For deep-sea creatures that live where there’s little to no sunlight, the rules of the game are simple: Eat, don’t get eaten and try to reproduce. Some of these animals — such as jellyfish — produce their own light. Other animals shelter bacteria that produce light for them, like anglerfish with their bacteria-filled lures. A variety of sea creatures release luminescent particles to distract or mark would-be predators, as National Geographic describes.
All of this bioluminescence in a very dark environment has led to extraordinary adaptations, such as eyes that can detect just a few photons of light. As a study in Current Biology reports, researchers have discovered another common tool for survival in the deep sea: ultra-black pigment.
The Blackest Natural Black
Scientists set out on research cruises in Monterey Bay, California, and the Gulf of Mexico to collect samples from 0 to 2,000 meters (about 6,562 feet) below the surface, using a trawl net or remotely operated vehicle. For comparison, commercial scuba divers with special equipment can reach depths of 600 meters (about 1,968.5 feet) before pressure from the water above becomes lethal for humans. The marine samples were collected at night, when deep-sea fish swim higher.
The team identified 16 species of fish with ultra-black skin that reflected less than 0.5% of light shined on them. The darkest species — a dreamer anglerfish — reflected just 0.051% of light. In contrast, regular black fish have 2% to 3% reflectivity, black paper has 10% reflectivity and white or silvery fish have greater than 50% reflectivity. Many transparent sea creatures reflect less than 0.4% of light.
According to the authors, land animals that produce ultra-black pigment include butterflies (0.06% to 0.5% reflectivity), birds of paradise (0.05% to 0.31% reflectivity) and jumping spiders. In these species, the ultra-black pigment is used to highlight bright colors, producing visual signals that warn potential predators or attract potential mates. In contrast, most of the ultra-black fish had black skin covering most of the body, suggesting that the main purpose is camouflage.
Several of the ultra-black fish, including the Pacific dragonfish, use bioluminescent lures. This suggests that the ultra-black is used to conceal the hunter from its potential prey. For one species, ultra-black was found only around the gut, which would be useful for hiding bioluminescence from recently eaten prey. Notably, these 16 ultra-black species represent seven distantly related groups of fish, which all have close relatives that are colorful or silvery. This is good evidence that ultra-black pigment evolved multiple times.
Tightly Packed Melanosomes Trap Light
When the skin of these fish was observed under a microscope, the explanation for the ultra-black coloration appeared to be surprisingly simple. Like mammals, fish produce a pigment called melanin, which is packed inside membranes to form melanosomes. In other animals, including ultra-black land animals, melanosomes are sphere-shaped and separated from one another by supportive keratin protein. When light hits a melanosome, it can be absorbed or reflected. In ultra-black fish, the melanosomes are larger, oval-shaped, and closely packed together.
Computer modeling shows that this arrangement is ideal for absorbing light. Since the melanosomes are so tightly packed, incoming light will almost always hit a melanosome. If that melanosome doesn’t absorb the light, the shape of the melanosome causes the light to be reflected sideways, so it’s likely to hit another melanosome. Thus, evolution has produced a super-efficient, super-thin light trap.
This simple strategy has caught engineers’ attention. Ultra-black surfaces are ideal for optical equipment, such as cameras and telescopes. The ability to absorb stray light can also improve solar panels, camouflage and night ops. Human-made carbon nanotubes are more black than ultra-black fish skins, with commercially available Vantablack absorbing 99.965% of incoming light and an even blacker carbon nanotube absorbing 99.995% of incoming light, MIT News reports.
However, carbon nanotubes are expensive to produce and fragile. Thus, mimicking the simple architecture of melanosomes in ultra-black fish could allow for the production of flexible, affordable and durable ultra-black substances. Popular Science reports that scientists are already experimenting with synthetic melanin particles. Natural melanin can also absorb X-rays, radiation and heavy metals, which could make it useful for many safety applications.
The Challenges of the Deep Sea
You might wonder why deep-sea creatures need ultra-black pigment. After all, their environment is extremely dark, and regular black fish reflect just 2% to 3% of light. Keep in mind that these fish reside in a cold environment where there’s no place to hide, little food and limited oxygen. Therefore, many of these fish are slow and have little energy to spare.
Furthermore, most of the ultra-black fish collected were small enough to easily hold with one hand. Altogether, this means that these fish aren’t strong predators and are vulnerable as prey. The authors used computer modeling to show that fish with 2% light reflectivity could be spotted from twice as far away as fish with just 0.5% light reflectivity (the cutoff point for ultra-black pigment) and six times as far away as the blackest fish with 0.05% reflectance. These can be life-or-death differences for small, slow fish.
We know less about the deep seas of our own planet than about the surface of Mars. A few fishing expeditions revealed an extraordinary adaptation that might improve telescopes and solar panels and reduce exposure to toxic substances. Just imagine what else is hiding in the deep, waiting to be discovered, and how those discoveries may impact and inspire the future of new technology development.
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