In darkness deep
ven when they are illuminated, some species of deep-sea fish remain essentially invisible to potential predators. This makes life more difficult for their would-be foes – and for photographers, who must dig deep into their box of tricks to image them.
The black dragonfish (Idiacanthus antrostomus), which is found in the Pacific Ocean at depths below 200 m, has a good claim to possess the most effective camouflage in the world. In its native habitat, it is to all intents (especially those of potential predators) and purposes, invisible. This is because its ultra-black skin absorbs more than 99.5% of the light that reaches its dimly lit home. This level of absorbance means that even species equipped with bioluminescent organs draw a blank. Their frustration can be easily imagined when one considers that most organisms reflect over 50% of the ambient light. Indeed, transparent animals manage a figure of 0.4%.
A group of researchers from Duke University in Durham (North Carolina), led by Alexander Davis, has now studied the phenomenon of near-total light absorption. In Monterey Bay of the coast of California and in the Gulf of Mexico, the marine biologists discovered no less than 16 different species of deep-sea fish whose skins are extremely black from depths on the order of 1500 meters.
They went on to show that the black dragonfish and its peers owe their ultra-dark colours to a very thin sheet of densely packed pigment cells located immediately below the outermost, epidermal layer of the skin. These cells possess specialised organelles called melanosomes, which are packed with light-absorbing molecules. In the course of evolution, a wide range of organizational patterns has been “discovered” for the storage of melanin, the major pigment synthesized in cells that contain melanosomes. Melanin is the pigment responsible for determining skin colour in most vertebrates, including humans.
The skin of very many species of fish is darkly pigmented. In most of them, however, the pigment cells do not form an unbroken layer, but are intermingled with non-pigmented cells. This arrangement allows significant proportions of the ambient light to be reflected. In contrast, according to Davis and his co-authors, the extremely dense packing of pigment cells found in ultra-black species such as the black dragonfish enables them to reduce the range of vision within which would-be predators can see them by a factor of more than six – relative to prey that reflect 2% of the incoming light.
Needless to say, this unusual feature considerably complicates the task of photographing these fish. Without reflection, there can be no image. Nevertheless, marine biologist Karen Osborn, who is at the Smithsonian National Museum of Natural History in Washington DC, took up the challenge, and succeeded in obtaining impressive photographic portraits of the black dragonfish. “Much of my time is devoted to photographing deep-sea fish,” she explains, as images are very useful for morphological and behavioural studies. But this time, no matter how I set up the cameras and the lighting, the fish absorbed virtually all of the light. I used four flash units for the pictures that appear in the report. Luckily, we were able to keep two specimens in an aquarium on board our research vessel, and this gave me the time to try out enough lighting configurations to allow me to capture morphological details of the animals.”
Osborn used a Canon EOS 5DS R and a 65-mm macro-lens to photograph her uncooperative subjects. “In order to reveal details, you have to use lots of lights and experiment with many different angles and camera settings,” she says. Post-processing was restricted to the use of a high-pass filter to reveal extremely fine detail. “It doesn’t always work, but I got lucky with these two specimens.”
The surface properties that make photographing ultra-black fish a nightmare for photographers could be of interest for technological applications. “In size and form, melanosomes are perfectly designed for maximal extinction of incoming light,” says Alwxander Davis. “Maybe we can someday make use of this astonishing feature of deep-sea fish for the development of ultra-black materials.”
Alexander L. Davis et al.; Ultra-black Camouflage in Deep-Sea Fishes.
Current Biology, 16 July 2020