Fish have essentially three options to survive in the deep-sea where there are few places to hide: be big, be fast or be invisible. Being big and fast takes a lot of energy, which requires a lot of food. Being invisible, on the other hand, may be a better strategy for escaping predators and moving through the water undetected by prey. In a new study published last week in Current Biology, researchers have discovered an evolutionary tactic that gives some fishes an invisibility cloak. For the first time, an ultra-black skin color or pigmentation that protects 16 varieties of deep-sea fishes has been documented and studying the efficient survival tactic could provide insight into developing new synthetic camouflage materials as well as have implications for the understanding and treatment of skin cancers.
Ninety-nine percent of habitable space on the planet is in the ocean, and we have only begun to understand the diversity of animals that call the sea home and how they have adapted to unique underwater environments. The “mid-ocean,” which is not at the bottom nor at the surface, is an especially challenging habitat to survive in.
“You have no place to rest, you have no place to hide, you have very little food available, and it's really dark,” says Karen Osborn, a marine biologist at the Smithsonian’s National Museum of Natural History, and one of the authors of the study. “There are all these weird things that are quite different than most of the other habitats that we think about, even the deep-sea floor. And subsequently, most of the animals that live out there look really weird.”
Enter the ultra-black fishes, several species that can move with stealth in this challenging environment. Osborn and her colleagues had been studying how the skin and surfaces of fishes and other animals help the animals to survive. Some crustaceans, for example, are transparent, but if light shines on them, they can be easily detected. These creatures have developed anti-reflective coatings on their shells to help reduce glare that would give them away to predators. While netting crabs, Osborn and her team happened to pull up a fangtooth fish, which, try as they might, the researchers just couldn’t get a good photograph of the creature. Why, wondered Osborn? It turns out, the skin of the fish was simply unphotogenic—the tissue was absorbing a whooping 99.5 percent of the camera’s light.
Other ultra-black animals, like birds-of-paradise, some butterflies, beetles and snakes carry the light-absorbing pigment along with bright, vibrant colors that combine to create an eye-catching display. Against the ultra-black, colors just pop. And the effect works to signal danger warnings to would-be predators and come-hither messages to potential mates. But rather than using the strategy to draw attention to themselves, the ultra-black fish in the middle ocean simply disappear.
The fangtooth fish was one of 16 species of ultra-black fishes that the researchers have since identified. To be classified as ultra-black, the bar was high. Like the fangtooth, the researchers were looking for fish skin that reflected less than .5 percent of light across the visible spectrum. They collected deep-sea fish specimens from 18 different species and used a special black-reflectance light probe to measure the angles and the amount of light that were absorbed. They found that 16 of the species qualified. By comparison, man-made black materials reflect ten percent of light, and other black fish reflect two to three percent, giving ultra-black species a six-fold advantage when it comes to hiding.
“It’s a splendid exercise in quantifying blackness,” says Peter Herring, marine biologist and author of The Biology of the Deep Ocean, who wasn’t part of the study team. “Deep-sea fishes are routinely described as inky black or velvet black, so it’s nice to have some numerical basis. On an intuitive level one might think that just two percent reflectance would be good enough, but if you get a six-times improvement then no doubt an evolutionary [advantage] could have occurred.”
After seeing the results of the reflectivity measurements, the researchers dug deeper to find out how the fishes were capable of such expert-level camouflage. Using microscopy to examine tissue samples from non-black fishes, black fishes and the ultra-black fishes, they found that the ultra-black fishes had unique patterns and organizing principles in the pigment cells of their skin.
Every fish produces melanin; it is the same chemical found in human skin that protects from UV light. Melanin is produced in much the same way across species. But when researchers examined the tissue of ultra-black fish skin, the researchers found that their melanosomes, or the cells that hold the pigmenting chemical, were different in three important ways. The cells were more densely packed, larger, and capsule-shaped rather than rounded. Because of this structure, photons of light that hit the surface of the fishes’ skin are absorbed not only by the cell they hit, but the light also gets sucked sideways into the cells next to it.
“So basically, by changing the shape and the size of those granules,” says Osborne, instead of letting light that’s not immediately absorbed escape and signal their presence, “they control it so that the light goes into the layer and side-scatters into the granules next to it.”
But given the vastness and darkness of the deep ocean, how much of a difference does it actually make if a fish absorbs three percent of light or .5 percent of light, and where is that light coming from anyway? Because very little sunlight reaches these regions, any light that is produced is typically coming from another organism—like those that use bioluminescence—and there is a good chance that that organism is looking for a meal.
“There is a ton of animals down there, but their density is relatively low, which means you probably very rarely meet your lunch. So, when you do meet your lunch, you want to make sure that you catch it,” says Ron Douglas, a marine biologist at the City University of London who studies visual systems and who was also not part of the study team.
Water molecules scatter what little light there is and so the sight distance for most underwater organisms is not very far, says Douglas. “We’re talking probably inches. But let’s say if you can be seen from six feet or one foot, that makes a hell of a difference in terms of [escaping]. Reflective percentages of a couple of percent doesn’t seem like a lot, but it’s very significant.”
The researchers investigating this evolutionary survival tool say that the tissue structure has wide applications. Melanin, a type of chemical that can release or absorb free radicals of oxygen that can damage cells, is packaged inside melanosomes, to keep it contained as it travels to the outer layers of the skin. Typically, these cells are loosely spaced out around the skin. In ultra-black fish skin, melanosomes somehow protect the skin without damaging the rest of the creature’s cells or organs even as they form a dense, continuous layer that might otherwise be indicative of disease. “Basically, these fish look like they have melanoma all over their body,” says Osborn. Oncology and dermatology researchers want to learn more about how the chemical is managed or controlled in fish skin.
In addition to creating a layer of camouflage, melanin can also absorb X-rays, radiation and heavy metals, which is why ultra-black skin in fishes has piqued the interest of material scientists. According to Osborn, Naval researchers, for example, are interested in how this discovery might aid them in developing coatings for submarines and other vessels. “If you were to make, let’s say, armor that had melanin on the outside, you would be great for night ops, or able to walk through Chernobyl and be safe,” she says.
“Everybody wants to be stealthy,” Osborn adds. And ultra-black fish have stealthy down to a science.