-deprived depths of the oceans, visual predators and prey often rely on light provided by bioluminescent organisms to hunt or to detect predators. Because of the low quantities of light present, the species which inhabit these dark environments are extremely sensitive to an object’s ability to reflect light, and even reflecting a few photons at a time could mean the difference between life and death.

The findings of two recent research expeditions to investigate deep-sea fishes, one in the Gulf of Mexico and one off the coast of Monterey Bay, California, have been published in Current Biology and demonstrate that at least 16 species of distantly related deep-sea fish display ultra-black colouration which allows them to absorb more than 99.5% of light that touches their skin. The researchers hypothesize that this ultra-black colouration has evolved to reduce reflectance of bioluminescent light rendering the fish visually undetectable and effectively camouflaging them against the darkness. 

Different Strokes For Different Folks

Although octopi are often acknowledged as the masters of camouflage in the ocean, many fish species also employ camouflage as a means to avoid detection. There are a number of different strategies used by marine life to remain undetected, such as transparency, mirrored surfaces, counterillumination and pigmentation. Transparency and mirrored surfaces work best in shallow surface waters where there is plenty of diffuse ambient sunlight as mirrored surfaces and white animals can reflect more than 50% of light helping them blend in. Although transparent species reflect low amounts of light (>0.4%), it is possible that it is still enough to allow for detection in low light settings.

Species employing counterillumination techniques emit light from specialised ventral photophores to replace the light from the sun that their bodies block making them appear invisible to viewers from below. Pigmentation is also a common camouflage technique in many pelagic species, which use colour to blend into their surroundings to escape detection. In the depths of the ocean, where there is very little light, reflecting even the smallest amount of bioluminescence can reveal a fish to potential predators or prey. Ultra-black colouration is therefore likely to be a very effective camouflage strategy in low light conditions, as any ambient light is absorbed by the individual allowing it to remain undetected.

On a side note, although many deep-sea invertebrates appear black, they are typically not black but red as red light only penetrates the topmost layers of the ocean and doesn’t reach into the depths.

Functions Of Ultra-black Camouflage

The researchers believe that the ultra-black colouration performs different functions across the different species that they collected. In the majority of the species, the ultra-black skin covers most of the body, suggesting that the primary function is to reduce the reflection of bioluminescence and therefore allow the species to avoid detection. By reducing the percentage of light reflectance, ultra-black fish can reduce the sighting distance of visual predators more than 6 times compared to fish with a 2% reflectance. Due to the intermediate size of the species, the researchers hypothesize that pressure to either avoid predator searchlights and bioluminescent prey defence mechanisms could be driving the development of the ultra-black colouration. 

Some species, such as Sigmops elongatum and Chauliodus macouni, are thought to use a combination of visual strategies to avoid detection as they possess ultra-black colouration above and below a mirrored stripe that runs the length of the body.

In ambush predators that possess bioluminescent lures such as Oneirodes spp., Eustomias spp., and Astronesthes micropogon, researchers believe that the ultra-black skin allows the fish to remain undetected, encouraging potential prey species to come investigate the light source and move within striking distance.

Interestingly, one of the species, Cyclothone acclinidens, only had ultra-black colouration on its gut which the researchers believe functions to conceal the bioluminescent glow from ingested prey items, allowing C. acclinidens to avoid detection while digesting its food.

How Do They Do It?

The researchers investigated the ultrastructure of the fish skin used a combination of light microscopy, transmission electron microscopy (SEM) and scanning electron microscopy (SEM) and found that ultra-black deep-sea fish rely on a continuous layer of closely packed melanosomes (organelles containing melanin – a dark pigment also found in human skin) which are optimized in size and shape to minimize reflectance by scattering light within the dermal layer to increase the optical path length and therefore light absorption by melanin pigments. Unlike other darkly pigmented fish where melanophores (pigment cells containing melanosomes) are separated by unpigmented gaps occupied by collagen and other cells, ultra-black fish have few to zero unpigmented gaps between individual melanophores.

Ultra-black colouration has also been detected in other species such as butterflies and birds of paradise. However, unlike these terrestrial species, which rely on both pigmentation and chitin or keratin structures to absorb light, ultra-black deep-sea fish species reduce reflectance by melanin pigmentation and melanosome arrangement only. Despite this, the low reflectance of the darkest deep-sea fish (Oneirodes sp.) surpasses the darkness of ultra-black butterflies and is on par with the blackest birds of paradise. Oneirodes sp. reflects 0.044% of light at 480 nm which is average wavelength of bioluminescence and ambient sunlight at >200m below sea level and has an average reflectance 0.051% across the visible light spectrum (350 to 700 nm), whereas butterflies typically emit at 0.06% – 0.5% reflectance and darkest birds of paradise at 0.05% – 0.31% reflectance. 

In comparison, the darkest man-made material is vertically aligned carbon nanotubes which reflect about 0.045% of light, but the scientists leading this investigation are hoping that further research into deep-sea fish will drive the development of other light-weight ultra-black material. They hypothesize that there are many more species of ultra-black fish still to be discovered in other parts of the ocean – and who knows what we might learn from them?